How Do OEM Dual Batteries Work?

[downslope]

I have a 2008 gas GM pickup with the factory dual battery option.
Are these just 2 batteries connected in parallel, or is there some kind of isolation between the two?
I had a dead battery situation (solenoid just clicks, maybe some light was left on). I connected the charger (Battery Minder) to the battery on the driver's side at 4 amp rate and in a few minutes it indicated fully charged and battery OK. So then I connected the same charger to the battery on the passenger side and it started to charge but indicated heavy discharge condition. It was getting dark and I left it charging overnight and will check in the AM tomorrow.

My question is does each battery have to be charged separately?

Also, how can I get rid of one of the batteries? I'm not in a cold climate and don't need two batteries for starting or anything else. When it comes time to buy a new battery (like maybe now) I would like to buy just one good battery and install it. Can I just tape over the unused cables? If I can go with one battery, which bay should I install it in, or does it even matter?

 

[Diamondpilot]

I think you have answered your own question. If you can not start with one battery charged then you need tow batteries. I am sure you could pull some stuff out and do some re-wiring but whats the point. One good big battery is going to push $100 or two of the smaller oem batteries at $60 each it just easier to leave it as is.

Chris

 

[downslope]

I think you have answered your own question. If you can not start with one battery charged then you need tow batteries. I am sure you could pull some stuff out and do some re-wiring but whats the point. One good big battery is going to push $100 or two of the smaller oem batteries at $60 each it just easier to leave it as is.

Chris

When I was charging the batteries they were still in the truck and I didn't have the cables disconnected. So I don't know why one would charge different than the other. Maybe one of them is shot and drawing the other one down, I really don't know how it's supposed to work. That is one reason I don't like two batteries, if one is going bad and can take the whole system down even though the other one is still good.

 

[jonyyuma]

My 1983 6.2 chevy had the two tied together directly with a cable atop the radiator. THEY BOTH CHARGED TOGETHER. I used two reg batterys in mine, and jumped the regulor circuit up in the alternator to put out a higher charge rate. You prob. have a serpentine belt, which is good. The v-belt on mine did not last long and was always squeeling. Be aware the glow-plugs pulled a lot of current and the fuel line had a electric heater under the intake manifold. So it had a large electrical demand on start-up..

 

[WM75Guy]

In my experience if one goes bad the other will shortly follow. The voltage is measured at one point. As one fails it causes the system to continually charge so the good one is overcharged and the bad one will show undercharged. In my diesels I replace both at the same time to prevent this from occurring with one new battery and one old battery.

 

[grsthegreat]

I have a 2008 gas GM pickup with the factory dual battery option.
Are these just 2 batteries connected in parallel, or is there some kind of isolation between the two?
I had a dead battery situation (solenoid just clicks, maybe some light was left on). I connected the charger (Battery Minder) to the battery on the driver's side at 4 amp rate and in a few minutes it indicated fully charged and battery OK. So then I connected the same charger to the battery on the passenger side and it started to charge but indicated heavy discharge condition. It was getting dark and I left it charging overnight and will check in the AM tomorrow.

My question is does each battery have to be charged separately?

Also, how can I get rid of one of the batteries? I'm not in a cold climate and don't need two batteries for starting or anything else. When it comes time to buy a new battery (like maybe now) I would like to buy just one good battery and install it. Can I just tape over the unused cables? If I can go with one battery, which bay should I install it in, or does it even matter?

I know on my diesel truck, you CANNOT lose the second battery. You need the amperage to help start rig. My ford is set up with a relay that sends power from one battery to a trailer if so equiped, so it only draws trailer power from 1 battery. As long as it doesnt draw down that battery till its dead, you could always start the rig. Im not sure what would happen on your gas rig if you removed 1 battery.

 

[Soundguy]

could be bad connection between batteries... that and one battery with a bad cell and a good battery, and there will always be a state of discharge..

soundguy

 

[Dmace]

Follow the cables from the batteries to see where they go, one battery should ground to the frame and have one power lead go to the starter and one go to the fuse box. The other battery should ground to the frame and have just one power lead running to the power lead of the first battery. Many times it's a special cable that's all one piece. I had this setup in my old 94 F350 diesel dump truck. They charge together and discharge together. I found one good Everstart Maxx 1200 CA battery was more than enough.

If you charged one battery and the other was still dead then it's because that dead battery is not holding a charge. That's why the charger showed a "heavy discharge". Since your truck has a gas engine, it really doesn't need the dual batteries so remove them both and replace a new battery in the spot that has the battery connected to the fuse box. Most likely the battery on the drivers side.

 

[Fixemall]

Hit up your local Interstate warehouse if you have one and you can pick up "blems" for around $40 a piece, they might be ugly but they will crank like the pretty ones. Make sure you remove the ground wire and clean it real good where it attaches to the frame, bad grounds are the express route to dead batteries.

 

[B7510HSD]

If these are starting batteries and not deep cycle which they should be I would check on the warranty as they are 3 yrs old if this is an 08 and most batteries carry a longer warranty than 36 months~

 

[Bigkrank]

Don't know about the 2008 GM Gas truck but I had a 2004 GM HD gas w/ 2 batt option and only one was used for starting, my 2008 GM HD Diesel of course uses both, wired completely different than the gas was.

 

[Soundguy]

the 2 bat setup in my 99 f450 uses 1 long cable to hook both bat hots together.. suspect my f350 is the same..

soundguy

 

[downslope]

OK, I did some checking on the truck. The battery on the passenger side is mounted by the firewall. It has heavy gauge positive cables coming from it. One cable runs down to the starter and the other one runs to some big terminal type device (circuit breaker, isolator??) on the firewall and then splits into two wires going over to the driver's side. One of these two goes directly to the fuse box, the other runs thru a relay to the other battery on the left side.

So I'm thinking the firewall battery does all the starting because (1) it has the heavy cables and (2) is the one closest to the starter.

The battery on the driver's side only has a light gauge positive cable going thru some kind of fusible link and then over to the relay which connects to the other side. I have a feeling in the 3.5 years I've had the truck this battery has only been along for the ride. When I do something stupid like leave the dome lights on overnight it only runs down the starting battery.

This truck has the factory HD trailering package and also the snow plow prep package. I have a feeling if I had a plow installed or was using the accessory power to the trailer, then the 2nd battery would come into use, and if you ran it down thru the trailer plug with the engine off the starting battery wouldn't be affected.

Maybe it would be a good idea to switch the two batteries for now. I don't think either one is "bad" yet, but I did run the one down a few times by leaving something on in the truck overnight and I'm sure that doesn't do the battery any good. But what do I know, maybe I'll ask at the dealer when I get a chance.