battery-alternator problems

[schemi]

I was having a charging system problem yesterday. I stopped by autozone and had them test it. He told me the alternator was only putting out 12.9 volts. I took the alt off and took it in to have it bench tested and it passed at 14.7 volts. With the engine running I am getting 12.6v from the alt. I am getting 10.8v to the green exciter wire on the alt which seemed wrong. I then checked my batteries. Both were at 12.4v disconnected. I connected one and got 12.2v. Then I disconnected that battery and connected the other one and got 10.8v. I switched the batteries and the one battery always drops about 1.5v when connected no matter what side it is connected to. This may be a dumb question, but is it possible for a bad battery to lose 1.5v when connected? The low voltage battery was originally on the passenger side. Will 10.8v to the exciter wire cause a low output on the alt? I checked all grounds and cleaned all the terminals. Thanks in advance

[JSchuricht]

Looks like you have a bad battery. With the batteries disconnected you are seeing a surface charge of 12.4v, with them connected there is a light load and the true battery voltage is showing. Try charging it overnight, if you still have 10.8v then replace both batteries.

[schemi]

that is correct but only one battery does this. Will low voltage to the exciter cause low output from the alt?

[Diezel Dawg]

Originally Posted by schemi

that is correct but only one battery does this. Will low voltage to the exciter cause low output from the alt?

When you t n your ignition to the on position and your dash lights come in, does your battery indicator come on, or is it dimmer than the other. I had a similar issue and it was that indicator bulb being dimmer that caused my issue

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[JSchuricht]

The alternator is either on or off. Try this, disconnect the - on one battery, start the truck and see where the voltage is. I am guessing one battery is pulling the alternator down to 12.9v which will eventually kill the alternator. You should replace both batteries at the same time.

[schemi]

There was no dash lights. I took the batteries in and the one battery tested bad. Replaced both.

[Lincoln_Rig]

schemi......did replacing both batteries fix your problem ?

[schemi]

Yes it did. There was one bad battery. I replaced both but kept the good one for the jeep.

[Lincoln_Rig]

Thanks for the response, having a little trouble here, new alternator, one new battery, not charging at all.
The old alt. started over charging and seemingly draining batteries when not running so I replaced it. One battery was bad so I replaced that as well.
I sent the alt. back, the guy tested it and says it's fine.......not quite sure what the problem is yet.

[schemi]

Get your other battery tested. It could be bad dragging everything else. that is what my problem was