yeah i know more dual battery questions!

[gu town]

i've had a search or two about these issues but not quite what i was after.

1) i've heard that you shouldn't, or there is an issue with running two different size or type of batteries in your dual battery set up. i've been told they can "fight" when charged or linked in parallel for any period of time?


2) would there be any drawbacks in using a smaller (say 75amp) dual battery isolator to actuate a larger amp rated solenoid (old winch solenoids) to be able to get the current ratings up around 400amp to winch off both batteries?

i have a fairly good knowledge of power and the like so dont hold back on the tech stuff

cheers

[its aford not a nissan]

you dont need a smaller relay of 75 amps to activate the bigger one , a switch will be fine , unless for some reason the big relay needs over 10 amps to activate ( which i doubt ) normally it is less than 1 amp draw for any size relay

[PBBIZ2]

GU Town,

I have run triple aux battery sustems over the last 15 years with the following observations. My application might be different to yours, but the 'mechanics' should be similiar.

1) you should not run different 'styles' of batteries in parallel as they will likely have different charging profiles. One battery will inevitably not get the charge it needs or the other will get over charged, either way you risk shortening the life of at least one battery. I only couple like batteries of identical make, just to try and get the closest 'like fit' with the charge/discharge profile.
I have tried to charge a dual battery set-up with the aux. battery pack made up of a wet cell century 80A/Hr and a Sonneshein 200A/H gel battery, and the system was a disappointment - just couldn't get either to charge properly.
2) If you think about two batteries linked together in a parallel config, imagine they are just tanks with an interlinked pipe. It will depend how the piping feeding the tanks and taking power out of the tanks is configured. If the feed and link pipes are the same size and the discharge goes out the same feed pipe - hope you are following this convoluted description! - then you should in theory be drawing each battery level down evenly, provided the drawdown rate doesn't exceed the free flow rate of the inter-connecting pipe. If however you have a single feedline for both batteries and dual power take-off, if you draw evenly from both batteries, the smaller one will be depleted quicker due to reduced capacity. Also, as the voltage drops, higher amps, more heat, higher resistance, more losses - etc. In the end, the smaller size battery will crash quicker, and with the parallel linking the larger battery will try to equalise or send power across to the smaller battery - maybe what you said about 'fighting each other' perhaps.
This will be very evident in high current applications such as winching. For low current draw such as fridges and lights, which is my application, the discharge is so low that the large battery compensation just isn't a real issue and something I live with. I run an 80 amp under the bonnet and an 80 a and 55a behind the winch between the chassis rails on a GQ. The winch is a low mount, so it all fits, but very tight and a pain to install and maintain.

Sorry, can't comment on your second question as don't understand the issue to be honest.

Hope this helps

[-Scott-]

1) you should not run different 'styles' of batteries in parallel as they will likely have different charging profiles. One battery will inevitably not get the charge it needs or the other will get over charged, either way you risk shortening the life of at least one battery. I only couple like batteries of identical make, just to try and get the closest 'like fit' with the charge/discharge profile.

2) If you think about two batteries linked together in a parallel config, imagine they are just tanks with an interlinked pipe. It will depend how the piping feeding the tanks and taking power out of the tanks is configured. If the feed and link pipes are the same size and the discharge goes out the same feed pipe - hope you are following this convoluted description! - then you should in theory be drawing each battery level down evenly, provided the drawdown rate doesn't exceed the free flow rate of the inter-connecting pipe. If however you have a single feedline for both batteries and dual power take-off, if you draw evenly from both batteries, the smaller one will be depleted quicker due to reduced capacity. Also, as the voltage drops, higher amps, more heat, higher resistance, more losses - etc. In the end, the smaller size battery will crash quicker, and with the parallel linking the larger battery will try to equalise or send power across to the smaller battery - maybe what you said about 'fighting each other' perhaps.

Hope this helps

No, it doesn't. It's primarily crap. Fundamental flaws in the physics. Your understanding of how lead acid batteries charge and discharge is simply wrong.

Almost all lead acid batteries can be paralleled up - don't mix "calcium" batteries with "standard", but other than that, 12V batteries can be mixed with 12V batteries. They are the simplest batteries to charge using a simple float charge, and, unless you try to charge them too fast, each will charge itself at its own rate until it's happy. Similarly, under discharge, their terminal voltage will drop together (they're electrically connected) and each will contribute whatever current it can at that terminal voltage.

They will not "fight" when charged, you cannot harm them by linking them in parallel - unless you try to use a sophisticated multi-stage fast charging regime on the combined pair. If you want to do this, then PBBIZ2 is right - it's best to have two identical batteries. Or charge each separately.

[chimpboy]

1) i've heard that you shouldn't, or there is an issue with running two different size or type of batteries in your dual battery set up. i've been told they can "fight" when charged or linked in parallel for any period of time?

There are issues with running two different batts in series (ie, for a 24V system), or putting different loads on them. And somehow people have confused this with parallel set-ups where there is no particular issue.

In most dual battery systems the whole idea is to have two different batteries, for example one cranking battery and one deep cycle battery.

[PBBIZ2]

Scott,

I have re-read the original question and poorly configured my answer. I interpreted the 'mixing of battery types' not on different charge legs, but on individual legs, ie, crank- wet acid, aux - maybe wet acid and gel as a combination.

This now appears to not be the question at all, so my comments don't stand for the specifics.

This application is for a vehicle I believe, probably using a standard 'bulk charge alternator' so without the interface of some smart controllers you won't be able to trickle charge the batteries is a slow way to bring them up to peak or allow them to equalise. The standard alternators are designed to dump high current into a battery over a short period to replenish the cranking/startup loss - as you would be aware. They typically have a duty cycle rating of 3-5%. I don't know of any alternators alone that have the ability to perform the trickle charge function - if there are, pls let me know as would be keen to get my hands on one.

I don't know anything about calcium batteries, as have stated before in previous threads. I do know that when you set up a smart battery controller on a vehicle alternator charge circuit, you need to select the correct battery types for each charge leg as both the charge voltages, current limits and charge time vary greatly. It is very easy to shorten a battery life on one of these controllers if the 'dip switches' are not in the right spots.
You are not able to have your auxilliary battery bank made of of different types of batteries(such as wet +gel) as the controller will only charge to the one selected profile, and has no way of detecting the type or change of batteries.
Lastly, I withdraw the comment about 'fighting' as the question related to charge and I was speaking about discharge.

Happy to hear back if you have further comments of value for sure - always glad to learn something.

[presto]

too much reading and decyphering for me in the last couple of posts, its doing my head in! but heres my two cents and an attempt at simple info.

queestion one
running different sized batteries in a dual battery system is fine - provided they are BOTH kept charged at their capacity. alot of people have told me they have a 'smart' isolator that charges the flatter battery first and when i look, its just a voltage sensitive relay (ie redarc isolator - senses a certain voltage and joins batteries together, seperates once reaching a specific lower voltage)
one thing people need to understand is that if you run accessories of your second battery while the isolator is isolating, ie the car is turned off and you are running a fridge for two nights, driving the car for a couple of days will NOT charge the aux battery up properly. it should be put on a charger while isolated when possible to bring it back up to full capacity - equal to the cranking battery so there is no 'fighting' between the batteries when charging.
if one battery is in a lower state of charge than the other, it will draw from that battery to try and bring them to the same level of charge.
so as long as both battries are charged, there is no problem with having different sizes.

question two
you can use two heavy duty solenoids (cole hersee or equivalent) to run heavy battery cables to your winch, these are rated up to around 200amps from memory and draw bugger all to switch them. tho if you plan on doing more winching than just the once every now and then get-out-of-a-tight-spot-whoops-i-didnt-mean-to-be-here-what-was-i-thinking situation, then id be using another cranking battery or a dual purpose battery to stop from destroying your deep cycle.

so what batteries are you using and what are you using them for is my question

[chimpboy]

one thing people need to understand is that if you run accessories of your second battery while the isolator is isolating, ie the car is turned off and you are running a fridge for two nights, driving the car for a couple of days will NOT charge the aux battery up properly.

How much driving would you say would be required before both batteries are fully charged? I would have said just a few hours at the absolute max, so that shows how much I know

[stuee]

one thing people need to understand is that if you run accessories of your second battery while the isolator is isolating, ie the car is turned off and you are running a fridge for two nights, driving the car for a couple of days will NOT charge the aux battery up properly.

How much driving would you say would be required before both batteries are fully charged? I would have said just a few hours at the absolute max, so that shows how much I know

This is something I'd like to know too. I think the survey drivesafe held recently (which I didnt see till the other day ) should give some feedback to that. I haven't chased it up to see what results he has received back yet though.

[presto]

the alternator will be putting out more than enough juice to charge the battery up, but because the cranking battery is full and the aux battery is low this will create the 'fighting' i referred to and it kind of plateaus between the batteries. if you go away for a week and run the aux battery flat, driving will charge it back up but not to 100 percent. it will be fine for the duration of the trip but the best way to get the most out of any battery and get the most life out of it is to keep it fully charged and happy. i usually recommend to anyone with a dual battery system to give the aux battery a good charge while isolated when they get back from the trip just to make sure both batteries are kept at 100 percent. also the advantage of putting it on a charger when you get home is that the charger will most likely be anything from a 3 to 8 stage charger which will recharge the battery, desulphate, trickle, whisper sweet nothings to it, buy it flowers and all the rest of it to make the battery happy and live a long and prosperous life

[-Scott-]

if one battery is in a lower state of charge than the other, it will draw from that battery to try and bring them to the same level of charge.

No. This is simply not possible.

Lead acid batteries use a chemical reaction. During discharge, the reaction goes in one direction (for want of a better description) and produces voltage. During charge, a higher voltage is used to force the chemical reaction to reverse.

If you have two healthy 12V lead acid batteries connected in parallel, one cannot charge the other. The discharge reaction cannot produce sufficient voltage to reverse the same reaction in the other battery.

[-Scott-]

the alternator will be putting out more than enough juice to charge the battery up, but because the cranking battery is full and the aux battery is low this will create the 'fighting' i referred to and it kind of plateaus between the batteries. if you go away for a week and run the aux battery flat, driving will charge it back up but not to 100 percent. it will be fine for the duration of the trip but the best way to get the most out of any battery and get the most life out of it is to keep it fully charged and happy. i usually recommend to anyone with a dual battery system to give the aux battery a good charge while isolated when they get back from the trip just to make sure both batteries are kept at 100 percent. also the advantage of putting it on a charger when you get home is that the charger will most likely be anything from a 3 to 8 stage charger which will recharge the battery, desulphate, trickle, whisper sweet nothings to it, buy it flowers and all the rest of it to make the battery happy and live a long and prosperous life

I have no argument with the concept of putting the aux battery on a fancy multi-stage charger which will treat the battery better than it has ever been treated before. Definitely a good thing.

But, call me cranky if you like, it is NOT necessary. I get sick of reading this over and over again that an alternator is not capable of charging a battery to 100%. A healthy alternator system is capable of not only fully charging a 12V battery, but it will, if anything, overcharge the battery - if given enough time.

Go and look at battery specifications from the manufacturer. Most, if not all, recommend a constant voltage charge as the best charge method, and generally recommend 2.2 - 2.3V per cell as the constant voltage to apply. An automotive 12V lead acid battery has 6 cells, so the optimum float charge voltage is 13.8V (at 25 deg C).

But, as most people in this thread have recognised, a 13.8V charge takes time to charge the battery. I just had a quick look at a Full River spec sheet (because that's the battery I've been cooking under my bonnet for 3 years) and that indicates up to 5 hours to full charge (after dropping out of constant current mode) depending on other conditions. What's that? To and from work every day for a week?

9 hours to fully charge is too long for automotive applications, so most alternators will charge around 14.5V - because that will bring the battery back to full voltage faster, and, given time, actually cook it. I have noticed that my 4wd will normally indicate battery voltage over 14V initially, but after a few hours of driving, that voltage comes back to below 14V. I have NFI what causes the change, other than perhaps temperature compensation?

If you stationary camp and only drive for an hour or two every few days your alternator will not keep your auxiliary battery fully topped up.

If you're a tourer who spends 4-6 weeks touring each year and drives 6-8 hours every day (even with breaks in between) it really doesn't matter whether your aux battery returns to 90% or 95% every day - because your battery should be sized so that you're using less than 50% of it's capacity anyway. You should still get longer life from your auxiliary battery than you will from your starter. In between times, when you're not discharging your aux battery but still driving the vehicle regularly the aux battery will reach 100% charged - after a few days. If you want to take the battery out and stick it on a smart charger that would be a good thing - but I don't, and I don't feel the slightest bit guilty about it.

If you're a grey nomad, or recreational professional of some description, who is cycling the aux battery every day of the year you should be receiving advice from professionals - not from a public 4wd forum. That's when expensive batteries and sophisticated charge controllers really pay for themselves - if you get the right system for your particular application.

[presto]

if one battery is in a lower state of charge than the other, it will draw from that battery to try and bring them to the same level of charge.

No. This is simply not possible.
quote]

so your telling me, if you get two identical batteries - one fully charged, one flat and you connect these in parallel then one will stay fully charged and the other discharged? no, the discharged battery will draw off the charged battery and they will level out. same as if you have a battery with a dead cell, it will draw from the fully charged battery too.
im not saying that the alternator will not charge the battery right up... it will, but it wont be optimum. it wont be at 100 percent.

an alternator will put out a regulated voltage and whatever current it is capable of producing provided the battery accepts it. if the battery is charged, obviously the amps produced by the alternator will decrease.
with that in mind, the amps produced steadily drop as the battery reaches its charged state... using a charger whether it be a cheap $80 battery saver or an expensive $400 c-tek puts the battery thru stages and calculates what the best move next is (absorbtion, bulk, desulfation, pulse, etc - all at different voltages and cycles). it doesnt just pump a whole heap of amps into it and steadily drop off like an alternator would. This WILL improve performance of the battery and it WILL increase battery life.
i totally agree with you that if you are touring and you spend 5 or 6 hours driving and bring the battery back up to 90 or 95 percent then there is nothing to worry about, but what i said is when you get back from a trip it is always a good idea to put it on a charger to bring it right back up to scratch because an alternator simply cannot reverse the discharge process like a charger can. sorry if it sounded like i was having a go at alternators for doing a crappy job, thats not the case i just might have been a bit heavy in emphasising my point

[-Scott-]

if one battery is in a lower state of charge than the other, it will draw from that battery to try and bring them to the same level of charge.

No. This is simply not possible.

so your telling me, if you get two identical batteries - one fully charged, one flat and you connect these in parallel then one will stay fully charged and the other discharged?

Will one stay fully charged? I don't know, but I suspect so.

Will the other charge? No. It's all about the chemistry. Did you read that?

no, the discharged battery will draw off the charged battery and they will level out.

No. It's all about the chemistry. One lead acid cell cannot, during discharge, produce sufficient voltage to reverse the same chemical reaction. If one cell could charge another cell external to itself it could also charge itself internally. That's simply non-sensical. A chemical reaction producing voltage to reverse itself.

same as if you have a battery with a dead cell, it will draw from the fully charged battery too.

Not the same. Different scenario. Go back and actually read my post. Specifically, this bit:

If you have two healthy 12V lead acid batteries connected in parallel, one cannot charge the other. The discharge reaction cannot produce sufficient voltage to reverse the same reaction in the other battery.

In your scenario you no longer have two healthy 12V batteries. You have a healthy 12V battery and a 10V battery with a random internal resistance. The 12V battery is able to produce 12.6V (or more - depending on current and state of charge) during discharge, which is more than enough to charge a 5 cell string (5 x 2.3 = 11.5V nominal float voltage). That's why one sick battery in a bank can deplete healthy batteries.

im not saying that the alternator will not charge the battery right up... it will, but it wont be optimum. it wont be at 100 percent.

I think this is the only bit on which we really disagree. A 13.8V float charge (at 25 Deg C) WILL 100% charge a conventional lead acid storage battery. It IS the preferred charge method for most manufacturers when charge time is not an issue.

an alternator will put out a regulated voltage and whatever current it is capable of producing provided the battery accepts it. if the battery is charged, obviously the amps produced by the alternator will decrease. with that in mind, the amps produced steadily drop as the battery reaches its charged state...

Yes.

using a charger whether it be a cheap $80 battery saver or an expensive $400 c-tek puts the battery thru stages and calculates what the best move next is (absorbtion, bulk, desulfation, pulse, etc - all at different voltages and cycles). it doesnt just pump a whole heap of amps into it and steadily drop off like an alternator would. This WILL improve performance of the battery and it WILL increase battery life.

In the context of this discussion, most likely. However, this is a whole new can of worms, and I would put more faith in the $400 C-Tek than a cheapy - different batteries have different requirements (charge current and voltage profiles) so picking the wrong combination could shorten the life of the battery. Personally, I think these devices are great for extending the life of a tired starting battery, but I'm happy with my 20A 13.8V regulated power supply - $99 from Jaycar.

i totally agree with you that if you are touring and you spend 5 or 6 hours driving and bring the battery back up to 90 or 95 percent then there is nothing to worry about, but what i said is when you get back from a trip it is always a good idea to put it on a charger to bring it right back up to scratch

I'm with you all the way up to here. "Good idea" is a subjective term, but it is certainly not a bad idea.

because an alternator simply cannot reverse the discharge process like a charger can.

And here is where we differ.

The super-dooper U-beaut charger is less likely to overcharge a battery like an alternator might, but if an alternator is going to overcharge the battery it's more likely to do it on the long on the highway during a trip than during around-town running. So the battery is likely to cook under the bonnet regardless of what happens in between trips.

Simply getting the battery out from under the bonnet is probably the most important step, as it prevents repeated cooking during day to day use. Preventing discharge while it's on the bench is the next most important step.

These multi-stage chargers will do this well, but a simple 13.8V regulated supply will also suffice.

I do not dispute that a good multi-stage charger is kinder to an aux battery than a mass-produced alternator charging system (particularly when you connect the alternator to two batteries at different states of charge, like we tend to) and in this respect, the multi-stage charger is better. But to claim that an alternator is unable to charge a battery to 100% is simply wrong.

If somebody wants to remove their aux battery they should do it to prevent over-charging - not under-charging.

Sorry if I'm being pedantic here, but it's too deeply ingrained in my nature, and I'm too old to change now.

[presto]

haha its ok, i enjoy the banter and its making me think which is always good - especially about this kind of thing which i love.
if i remember, tomorrow i'll flatten a battery, and charge another one. then measure the voltage of both, connect them in parallel and measure the voltage. leave the batteries joined for the day and then measure the voltage, then disconnect them from one another and measure the voltage of each. now from what i can remember, the chemical reaction your referring to will be there whenever there is a 'potential difference' apparent. whether that be a collapsed cell in a parallel battery, a globe across the terminals or a battery with a different voltage.
or is your argument that the electrons will only flow in one direction, meaning the last example wont work since it cannot shift into something parallel unless there is something creating the chemical reaction to flow in the opposite direction? i think im gonna have to pull out my old tafe books tomorrow and relearn myself. not sure if i worded any of that well enough for you to understand but it kinda makes sense to me so hopefully you can work out what im thinking

everything else you wrote i agree with, but im not sure if it was sales orientated or not that i was taught that having dual batteries at a different state of charge will not charge the lower battery properly and it is better to charge it when you get back from a trip. im not saying it is detrimental to the survival of the battery, i know lots of people who have got 3-4 years easily out of a battery and never putting it on a charger which, i know, contradicts what i claimed earlier of the battery not getting charged properly. but this is what i was taught. from talking to people who have used a charger they have recieved better performance from their battery and extended life. like i said i before i emphasised my points probably too agressively to make them obvious and might have made it sound like a very serious problem.

[presto]

if one battery is in a lower state of charge than the other, it will draw from that battery to try and bring them to the same level of charge.

No. This is simply not possible.

Lead acid batteries use a chemical reaction. During discharge, the reaction goes in one direction (for want of a better description) and produces voltage. During charge, a higher voltage is used to force the chemical reaction to reverse.
.

wouldnt a battery at 12.5 volts signify a higher voltage than a battery at 11.5 volts thus forcing the chemical reaction to reverse in one battery, charging it and discharging the other? the only time there isnt a higher voltage to create this chemical reaction is when they have reached the same level

[-Scott-]

if one battery is in a lower state of charge than the other, it will draw from that battery to try and bring them to the same level of charge.

No. This is simply not possible.

Lead acid batteries use a chemical reaction. During discharge, the reaction goes in one direction (for want of a better description) and produces voltage. During charge, a higher voltage is used to force the chemical reaction to reverse.
.

wouldnt a battery at 12.5 volts signify a higher voltage than a battery at 11.5 volts thus forcing the chemical reaction to reverse in one battery, charging it and discharging the other? the only time there isnt a higher voltage to create this chemical reaction is when they have reached the same level

Yes. I tried to explain that, but perhaps I didn't do a good job. If one battery has a defective cell, then (depending on how it has failed) the defective battery can behave like a 10V battery rather than a 12V battery. In this case, the fully charged, healthy 12V battery can charge the 10V battery, and discharge itself in the process. I expect that (depending on temperature) the "good" battery will continue to overcharge the dud battery until it's terminal voltage reaches around 11.5V, at which stage I would expect an equilibrium to be reached. End result, the "good" battery is significantly discharged (but not to the point of damage) and the "dud" battery has 5 fully charged cells and one dead cell.

[chimpboy]

I feel like I might be missing something, but anyway, afaik:

If you have a fully charged battery and you hook it straight to a partially flat battery (both batts in good condition otherwise), the fully charged battery is going to partially discharge until they level out together. So if you do this:

tomorrow i'll flatten a battery, and charge another one. then measure the voltage of both, connect them in parallel and measure the voltage. leave the batteries joined for the day and then measure the voltage, then disconnect them from one another and measure the voltage of each. now from what i can remember, the chemical reaction your referring to will be there whenever there is a 'potential difference' apparent.

... then you are going to see the batteries equalise and both will be X% charged at the end. If A was at 100% and B was at 20% you'll end up with both at around 60% at the end.

But... this is different to a dual battery situation where the alternator is in on the deal. The VSR type dual battery controller is probably the most common. It will make it so that even with all the equalising going on and a completely flat B battery, the A battery can't drop below (say) 90%, because if it does then the B battery gets disconnected again. Then the A battery gets charged up some more, and the process cycles.

This is the sole reason why the VSR tye controllers are better than a simple solenoid hooked to the ignition switch. It's the whole point of their existence.

.... I think!

[-Scott-]

haha its ok, i enjoy the banter and its making me think which is always good - especially about this kind of thing which i love.
if i remember, tomorrow i'll flatten a battery, and charge another one. then measure the voltage of both, connect them in parallel and measure the voltage. leave the batteries joined for the day and then measure the voltage, then disconnect them from one another and measure the voltage of each. now from what i can remember, the chemical reaction your referring to will be there whenever there is a 'potential difference' apparent. whether that be a collapsed cell in a parallel battery, a globe across the terminals or a battery with a different voltage.
or is your argument that the electrons will only flow in one direction, meaning the last example wont work since it cannot shift into something parallel unless there is something creating the chemical reaction to flow in the opposite direction? i think im gonna have to pull out my old tafe books tomorrow and relearn myself. not sure if i worded any of that well enough for you to understand but it kinda makes sense to me so hopefully you can work out what im thinking

everything else you wrote i agree with, but im not sure if it was sales orientated or not that i was taught that having dual batteries at a different state of charge will not charge the lower battery properly and it is better to charge it when you get back from a trip. im not saying it is detrimental to the survival of the battery, i know lots of people who have got 3-4 years easily out of a battery and never putting it on a charger which, i know, contradicts what i claimed earlier of the battery not getting charged properly. but this is what i was taught. from talking to people who have used a charger they have recieved better performance from their battery and extended life. like i said i before i emphasised my points probably too agressively to make them obvious and might have made it sound like a very serious problem.

It's getting late, and I've been polishing off left-over bottles - so I hope I can still make sense.

I enjoy these debates too, as it makes me think about my "beliefs", and it's a good way to find out when I'm wrong.

Please experiment and publish your results. Expect to be peer reviewed.

I'm not sure what "transient" conditions you might find, but I expect that when the two batteries (on charged & one discharged) are connected they will (obviously) both show the same terminal voltage. Even when disconnected, there may be some initial "surface charge" on the depleted battery which will make it look like it has charged, but I expect this will dissipate over time. Think of the battery (in this instance) as a capacitor: when hooked to a voltage source it will "charge up", but over time, this collection of charge dissipates (through the battery's internal resistance) and the terminal voltage will reduce.

If this helps:

In my experience of charging & testing lead acid batteries (using a float charge) the battery will accept whatever current it can.

If charged using less than 13.8V the charge current will eventually stop, indicating that the battery has accepted as much charge as it can, at that charge voltage. If the voltage is increased, current will flow again, until the battery is once again "full".

In my experience, once 13.8V is exceeded, the charge current never stops - it may reduce, but some current continues to flow, and is basically "cooking" the battery. This is bad.

When you conduct your experiment, make sure you "rest" the flat battery until its terminal voltage stabilises before you hook the two batteries together. When you connect them, measure the current between the batteries as well as voltage of each (if you have enough meters - I would watch the current initially, then measure each battery voltage).

I expect that you will see some current flow initially, as the flat battery effectively "charges its capacitor", after which current should stop and both batteries will sit at some form of equilibrium. To conduct further current would require a parallel path (ie a "leaky" battery), or for the cells to charge - which is (theoretically - based on the chemistry) impossible.

Once disconnected, the "flat" battery may hold its voltage for some time, but I expect the voltage will dissipate over time. If you don't want to wait, add a small load (say, a brake light globe) to each battery and monitor the terminal voltage. I expect the good battery to hold its voltage around 12.6V for a long time, while the flat battery will quickly (within seconds) drop to its "pre experiment" terminal voltage.

I hope this made sense - feel free to ask more questions, as I'm happy to help if I can. I'm really interested in what will happen, even if it means I'm horribly wrong - again.

[chimpboy]

Once disconnected, the "flat" battery may hold its voltage for some time, but I expect the voltage will dissipate over time. If you don't want to wait, add a small load (say, a brake light globe) to each battery and monitor the terminal voltage. I expect the good battery to hold its voltage around 12.6V for a long time, while the flat battery will quickly (within seconds) drop to its "pre experiment" terminal voltage.

Ah, so my post before was wrong. The 12.6V in the fully charged battery isn't beefy enough to kick down the door and actually charge a flat battery; you need 13 plus (or whatever) to actually get the current flowing across. So when I said they would equalise I was incorrect?

[Thylacine]

Why do you self-taught blokes continue to argue with those with more knowledge trying to help you?
Listen to Scott.
And if you only "think" that you know the answer, why reply at all?





ed

[presto]

I feel like I might be missing something, but anyway, afaik:

If you have a fully charged battery and you hook it straight to a partially flat battery (both batts in good condition otherwise), the fully charged battery is going to partially discharge until they level out together. So if you do this:

tomorrow i'll flatten a battery, and charge another one. then measure the voltage of both, connect them in parallel and measure the voltage. leave the batteries joined for the day and then measure the voltage, then disconnect them from one another and measure the voltage of each. now from what i can remember, the chemical reaction your referring to will be there whenever there is a 'potential difference' apparent.

... then you are going to see the batteries equalise and both will be X% charged at the end. If A was at 100% and B was at 20% you'll end up with both at around 60% at the end.!

exactly what im saying, it doesnt matter if there is a fault in the battery or not just as long as there is pd (potential difference) if there is a difference in voltage, as long as there is a circuit then there is potential for electron flow and the chemical reaction to take place.
easy way of looking at it is consider the lesser battery as a capacitor. a capacitor is just a battery anyway, it stores a charge. if you connect a capacitor to a battery, it will draw from it and charge. same with a flat lead acid battery, it will charge because there is the potential for the current to flow.

But... this is different to a dual battery situation where the alternator is in on the deal. The VSR type dual battery controller is probably the most common. It will make it so that even with all the equalising going on and a completely flat B battery, the A battery can't drop below (say) 90%, because if it does then the B battery gets disconnected again. Then the A battery gets charged up some more, and the process cycles.

This is the sole reason why the VSR tye controllers are better than a simple solenoid hooked to the ignition switch. It's the whole point of their existence.

.... I think

again i agree, the VSR will charge the starting battery first as it is more important than the aux and will measure the voltage of this battery to cut in / out. hence my comment earlier about people having the wrong idea of how these work and think it charges the flattest battery first.
im still sticking to my original claim that the aux battery will not be recharged to its optimal level just by driving. sure it will do the job i agree on that but it still will not do the same as if you put your battery on charge when you get home to give it a good reco. this will bring the battery back to 100 percent. and i mean 100 percent of its potential condition, not just 100 percent of charge. the best way to store any battery is fully charged, the more you keep it on charge (obviously a trickle charge not a constant charge like from an alternator) the less sulphation occurs and the less work the battery has to do in regards to staying charged, even if only from a light workout or a couple of days sitting.

[presto]

Why do you self-taught blokes continue to argue with those with more knowledge trying to help you?
Listen to Scott.
And if you only "think" that you know the answer, why reply at all?

im C.I.T and battery world taught.. cant vouch for what kind of job they did or not tho everyone here seems to have an understanding of whats going on so it makes for interesting conversation plus its educational (i hope) for anyone reading in - we can teach eachother. by offering your opinion with your 'thoughts' its easy to get it explained/confirmed/denied with reasonable explainations and learn in the process.

sorry for stealing your capacitor example, i swear i wrote it first just you published it before me

i'll post results tomorrow, i'll give the fully charged battery a quick load up to remove any surface charge and i'll use our jump battery (which is in need of a good charge.. forgot to put it on over the weekend ) to make sure were looking at reasonable differences rather than hooking up a 12.2 volt to a 12.5 volt battery and expecting astonishing results.

[-Scott-]

I feel like I might be missing something, but anyway, afaik:

If you have a fully charged battery and you hook it straight to a partially flat battery (both batts in good condition otherwise), the fully charged battery is going to partially discharge until they level out together. So if you do this:

tomorrow i'll flatten a battery, and charge another one. then measure the voltage of both, connect them in parallel and measure the voltage. leave the batteries joined for the day and then measure the voltage, then disconnect them from one another and measure the voltage of each. now from what i can remember, the chemical reaction your referring to will be there whenever there is a 'potential difference' apparent.

... then you are going to see the batteries equalise and both will be X% charged at the end. If A was at 100% and B was at 20% you'll end up with both at around 60% at the end.

But... this is different to a dual battery situation where the alternator is in on the deal. The VSR type dual battery controller is probably the most common. It will make it so that even with all the equalising going on and a completely flat B battery, the A battery can't drop below (say) 90%, because if it does then the B battery gets disconnected again. Then the A battery gets charged up some more, and the process cycles.

This is the sole reason why the VSR tye controllers are better than a simple solenoid hooked to the ignition switch. It's the whole point of their existence.

.... I think!

chimpdad, haven't you got a family to look after? Or are you awake for the feeding?

I think you're considering the batteries as simply capacitors; hook the two in parallel and charge will transfer.

In many ways batteries behave like capacitors, but they don't store charge in the same way that true capacitors do - they store charge as chemical energy, not electrical energy.

To convert the chemical energy to electrical energy requires a chemical reaction, which is not 100% efficient - some energy is lost as heat.

To convert electrical energy back into chemical energy is also less than 100% efficient (somebody's law - basically, heat is always produced somewhere in a system - entropy is always increased), so reversing the chemical reaction always requires more energy than was released in the initial reaction.

Basic chemistry dictates the voltage which is generated in the reaction, and charge is literally charge.

When a battery is "discharged" a certain amount of charge (Coulombs - the area under the current-time curve) leave the battery. To replace this charge (Coulombs) the area under the current-time curve must be the same.

Presuming both curves (charge & discharge) are the same shape (for ease of "picturing" the explanation), if they both have the same voltage curves then there is no loss of energy anywhere. That just ain't gonna happen.

For one battery to charge another the time-current curves must be the same for both (because they are electrically connected). Chemistry dictates that the discharge reaction in one battery generates less voltage than the voltage required to reverse the same reaction in the other battery.

Same current in both batteries - it's the same circuit.

To drive this current in the circuit the voltage in the "good" battery must be more than the voltage the "flat" battery.

But, to drive the "charging" chemical reaction, the flat battery requires a higher voltage than the good battery can produce during the "discharging" chemical reaction. We need to be pushing current up the voltage hill.

If the current through both batteries is the same, and the flat battery requires a higher voltage than the good battery, then the power going into the flat battery is more than the power coming out of the good battery. This is before we have any losses of energy in either battery.

Whichever way I look at it I can't find a way that this can work.

[presto]

okay thay was way too much for my brain to handle at this hour, im going to try and sleep on it.. tho i think i'll have trouble because this will be doing my head in all night thinking about it. i'll do the test tomorrow and see if we can stop this theory business before my head explodes

[-Scott-]

Once disconnected, the "flat" battery may hold its voltage for some time, but I expect the voltage will dissipate over time. If you don't want to wait, add a small load (say, a brake light globe) to each battery and monitor the terminal voltage. I expect the good battery to hold its voltage around 12.6V for a long time, while the flat battery will quickly (within seconds) drop to its "pre experiment" terminal voltage.

Ah, so my post before was wrong. The 12.6V in the fully charged battery isn't beefy enough to kick down the door and actually charge a flat battery; you need 13 plus (or whatever) to actually get the current flowing across. So when I said they would equalise I was incorrect?

Vivid analogy! To charge a flat battery the door needs to be kicked open, to let charge in. But a charged battery isn't strong enough to kick the door open.

Anyway, reading back through my posts I don't think I'm explaining things that well. So I'll go to bed now, and see where we are tomorrow (oops - today.)

[chimpboy]

chimpdad, haven't you got a family to look after? Or are you awake for the feeding?

That's it. Expect more at around 3.30am

[Thylacine]

everyone here seems to have an understanding of whats going on so it makes for interesting conversation plus its educational (i hope) for anyone reading in - we can teach eachother. by offering your opinion with your 'thoughts' its easy to get it explained/confirmed/denied with reasonable explainations and learn in the process.

The problem is that some have an understanding of what's going on, and others think that they do. This is physics/chemistry and there are *very* few grey areas, it's all pretty well black and white.

sorry for stealing your capacitor example, i swear i wrote it first just you published it before me

Not my example, as it's a poor one


There's a lot of knowledge being offered here, and posts by people who "think" that they know what's going on only drowns that data in crap.

Honestly, it get's tiring offering the same advice over and over, and being disputed by someone who doesn't fully understand the concepts that they are speaking of. I stopped years ago trying to correct people's usage of the term A/Hr, as it has no logical meaning, and worse, engenders a general misapprehension regarding batteries in that they are just a "bucket" full of "amps" which can be emptied and refilled as suits.
And please , fellows, don't talk about electrons flowing unless you do know something about it. This has a specific meaning and only muddies the waters (apart from showing that you haven't a clue about what you are saying).

This forum is very lucky to have a mod that actually *knows* something about 12VDC systems, and members should be grateful and respecting of the information offered here. This is one of the very few fora left where 12VDC issues can be discussed without being drowned in misinformation.





ed

[gu town]

i think i'm starting to regret writing "dont hold back on the tech stuff"

in a nutshell Q.1. batteries mainly have the "fighting" problem with different types and sizes in a series (24V) system and so long as i give my aux battery a charge after a good workout i'll be fine.

i've been running NO7Z's with a cheap VSR between them but i would like to go with an AGM as aux for fridge work as i belive they can handle the deep discharges alot better.

Q.2. is easlily do-able

cheers ppl

[chimpboy]

i think i'm starting to regret writing "dont hold back on the tech stuff"

in a nutshell Q.1. batteries mainly have the "fighting" problem with different types and sizes in a series (24V) system and so long as i give my aux battery a charge after a good workout i'll be fine.

i've been running NO7Z's with a cheap VSR between them but i would like to go with an AGM as aux for fridge work as i belive they can handle the deep discharges alot better.

Q.2. is easlily do-able

cheers ppl

Correct... no probs with different battery types in a 12V dual battery system (other than some more exotic batteries), and no there wouldn't be a problem with using a "smart" isolator to control a heavier solenoid.