# Series Battery Q's?

Discussion in 'General Electronics Chat' started by pntrbl, Jul 6, 2008.

1. ### pntrbl Thread Starter Senior Member

Apr 21, 2008
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I'm gonna be using a pair of 12v battery's in series for 24v. I do have 12v requirements tho and I'm wondering about total power dissipation between the two.

Say I need 3A on the 24v for 72 watts.

Call it 5A on the 12v for 60 watts.

Is the total power 132 watts?

The other thing is I'll be charging these battery's with 2 GM alternators, one on the high side, one on the low, and I'm wondering if one alternator will be working harder than the other. Or will the battery's equalize and place equal demands on the alternators?

Just for the sake of education and my personal understanding, given the above numbers, what amount of current would flow between the battery's during normal operation?

I'm thinking the 12v is supplying half the watts for the 24v line which would be 36 watts. 12v/36w=3A? That doesn't hardly seem fair! LOL! We're only looking at 3A total on the 24 anyway!

I got a feeling this all works out from the battery's equalizing but I'd like to know for sure.

Thanx for any help in understanding what's gonna be happening when I get this thing fired up.

SP

2. ### beenthere Retired Moderator

Apr 20, 2004
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Of the two batteries, the lower on (12 volt supply) will be supplying 8 amps, while the upper will only have to handle 3. In automotive practice, this is a very small load, both to the batteries and to the alternators. Just make sure the alternator charging the upper battery uses the common point between the two batteries as the ground reference.

3. ### SgtWookie Expert

Jul 17, 2007
22,201
1,807
Any chance you could split that 5A load on the lower battery between the two batteries? That would decrease the cycling of the generator, saving you fuel.

Either that, or use a low-side battery that has nearly twice the AH capacity of the high-side battery.

The low-side battery will supply 5A, the high-side 3A, for a total of 8A current in the circuit.

You might just get three batteries with the same AH capacity, and put two in parallel for the low side.

4. ### pntrbl Thread Starter Senior Member

Apr 21, 2008
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I picked the numbers in the previous example out of the air, the real power requirements aren't yet known, but it sounds like it would behoove me to balance the load as much as possible.

The equipment to be powered is security cameras, a DVR, a SAT link, motion detectors, fans, lights, bells, and who knows, maybe a whistle! LOL! My son-in-laws are in charge of all that. I'm just the power guy.

Greg is fabbing a trailer that'll resemble a Brinks armored car with a mast on it to hold it all and Steve's the security CCTV guy. They have the idea that they can lease portable high tech security systems for a profit. Sounds good to me .....

I can't think of a reason why a 12v device couldn't be wired in on the high side tho. If I just pay attention I'll bet I can get the balance reasonably close.

If life will get out of our way long enough to get the 1st one built we'll find out.

SP

5. ### SgtWookie Expert

Jul 17, 2007
22,201
1,807
Yep, getting the balance close is pretty important from the standpoint of fuel economy.

If you have to choose, you'll want the low side to have a slightly higher current draw, as that's presently where you're monitoring the voltage using the circuit we whipped up - unless you changed things around.

If both batteries get depleted about the same amount, and then the generator charges them both back up, you'll get at least reasonable efficiency. However, if one side (upper/lower) is getting discharged more than the other, the alternator on that side will be idling and just causing drag while the other side will be working hard to charge the battery back up.

6. ### pntrbl Thread Starter Senior Member

Apr 21, 2008
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Nawh, still pretty much the same circuit you steered me thru Sarge. I've switched to PNP's to use a Ford solenoid and I "think" I've come up with an ignition kill. We'll see.

SP