I have two six volt golf cart batteries that I run in series and have the option of adding two deep cyle 12 volt to the battery pack. Knowing the 12 volt must run in parallel and the 6 in series can I hook them together for a 4 battery pack?

(As an elecrical Deidgner)

CAN you? Yes.

SHOULD you? NO!

If you do, it will work for a little while - BUT you will kill (damage) you 12v batteries quickly.

As a General Rule, all batterys in a bank should be of the same size and capacity.

If you do hook them all together, here is what will happen:

Assuming all your connections are done properly, any load will be equally spread across all batterys. However, those 6V botterys have a much larger Amp/hour rating. That amp hour rating tells you how big the "Gas tank" is - how many hours that battery will provide one amp, till it gets below target voltage. (or how many amps it can deliver for one hour, same diffrence). Say you have a 30 amp load - the three banks (the 2x6v batts make one bank) will each see 10 amps. XX hours later, the two 12v batterys are "Dead", but the 2x6v bank is still at 50%, and running strong. Each battery will still see load (as a battery dies, it's resistance goes up, so it will draw less) and the "Dead" 12v batterys will get discharged further. As we know, if you leave a load on a dead battery, you risk permanate damage. In ther trailer, you may not notice you lights are not quite as bright (because the 2x6v bank is supplying 90% of the load), but outside, your 12v batterys are being cooked to death.

You may not kill them you first trip, or the fifth, but they will die an early death.

If you need the extra capacity, buy a transfer switch - run the 2x6v bank till depleted, then switch over to the 2x12v bank.

Jerry

Tamid, you should have no problem connecting the four batteries as you have stated.

As an electrical engineer with experience in design of lead acid battery systems used for un-interuptible power supplies for computer systems, I suggest the following:

Think of the battery pack (2 ea. 6 volt batteries in series connected in parallel with two ea. 12 volt batteries) as an assembly of water tanks. When initially connected each tank may have more or less water than the others. Water will flow between them until they reach equilibrium. At which point they will provide proportionately sufficient water to maintain that equilibrium. As you drain water from these tanks they will continue to maintain this equilibrium until all the water is drained from them. One tank will not become empty (dead) before the others. When you replenish the water it will flow into the tanks propotionately until each tank is filled to its capacity.

I agree with Jerry Gw that, "As a General Rule, all batterys (sp) in a bank should be of the same size and capacity.", but it is not a "design law".

(as an electronic engineer)

I don't think I totally agree with this.

Batteries connected in parallel, no matter the AH rating will maintain the same output voltage. But, what you get when connecting batteries of different AH (Amp Hour) rating, assuming they're both of the same battery technology, is as they are discharged the larger AH rating batteries will put some of their energy into the lower AH rating battery. The end result is that you only get the AH of the higher rated batteries. With copper losses and other inefficiencies you probably get less Amp Hours than you would without the lower rating battery.

???

"...assuming they're both of the same battery technology, is as they are discharged the larger AH rating batteries will put some of their energy into the lower AH rating battery..."

Besides not being true, it even doesn't sound logical.

Again with the water analogy thing (remember amps are capacity and volts are pressure) - your saying that if I have two 10 gallon tanks of water in series connected in parallel with a 15 gallon water tank, I can never get any water from the 15 gallon tank because the 2 ten gallon tanks in parallel equal 20 gallons (i.e. greater than the 15 gallon tank).

Ummmm...

Once the batteries are connected (series/parallel) with no load and they reach equilibrium (i.e. the voltage stabilizes and there is no current flow between the batteries), when the load is connected the current flow into the load will come from all three batteries proportional to the internal resistance of each battery (whether in series or parallel) and the total AH of the battery bank will be the sum of their individual AH less any losses due to their internal resistance and the resistance of the connecting wires.

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(Nothing beats "A rivet runs through it"... YET.. this one is close )

I stand corrected.

However, it does seem that there might be a charging problem. Any thoughts there?

We exceeded my skill level on this, but the knowledgeable folks who put in BIG battery plants in their BulgeMobiles recommend against mixing batteries like this; in fact, they highly recommend getting 6V batteries at the same time so as to choose them from the same manf lot so they are as alike as possible. They also recommend replacing both batteries if one goes bad.

However, if you already have all the batteries and have no alternative uses for them, then you don't have much to lose in trying it out. Don't forget to give a report occasionally

Why wouldn't you just throw in a Battery Bank switch and call it a day. Run your 6'ers as a bank and your 12's as a bank. Whichever has the most staying power, make bank one. Run one down and then flip the switch over to bank 2. Least this way you'll know when you are into your reserve power.

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If you buy two at the same time and a cell goes bad later, it is a good idea to replace both. Why, same lot, same history, probably a very similar MTTF.

I would go with the 2 bank and switch for the same reason I like a real reserve tank in an airplane or diving. Gages and time estimates are not as valuable as a real reserve. That reserve is very nice even though the outcome is not as black. OK a little joke there.

This may help clarify the mud.

Parallel Series Batteries

Sean, excelent discription of the problems associated with connecting batteries in serial/parallel.

As far as I am concerned this is the final word on this thread.