[garyhgaryh]

I have an '02 Chevy Silverado 2500HD crew cab plus long bed. Love the truck.
My alternator goes out, but I decide to drive from Milpitas to Santa Cruz and back. It's a 2 hr drive round trip and I made alot of stops in between. I know I'm pushing it as the voltage is below 12 volts and the truck is starting to act funny. It doesn't help when I drive back in the evening because the lights come on. Anyways, I finally limp home and I decide to charge my battery. I decide to charge the one on the passenger side. After about an hour it's 12.6 volts. I assume the batteries are in parallel. To check my theory, I measure the voltage across the driver's side battery. It should show 12.6 volts, but it does not! It measures 11.6 volts. Now I have to charge two batteries separately. If they are in parallel, I wouldn't have to do this.

Are the batteries on different circuits? If so what do each of the batteries control? I've heard that you can run on one battery (does it matter if it' the left or right one?).

Luckily I have the optima red top batteries which allowed me to get home w/o running out of juice. It helps our trucks are diesels because there is no spark plugs to spark which would have killed the truck in less than 2 hrs.

Anywayone have the info on the dual battery setup?
Thanks,
Gary

 

[chrisuns]

They're on the same circuit. Out diesel probably draw more power the gas engines as well. More computers and not the mention the injectors have a decent current draw. I'd get that fixed asap. Computers are known to bug out from low voltages on these trucks.

You can also run on one battery, but as always I will recommend keeping the two unless it's a race truck.

 

[garyhgaryh]

They're on the same circuit. Out diesel probably draw more power the gas engines as well. More computers and not the mention the injectors have a decent current draw. I'd get that fixed asap. Computers are known to bug out from low voltages on these trucks.

You can also run on one battery, but as always I will recommend keeping the two unless it's a race truck.

Thanks! What you said matches up to what others have said. What I don't understand is that if I go and measure the terminal voltage of both batteries they are different. If the batteries are in parallel they should be one voltage.
That's what's getting me.

Because they are in parallel, charging one should charge the other. I charge the battery by putting the charge clamps directly on the two post of the one battery (I've done it for years and I know someone's going to tell me that's not the right way to charge a battery).
Gary

 

[19psi]

well.... if they're in parallel and one is 12.6v after charge, and the other is probably about 10.8V, they would average about 11.6. After all, they are connected in parallel, so think of them as one battery that you only charged half of...

 

[garyhgaryh]

well.... if they're in parallel and one is 12.6v after charge, and the other is probably about 10.8V, they would average about 11.6. After all, they are connected in parallel, so think of them as one battery that you only charged half of...

I don't think I'm being clear. I have not disconnected them. They are still in the car, but both read a different voltage. I should only get different voltage if I disconnect the batteries from each other. Yet, if I measure the terminals of one battery, it's different from the other.

I'm an EE / CE guy so this does not make sense to me if they are connected in parallel and have different voltages.
Gary

 

[Durallymax]

Check your connections and grounds. Give them all a good cleaning. Ive had it happen a lot where they just get corroded up which will leave one battery operating fine and not charging the other.

The computers do not like being run low and their cutoff is around 9 volts.

 

[19psi]

I don't think I'm being clear. I have not disconnected them. They are still in the car, but both read a different voltage. I should only get different voltage if I disconnect the batteries from each other. Yet, if I measure the terminals of one battery, it's different from the other.

I'm an EE / CE guy so this does not make sense to me if they are connected in parallel and have different voltages.
Gary

There must be a resistance somewhere then. Corrosion inside one of the big cables possible. bad terminal ends, bad contacts at the terminals are where I would look.
Check the resistance of the main cables.

 

[djmercer1]

I don't think I'm being clear. I have not disconnected them. They are still in the car, but both read a different voltage. I should only get different voltage if I disconnect the batteries from each other. Yet, if I measure the terminals of one battery, it's different from the other.

I'm an EE / CE guy so this does not make sense to me if they are connected in parallel and have different voltages.
Gary

you are right---both batteries should show the same voltage. i had a bad battery last fall and i couldnt boost the truck from the driver side battery, only the passenger side. when i replaced the batteries i found a bad clamp on the drivers side battery....