Two Batteries

It doesn't, and it can't. The alternator just supplies an amount of current in a voltage range, normally a little higher than the battery voltage of 12v, and the battery charges. As long as the batteries don't draw more current than the alternator can supply the alternator won't be damaged. You should get a battery isolator from an RV supply to keep one of the batteries charged up enough to get the engine started.

JazzMan

It doesn't and because of that you will soon have one fried battery.

The best way to wire them is to get a battery isolator unit from an RV shop. These allow the alternator to charge one battery at a time or at least when one is fully charged, it shuts off that one so the primary is just running things and getting charged up.

They also allow one battery to go dead and keep one in reserve for starting.

They make all kind of different configurations, check with the RV shop.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Flyer wrote:

Actually, because they are tied together, you'll have two fried batteries. Get an isolator as mentioned.

However, an isolator does create a small problem in that the alternator can only take input voltage from one battery in its response to how much current to generate for charging purposes. The other battery gets what it gets so the best thing to do is to use the voltage from the battery that does the most work. Then what I do is swap batteries (actually, just the positive cables) from time to time so that each battery gets a similar workload over time. -PapaRick

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Mike Roma> It doesn't and because of that you will soon have one fried battery. >

The trick is to use 2 identical batteries and purchase/install them at the same time. A 1-BOTH-2-OFF battery selector may also be used in some truck (particularly emergency vehicles) & marine applications for manual selection/isolation. The isolators are best for a cranking battery+deep cycle battery applications (it will keep loads on one from discharging the other and vice virsa), but may also be used in other applications.

I agree with the posters saying no problem. The alternator puts out 14.x volts and has some max current capacity. If one battery is charged and one needs charging, the current will flow to the one needing the charge.

The only risks I see are if both batteries are dead, the alternator will be putting out max current for much longer than intended. This shouldn't harm anything, but could push an old or light duty alternator over the edge. The other risk is if one battery were to fail with a hard short, it will drain the other battery if you do not have an isolator (had something happen once that took out the battery, alternator and voltage regulator (external on Ford). Can't say what went first, but guessing something shorted and fried everything. I blamed the battery, but can't say for sure).

You are incorrect.

Lead acid batteries can short out sucking an alternator to the cooking point or pinning out a battery charger on full amps to the point of smoking the charger if left on.

Or they can go open with 0 volts or one cell can bridge making them 10 volt batteries.

If they go 10 volts, the 12 volt system will keep trying to rise the voltage or charge them up and the second battery will boil dry.

I see tons that go 10 volts with an almost pinned volt gauge once boosted and going because I hang with a group that off roads and batteries don't much like vibration.

I went Optima lead acid coil cell that is packed tight enough the acid can't even leak out after my 3rd '10 volter' in a year. 3 years and counting on the Optima now.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Maybe....maybe not.

Well I'll disagree. You can and do get circulating currents between them. I'm watched same... and shown same to fellow EE's who would not believe they existed. I agree the alternator won't care.

And oops, the two chemistries have different float voltages...

At the worst, you want a contactor that hard-parallels the batteries when charging and isolates them when not. But the isolator is a better idea. And now likely cheaper than a 100A contactor.

Hopefully far better than GM did on their infamous 350 diesel. There, since it was a kludge,[1] they used two 12v's in parallel, one on each side:

Which the isolator deals with, unless its an ultra-cheap isolator.

Knife switches in that rating are very cheap. Not automated, but cheap :-)

More seriously, knob-operated main battery switches are available at speed shops for use on race-cars as emergency disconnect switches. One of those between the two batteries is another good low-buck manually-operated solution.

Even better for a place for a knob type battery switch is your local marine supply shop.

Tom