Dual battery system-Will my idea work???

[Shark]

OKay i KNOW that this topic has been discussed over and over but i had an idea and i wanted the thoughts from you guys on wether it would possibly work.

Im sure everyone can remember those marine battery switches with the choice of charging two batteries independantly or both together?? Well my idea is along those lines.

What i want to do is "split" the alternator charge so it goes to two solenoids, those 100amp jobbies you can get from most electronic places and ebay and the like. Then linked to two switches on the inside of my truck. So i can basically charge either battery seperatley or together from the drivers seat.

I know this idea is a manual style and not an auto/smart style but i prefer this type as it suits my needs better..

Does it sound like it could work???

Oh and a 1985 MK patrol, 12v.........

Marc

[chimpboy]

If you call the batteries A and B, then whether the set up is manual or "intelligent", the usual deal is that the alternator is either connected to A, or to A+B, but not to B.

I can't see a super compelling reason why you couldn't do it your way instead. I am not 100% sure, though, on what happens during the split second when neither battery is connected to the alternator. Remember the alternator relies on battery sensing to regulate output voltage. My suspicion is that it would be okay on a carby vehicle but perhaps unwise on something with lots of electronics on board.

Overall though I am struggling to see a situation where the "A or A+B" design isn't the most suitable. IMHO, you pretty much always want to give your cranking battery #1 priority because you always want to be able to start the car when you need to.

[familybus]

If you call the batteries A and B, then whether the set up is manual or "intelligent", the usual deal is that the alternator is either connected to A, or to A+B, but not to B.

I can't see a super compelling reason why you couldn't do it your way instead. I am not 100% sure, though, on what happens during the split second when neither battery is connected to the alternator. Remember the alternator relies on battery sensing to regulate output voltage. My suspicion is that it would be okay on a carby vehicle but perhaps unwise on something with lots of electronics on board.

Overall though I am struggling to see a situation where the "A or A+B" design isn't the most suitable. IMHO, you pretty much always want to give your cranking battery #1 priority because you always want to be able to start the car when you need to.

id say that split second woudnt matter as ive started cars and takin the battery out to put in another and or tested the alternater by removing the battery and see if it still runs and no damage!

[Shark]

When it comes to charging the " B " battery or AUX i would only switch it on on long drives and possibly go A+B and the just B...

Im pretty sure my idea will work a couple of concerns are wether those 100amp solenoids can handle the charge from the alt?? Im pretty sure i read somewhere they can handle up to 400amp spikes??!!??

Also is real simple laymens terms no matter what dual battery solenoid you have you still at the end of the day end up charging two batteries instead of one, does this put extra stress on your alternator??

And last one, when using cable for these systems, remembering that mine is not going to be flash more functional than anything else. Rather than using say wire with a 10mm core could you use a 4core wire with a combined diameter of say 14mm?? Would that still work??

MArc

[craz3d]

Could use a 3 stage switch, go A to A+B to B, that way there's always a battery in the loop.

[Shark]

Yeah thats what i meant:)

[-Scott-]

When you switch you need to switch the sense wire too, so the alternator can sense the voltage of the battery it is charging - but I think you know this.

Some regulators will max the alternator output if the sense line fails - Gen 3 Pajeros are known to do this if they blow the hazard fuse, and pump the voltage to over well over 15V.

If you disconnect the sense line from one battery before connecting it to the other there is a risk that the regulator will max the alternator output, with unknown results. On a carby car there's less to go wrong, but on a modern vehicle with excessive electric dohickery there are many opportunities to kill something.

You could try a circuit which uses "make before break" contacts, so that the sense line is never disconnected - which I think is what craz3d was suggesting with his A - A+B - B scenario.

You need to make sure that the alternator sense is never disconnected - or take your chances with the outcome.

[Shark]

Will a 1985 nissan patrol have this, i really dont think its got too much in the way of computor and stuff..

Marc

[chimpboy]

Will a 1985 nissan patrol have this, i really dont think its got too much in the way of computor and stuff..

Marc

It's just how the alternator works, so yes it will have it. I think you could just hook the batt sense wire to the main charging wire though and solve the problem that way... in other words you hook your small "S" terminal on your alternator to the main B+ lug on the alternator rather than running a separate wire to the battery. But I may be corrected on that.

[Shark]

I actually didnt know about that " sense " wire. Is there an easy way to find out which one it is. Is it easy enough to split that wire to so it goes to both batteries or will that not work when u charge A+b..

Your idea sounds like it could work chimpy but i love some more info on this, worse case scenario i will just split the sense wire into two and make one got to each battery, although i dont know if that will work when i connect both batteries together, but then it would read them as two batteries in paralell so like one biug battery..

Correct me if im wrong guys,. i wanna know it all before i start..

Marc

[chimpboy]

I actually didnt know about that " sense " wire. Is there an easy way to find out which one it is. Is it easy enough to split that wire to so it goes to both batteries or will that not work when u charge A+b..

If you have a look here it is basically the "S" wire on all of these diagrams. I *think* the alternator types with no S terminal just sense the voltage via the big B+ terminal.

I have always wondered if the S terminal can possibly be more than a tiny bit different to the B+ terminal, as they both just go straight to the battery.

[Shark]

You bring up a good point there mate, if they both go to the same thing then i dont see any reason why it wouldnt work..

Interesting

[Ruffy]

I can understand what you are trying to achieve, however it's not as simple as it seems.
Firstly putting to Sense wire directly to the B+ terminal will work, to a degree. Due to some inevitable voltage drop through the solenoids and connections the alternator will "think" that the battery is more charged than it actually is. Also the indicator light will only operate if the alternator voltage drops not the battery voltage.
Secondly, with this system the alternator will only charge until one battery is fully charged and then it will only trickle charge the other battery.
I don't know why you want the system to work this way but it isn't ideal.
Dan

[Shark]

OKay this is the reason i love forums so much as questions and answers get raised and it helps me understand what is going on more..

i know there will be a small ammount of volatge drop but i would assume it will be the same for both batteries. What i want is a driver controlled dual battery system, not an auto system. I want something similar to the old marine battery switch with the A or B or A+B or OFF option BUT i want it via solenoids and switches...

Also the indicator light will only operate if the alternator voltage drops not the battery voltage.


I dont understand the above comment.. There is no indicator in the system i have thought of.

Secondly, with this system the alternator will only charge until one battery is fully charged and then it will only trickle charge the other battery.

With this comment i am again confused, my system is driver controlled, once A is fully charged then I can switch over to B, i wont be charging A and B together, even though thats an option. So for most of my daily commute i wont be charging B..

If i have any of this wrong please let me know, i value all input:D

Marc, oh and sorry i dont know how to use the quote thing properly

[-Scott-]

I'm not following everything here either, but I think I understand the comments about S vs B+ terminals.

The regulator adjusts the alternator output to maintain a particular voltage at the battery. If the alternator is putting out a large amount of current that will create a significant voltage drop between the alternator and the battery, so the voltage at the battery will be less than the voltage at the B+ terminal at alternator.

A separate Sense wire, connected direct to the battery, has negligible current flow, so no voltage drop. This means the regulator knows EXACTLY what's happening at the battery, and can adjust the alternator output to compensate for the drop in the B+ charging cable.

[chimpboy]

A separate Sense wire, connected direct to the battery, has negligible current flow, so no voltage drop. This means the regulator knows EXACTLY what's happening at the battery, and can adjust the alternator output to compensate for the drop in the B+ charging cable.

Given that there's about 800mm of 6mm wire from the battery to the B+ terminal, what sort of voltage variance is possible between the two points? I'm not disputing what you've said, I'm just curious.

Shark - I think what we're all struggling with is not so much the automatic vs manual element. That's really easy. What is harder is that conventional systems have A or A+B modes, but no B mode. Main battery or both batteries can be connected, but not the aux battery on its own.

Trying to have the complete, 100% swap-over actually makes the system more complicated and I still can't quite see why you want to be able to do it.

If just hooking B+ to S is not optimal it would not be hard to rig a relay to switch the Sense wires while the solenoid is switching the main wires. So it can be done but it seems worthless.

[-Scott-]

A separate Sense wire, connected direct to the battery, has negligible current flow, so no voltage drop. This means the regulator knows EXACTLY what's happening at the battery, and can adjust the alternator output to compensate for the drop in the B+ charging cable.

Given that there's about 800mm of 6mm wire from the battery to the B+ terminal, what sort of voltage variance is possible between the two points? I'm not disputing what you've said, I'm just curious.

Cable alone isn't significant, but there's also connections at each end of the cable which add resistance. And we're not talking just battery current - this cable is carrying all the current being used by engine management, lights, fans, stereo, fridge. There's a reason modern vehicles have 80A+ alternators.

If the cable (and connections) is only 0.01 Ohms, but carrying 40A, that's a 0.4V drop. If the regulator is regulating at 13.8V (and some do - sometimes) at the alternator then the battery may only be seeing 13.4V, which won't be enough to charge the battery to 100% (as many would argue isn't possible anyway ).

If just hooking B+ to S is not optimal it would not be hard to rig a relay to switch the Sense wires while the solenoid is switching the main wires. So it can be done but it seems worthless.

I understand why he wants to do it, but I'm not sure of the best way to do it. As I mentioned earlier, I wouldn't be keen to have the sense wire open circuited at any stage, so a make-before-break system would be preferred. I suspect two relays should be used, one for each sense wire, energised when the respective battery is connected. Then you need to ensure that both relays are de-energised when the ignition is off, to minimise current drain.

[Shark]

OKay okay i can see where you are all coming from now, maybe there is a reason why people dont do it the way i have described..

So the most common practice is A and A+B and thats it...

So with the auto units like Redarc for example i wanted to know are they actually smart solenoids or just voltage sensitive relays. Could you rig up something similar with a Coles herse type solenoid.

What i dont understand is what safeguard do you have with a redarc when say battery B is at 10v and your starting is at say 12.6.. Wont your alt pretty much within say 5 mins get up to a charge voltage of 14 volts and then link the two thus draining your main..

Just curious on how it all all works.. I liked the idea of full manual control in the cab with both batteries but i cant run the risk of draining batteries or frying something..

Cheers for all your help so far guys, i really appreciate it

Marc

[-Scott-]

So the most common practice is A and A+B and thats it...

Most common, yes - but not only. My father's dual battery controller (Pirahna, I think) works similar to your plan; charges the starting battery, then swaps everything to the auxiliary battery - there are signs everywhere under his bonnet which emphasise that the main battery is NOT charged during normal running, and all accessories must be connected to the auxiliary.

(I believe the system monitors the main battery, and will switch back to top it up on a regular basis - but the alternator is only ever charging one battery or the other, and never both.)

when say battery B is at 10v and your starting is at say 12.6.. Wont your alt pretty much within say 5 mins get up to a charge voltage of 14 volts and then link the two thus draining your main..

This is how the other "dual battery thread" got hijacked. I'll try to choose my words very carefully.

As long as your alternator can maintain voltage above 12.8V (or so) your main battery won't discharge into the secondary battery.

Once the two are both connected to the charging source the "flatter" battery may draw more current than the charging source is able to provide. If this happens, the charging voltage could be dragged down to the point that the main battery will take a LONG time to charge properly, which we can all recognise is a bad thing. This is why most controllers ensure the main battery is adequately (not fully) charged before connecting the auxiliary battery (or batteries).

The chances of this happening? While everything is working properly, too small to consider. If this happens, you have a fault somewhere - fix it!