yeah i know more dual battery questions!

[presto]

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. 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).
ed

im a little offended that you reckon i havent a clue about what im saying because i mentioned electron flow, which is not all that hard to understand in its basic principles - whenever there is current flowing there are electrons moving.
the higher voltage battery will discharge into the lower one until both batteries are the same. the time this takes tho, depends on internal resistance. if you can provide an equation or workings to prove this wrong, or even a link to ohms law and show me what im not getting then it would be helpful.

i didnt do the test like i said i was going to because i didnt have time, but what i did do was connect a flat battery with a fully charged battery using an amp meter and there was current flow of nearly two amps. depending on the size and flatness of the battery i cant see any reason this wouldnt get up to even ten amps and decrease as the full battery charges the lower battery.
there doesnt need to be a huge difference in voltage to kick down any doors to instigate a current flow.

if i have time tomorrow i will do what i originally planned and post results.

[-Scott-]

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.

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.

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.

Thanks for the words of support ed, and I understand how you feel. But now that you've said I know something, presto will prove that I'm wrong - that's the way these things typically work.

This is a public forum, and everybody is entitled to offer their opinion. I have no problem with opinions, except when they're stated as "facts" which I don't believe. That's generally when I'll add my 2c, to try to keep the tech content of this site as accurate as possible - because I believe this is the best 4wd tech site in Australia, and I'd like to see it stay that way. I rarely visit other 4wd sites these days because I become too frustrated seeing the same incorrect information being regurgitated again and again.

But nobody is infallible, and I'm prepared to be corrected (again), if somebody can provide some evidence that I'm wrong - which presto is working on.

There is absolutely nothing wrong with heated arguments, as long as they are over technical issues. Once the arguments become personal (i.e. name calling, personal attacks) moderation is required and members may be banned.

Vent frustrations and argue over technicalities - but don't make it personal.

[-Scott-]

i didnt do the test like i said i was going to because i didnt have time, but what i did do was connect a flat battery with a fully charged battery using an amp meter and there was current flow of nearly two amps. depending on the size and flatness of the battery i cant see any reason this wouldnt get up to even ten amps and decrease as the full battery charges the lower battery.
there doesnt need to be a huge difference in voltage to kick down any doors to instigate a current flow.

Well, that's a good start. How long did you watch the current flow?

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.

Reading this again, I didn't express that too well. Batteries behave like capacitors because of the parallel plates, so they will store some charge in this manner, but the bulk of the energy stored is chemical potential energy, not electrical potential energy. Most of the electrons released during discharge come from the chemical reaction, not from a capacitor "charge bank".

So, we need to be sure that this current flow is creating a store of chemical potential and not electrical potential energy. To help figure this out it would be good to know a little about the size (ie amp-hours) and style (starting, deep cycle, gel, agm etc) of both batteries. Similarly, "at rest" voltage readings for both (before connecting) would be good.

if i have time tomorrow i will do what i originally planned and post results.

I'm looking forward to it.

[Thylacine]

im a little offended that you reckon i havent a clue about what im saying because i mentioned electron flow, which is not all that hard to understand in its basic principles - whenever there is current flowing there are electrons moving.

Current flow is the exact opposite of electron flow. It's basic principles such as this that professionals take for granted that the novice is unaware of. This often makes it very hard to explain operational theories etc, as it's impossible to know what the novice isn't aware of, and like-wise impossible to determine the depth of explanation.

the higher voltage battery will discharge into the lower one until both batteries are the same. the time this takes tho, depends on internal resistance. if you can provide an equation or workings to prove this wrong, or even a link to ohms law and show me what im not getting then it would be helpful.

Given enough time, what your saying is correct, in that they'll both eventually reach 0VDC. But connecting batteries of 12.6VDC and 12.0VDC will not give you two batteries at 12.3VDC
Have a look at http://www.batteryuniversity.com/partone-13.htm, then read Scott's posts again.

This is not personal presto, it's the presenting of erronous opinions as fact that I'm disputing.
And I truly cannot understand it. Sh*t, Id never post on a topic without a thorough understanding of it, let alone dispute someone with apparent knowledge.





ed

[presto]

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

i was just going over what ive written and i didnt explain myself clearly here which started the majority of this debarcle. i meant to bring the battery back to its best possible condition - thus increasing efficiency and extending battery life.

Well, that's a good start. How long did you watch the current flow?

not long, only about 20 seconds and it did drop off but not significantly. i didnt make an effort to pay much attention it was just a quick look before i hooked up the charger.

Reading this again, I didn't express that too well. Batteries behave like capacitors because of the parallel plates, so they will store some charge in this manner, but the bulk of the energy stored is chemical potential energy, not electrical potential energy

im not disputing the difference in behaivior or how they work, im on the same page as you with pretty much everything. i only used it as an example to explain what its doing - taking a charge from the larger power source/higher voltage.

Current flow is the exact opposite of electron flow. It's basic principles such as this that professionals take for granted that the novice is unaware of. This often makes it very hard to explain operational theories etc, as it's impossible to know what the novice isn't aware of, and like-wise impossible to determine the depth of explanation.
This is not personal presto, it's the presenting of erronous opinions as fact that I'm disputing.
And I truly cannot understand it. Sh*t, Id never post on a topic without a thorough understanding of it, let alone dispute someone with apparent knowledge.

i never said they flow in the same direction, i said that when there is current flow there is electron flow. i didnt think it necessary to distinguish the difference between electron and conventional but since we have now, there is still going to be electrons moving when there is current flow.
i know its only an opinion at the moment, but were working on figuring it out and hopefully if noone else, i will learn something since you seem pretty bloody sure tho and you guys have many a year of experience over me, im expecting to be embarassed and apologetic tomorrow for frusting you
i can see everything your saying scott and with the bits i didnt have a complete understanding of ie coulombs i looked it up to make sure i know exactly what your talking about. i really dont wanna come across as one of those people who sees something somewhere and takes it for granted as the holy grail. i have a strong interest in this kind of thing since its going to be my career and would like to learn as much as i can even if it means making a fool of me tomorrow. atleast i'll learn from it and be able to tell the next fool exactly what happens and it WILL be a fact
this is the only thing we disagree on, the reason i have this opinion is because i was taught by someone who i think has a firm understanding and it makes sense to me. now being told by someone else it isnt, how else to understand exactly what happens other than to do it myself?

apologies for the mahoosive story, its late and i ramble.

[presto]

i charged up a fresh N70ZZ and discharged a 58VT battery using a couple of 100 watt globes.
once the globes started fading to an orange, i removed them, loaded the battery and performed a hydrometer check.

i'll refer to the charged battery as battery a
discharged battery as battery b

hydrometer indicated battery a to be fully charged and battery b to be dead flat (less than 25 percent)

once loaded up, battery b dropped to 4 volts and was producing roughly 50amps (only held there for a second to get a reading so as not to damage the battery).
i then loaded up the fully charged battery for 5 seconds producing roughly 450 amps at 8 volts - this was to remove any surface charge the battery had from being on charge.

i then let both batteries sit for half an hour and checked the voltages.



battery a - 12.6 volts
battery b - 11.7 volts

i then joined the batteries in parallel with an amp meter and instantly there was a 5.5 amp reading.
this dropped off to 4.6 amps within a minute or so.

after half an hour this had dropped to 2.2 amps

after 1 hour - 1.46 amps
2 hours - 1.1 amps
3 hours - 0.86 amps

at this point i disconnected the batteries and gave a short load (50 amps for 1 second)
to remove any surface charge and let the batteries sit for 5 minutes

after 5 minutes battery a read 12.48 volts
battery b read 12.24

i connected them again and instantly there was a 1.3 amp reading which dropped to 0.97 after one minute.

one hour later - 0.73 amps
2 hours - 0.64 amps
3 hours - 0.56 amps

at this point i disconnected the batteries again and gave another short load to remove any surface charge.

after 5 minutes battery a read 12.44 volts
battery b 12.27 volts

i then performed a hydrometer reading and battery a read just over 1.222 which is a bit over 75% charged
battery b read between 1.145 and 1.186 which was between 25-50 percent

these readings were over a period of 6 hours, iv left the batteries connected overnight and will check them both in the morning and give a propper load test to see output of each.
i might even leave them again another night to simulate the batteries being left over a weekend and to get an even more precise indication of where battery efficiency (or lack thereof) will hinder the transfer. but as far as i can tell, my theory is correct - the flatter battery is drawing current from the charged battery.

having said this, it doesnt mean i think scott is completely incorrect, i totally agree with the fact that a voltage needs to be to in a way 'excessive' to produce the current flow.
leaving these batteries connected i dont think will cause them both to read exactly the same state of charge, as the voltage difference between them becomes closer there is less 'push' to transfer the charge from one to the other. so i still think the charged battery will read higher on the hydrometer even when the voltages become closer.

im not getting into technical terms or explaining every single detail to the wire because im hoping everyone can understand what im saying. i dont want to have to get into another discussion about all the other variables that come into account (temperature, heat loss, efficiency of the battery and internal resistance etc) so for all intents and purposes, can anyone see something wrong with my calculations? theory? reasoning?

[Thylacine]

Now do a rough calc of the Ahrs transferred to get from 11.7 to 12.27 and you'll see how much charge has actually moved.






ed

[-Scott-]

Good results.

I agree that there appears to be more charge transfer from flat battery to good battery than can be explained by surface charge alone. So much for my chemistry lessons.

What does this tell us? Does everything "add up"?

Looking at the current figures quoted above, and doing some rough calcs, I'd say a little under 5Ah was drawn from the good battery, over the first three hours. The next three hours was approximatley another 2Ah, or 7Ah (total) over 6 hours.

What's the capacity of the N70ZZ? I couldn't find an Ah rating for a cranking battery, and the RC is calculated at 25A, not 2A - so attempting to calculate one from the other is too rough to be reliable.

In the first 6 hours these batteries were connected the current measurements indicate approximately 7Ah of current draw. The hydrometer reading suggested battery a is 20%+ down. If 7Ah is 20% of the battery's capacity then total capacity is 5 x 7Ah = 35Ah. Isn't an N70ZZ significantly larger than 35Ah? The RC figure of 140 minutes give an Ah figure of close to 60Ah at 25A. At 2A, the capacity should be much higher. This doesn't add up.

Turning to battery b, the 58VT. Again, I couldn't find an Ah rating, so we can only work on the hydrometer readings.

It started out at less than 25%, and finished between 25% and 50%. What do we say? 25% increase? If 7Ah is 25% of the battery's capacity then total capacity is 28Ah. Again, this doesn't add up - a 58VT should have a larger capacity than that.

So, what's going on? I really don't know. I don't doubt presto's figures. The measurements he's made, and the way he's presented the results suggests he knows what he is doing. I suspect the batteries need longer "rest" periods before the SOC measurements are taken.

I know this would be a PITA, but could the experiment be repeated over a few days? Such as:

Day 0: Charge & Discharge batteries, then sit overnight.
Day 1: Measure SOC - voltage & SG readings would be good. Connect for 3 hours, recording terminal voltage and current every half hour? Disconnect, sit overnight.
Day 2: Measure SOC - voltage & SG again. Connect for 3 hours, recording terminal voltage and current every half hour? Disconnect, sit overnight.
Day 3: Measure SOC - voltage & SG again.

[presto]

In the first 6 hours these batteries were connected the current measurements indicate approximately 7Ah of current draw. The hydrometer reading suggested battery a is 20%+ down. If 7Ah is 20% of the battery's capacity then total capacity is 5 x 7Ah = 35Ah. Isn't an N70ZZ significantly larger than 35Ah? The RC figure of 140 minutes give an Ah figure of close to 60Ah at 25A. At 2A, the capacity should be much higher. This doesn't add up.

i made similar calculations with the same results. i agree according to ohms law these do not add up. i would expect the n70zz to be roughly a 70ah and the 58VT maybe 50ish (dont quote me).
i think a longer wait between disconnecting and taking notes is necesary as my 'loading' might not be sufficient to give an accurate charge reading after standing for 5 minutes.
here are the results from today:

this morning the meter had dropped significantly and indicated 0.17 amps.
6 hours later it had dropped to 0.13 amps.
at this stage i disconnected tha batteries and took a voltage reading

battery a read 12.45
battery b 12.42

i then loaded both batteries up to roughly 100 - 130 amps for 1 second to remove surface charge and took another reading after about 30 seconds.

battery a 12.36
battery b 12.23

i then left batteries disconnected for one hour and took another reading

battery a 12.44
battery b 12.34

i then performed a hydrometer reading on both batteries with similar results to the last time, however when loading the battery up it was not as stressed as it was yesterday - voltage didnt drop that low and still produced over double the output. also i could feel that significantly less force was used to produce this result (i used a screw type manual carbon pile loader)

i have connected the batteries back up and leaving them overnight again.

[presto]

amp meter read 0.07 amps this morning, i left it for an hour or so until it hit 0.06 and disconnected the batteries.
battery a read 12.44 volts
battery b read 12.43 volts

loaded both batteries to 150 amps for 1.5 seconds to remove any surface charge and rechecked voltage after about 20 seconds
battery a 12.33
battery b 12.28

after 15 mins
battery a 12.46
battery b 12.36

after one hour
battery a 12.48
battery b 12.35

after 4 hours
battery a 12.49
battery b 12.34

i then gave them both a good workout with the load tester
battery a 450 amps and rising at roughly 8 volts for 5 seconds
battery b 250 amps steady at roughly 8 volts for 5 seconds

i then took another voltage reading
battery a 12.32
battery b 12.22

after an hour
battery a 12.47
battery b 12.28

would like to hear peoples comments before i give my opinion.. at risk of being 'flamed'

[-Scott-]

Interesting results, but I'm still not sure what's going on.

In your first set of results, we had these readings:

at this point i disconnected the batteries again and gave another short load to remove any surface charge.

after 5 minutes battery a read 12.44 volts
battery b 12.27 volts

After the two batteries were connected for another day, we get these readings:

after an hour
battery a 12.47
battery b 12.28

Battery a has increased by 0.3V and battery b by only 0.1V. To me, that's an indication that any change in SOC of either battery is less than "measurement error", most likely a result of temperature variations.

I too have a theory, but would like to see if anybody other than the pair of us is still reading this thread.

One thought which has only just occurred: how accurate is your ammeter? If that's dramatically under-reading, and the actual current is higher (say, double), then earlier results add up a little better.

[presto]

i tested the meter before and during - i forgot to swap the leads over from amps to volts and thought i might have blown the fuse
it was reading accurately throughout as far as i can tell (tested using a globe in series and was within specs for current draw) anyone else got an opinion? is anybody else even interested
i think we can consider this thread well and truly hijacked

[chimpboy]

would like to see if anybody other than the pair of us is still reading this thread.

I am, but that doesn't mean it's interesting; I'll read anything.

[-Scott-]

would like to see if anybody other than the pair of us is still reading this thread.

I am, but that doesn't mean it's interesting; I'll read anything.

Really? What are you like at reading palms?

[chimpboy]

would like to see if anybody other than the pair of us is still reading this thread.

I am, but that doesn't mean it's interesting; I'll read anything.

Really? What are you like at reading palms?

You've already hijacked this thread once Scott, surely that's enough!

[PBBIZ2]

I'm reading it with interest, especially since I know nothing about battery chemistry!
Good stuff, picking up a few bits and pieces.

[gu town]

yeah im checking in now and then. i've got my answer and happy with them, now just seeing what else i can learn

[Thylacine]

Been in horse-piddle for a few days and still not clear enough in the head to make comments diagnostics-wise, but definitely still thinking about this.
I'm suspecting that we're either missing something obvious/simple, or accumulative errors (variations in ambient temp, meter accuracy, etc) are playing games with us.




ed

[presto]

for anyone who is a bit iffy (me included) about what we are seeing and comparing here, this is my interpretation. please add or pick me up on anything iv got wrong or missed.

there was obviously a current draw as the flat battery attempted to charge from the full battery, this is apparent throughout all the readings. when the batteries were disconnected, they showed a similar voltage hence loading the battery up to remove this 'surface charge' and get a more accurate idea of what the batteries actual voltage was.
the surface charge building up in the flatter battery, and the decrease in the charged one was slowing down the transfer significantly - as we could tell by the steady drop off in amps.

the reason i stopped it at 0.06 amps is because thats usually around the maximum constant drain id like to see on a vehicle. its dick all. every now and then i disconnected the batteries for a moment and the voltage variation increased slightly (0.01 - 0.04v after a couple of seconds) and when connected again there was momentarily a higher amp reading, but that quickly dropped down as surface charge built up again.

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".

there is too much internal resistance to create the chemical reaction to produce a consistant current flow, hence building up this surface voltage but not actually charging the battery as effectively over time - as seen by hydrometer readings..
using ohms law, these results iv taken dont add up but there are alot of variables that havent been taken into account (as specified in previous posts).

id imagine temperature had a significant influence to the readings but i cant find correction charts to plot density against temperature - plus i didnt measure the temperature at all, let alone at each reading. i dont have access to a temperature compensating hydrometer either, so i think the only way to get a more accurate idea of exactly whats going - without the chart and more info or knowledge (help me here scott ) is to do the following. i'll try and organise it over the next week or so, so if anyone has any suggestions or things you want me to check at the same time please let me know.


take two identical cranking batteries and fully charge them. get two identical deep cycle batteries and discharge at an equal rate until they are both discharged to the same level. this time it will be a propper, deep discharge rather than a couple of globes over an hour or so. leave the four batteries overnight to give them a chance to 'level out'. record voltages the next day then connect one flat battery to a fully charged one using an ammeter and leave all batteries to sit for a couple of days, with recordings at intervals obviously.

having a battery sit fully charged will give us a voltage to compare to and see how much is being drawn from the now parallelled one. having a battery sit fully discharged will allow us to see just how much the flat battery regenerates (for want of a better word) by itself. i think this was also a big contributor to the results, hence giving the deep cycle batteries a propper discharge this time. ultimately, we will have isolated voltages to compare the parallelled batteries with which also means, since they'll all be subject to the same temperature variation, i dont think temperature compensation in the hydrometer readings is necessary in making comparisons and calculations.

please anyone who has any comment, something to add or sees something iv read wrong, do your bit and give us some feedback and ideas im probably going overboard with this, but im intrigued and wanna know the answer

[-Scott-]

I love it when somebody writes longer posts than I do.

Makes me feel not so bad.

I think you've summed it up very well, and I like your plan for the next test - complete with "control" batteries, an excellent idea!

Although I'm still not convinced that the flat battery is charging, and I'm not totally surprised that the charged battery has discharged, I am surprised by the size of the current flow. I really don't understand what's happening; what the current is doing. As I posted earlier, when charging SLA batteries, I am used to seeing the current drop to literally zero - nothing measurable. But, that's applying a voltage sufficient to charge, so this may be the difference - if we're not charging, something different is happening.

At some stage (either the end of the next test, or with the existing test pair) I'd like to know what happens if an external DC source is applied to the "charged" battery, and steadily increased from say 12.6V. I would expect current between the two batteries to slowly increase with voltage increase, then increase faster somewhere around 13V - as the second battery starts to actually charge. But I'll probably turn out to be wrong again.

Last thought is: how did we hijack this thread, and take it in this direction? Although I'm interested in the outcome, I suspect it's fairly academic, as we typically isolate the "auxiliary" battery from the cranking battery except when we're charging both. How often do we parallel a discharged battery with a charged battery without connecting to a charging source?

In this case current should be flowing in the same direction through both batteries, and the "flat" battery can't be drawing charge out of the "good" battery - because the charging circuit is pumping charge into both.

[presto]

i think i mentioned it in relation to the importance of an isolator early in the thread. buggered if im going thru all of this to find out where tho

At some stage (either the end of the next test, or with the existing test pair) I'd like to know what happens if an external DC source is applied to the "charged" battery, and steadily increased from say 12.6V. I would expect current between the two batteries to slowly increase with voltage increase, then increase faster somewhere around 13V - as the second battery starts to actually charge.

yeah i think the same thing will happen, the flatter battery will be drawing more, much like the dual battery system in a car - the battery will accept whatever charge it can when flat, provided the power source is a higher voltage to create the 'push'. very interested to see what voltages obtain different states of charge, unfortunately i dont have a variable power supply but might have a look into getting one.

[presto]

I really don't understand what's happening; what the current is doing. As I posted earlier, when charging SLA batteries, I am used to seeing the current drop to literally zero - nothing measurable. But, that's applying a voltage sufficient to charge, so this may be the difference - if we're not charging, something different is happening.

SLA batteries are much smaller, maybe the greater surface area of the plates in the car battery means less internal resistance and is more efficient?

[MICK77]

I love this sh1t.

It is great to see what are obviously two very well educated (in this field) people have a serious thought provoking "conversation" about a topic which I thought I knew a bit about. I can say without hesitation that I am now smarter for having read this thread. (Ha, I'm a poet and didn't know it!)

For what it's worth, I thought the full battery would charge the flat battery to the point of equilibrium(sp?)

Anyway, where are we at with the testing?

Cheers,

Micko