dual batteries

tmf6902 · September 29, 2003

I have the factory dual battery system on my truck. I am wondering, when I do things that require unhooking the battery, do I unhook both, or just one of them?

TMF

BlackSilver · September 30, 2003

I haven't sorted out how they're physically cabled together, but I'd almost certainly think you'd need to disconnect each one separately.

Bob G. · September 30, 2003

TMF,

Ok, I'm not sure if this applies to your truck or not. My 1500HD came with the factory aux battery and it is isolated from the the main battery when the ignition is off. When the ignition is on, the two batteries are in parallel and the alternator charges both. I'm not sure why the dealer ordered this truck with the aux battery, but it allows me to use it for accessories without draining the main battery.

Bob G.

SoMnDMAX · September 30, 2003

On the diesel trucks you need to unhook both.

I'm assuming the 8.1 is the same way, unless they use the same setup Bob G is referring to.

Regardless, unhook both grounds and you can't go wrong.

bigdogs · September 30, 2003

On a kinda related topic, how are the batteries on the duramax wired, Does it give 24v to the starter or 12v with just more amps?

When in Acc mode does it draw off one of the batteries or both equally?

Thanks

AlanTDickey · September 30, 2003

Bob:

I have a 2003 1500HD with the aux battery.

You may be correct, but someone on the GMC techline told me that the aux battery on mine is only hooked up to the auxillary trailer connection.

He told me that the spare is recharged by the alternator, but is completely independent from the main battery at all times.

Thus, according to him, unless I do any alterations to the system, the only advantage of the auxillary battery is that it would run trailer lights, and I could always jump the main battery from the spare if I ran the main one out (which is a good backup). Also, if you have the camper-connection option thingy, it might run that too.

I hope you're right that the spare runs in parallel when the ignition is on (makes sense), but that's not what GMC told me.

If anyone else knows for sure, I'd appreciate the info.

Thanks in advance.

Alan

BlackSilver · October 1, 2003

On a kinda related topic, how are the batteries on the duramax wired, Does it give 24v to the starter or 12v with just more amps?

When in Acc mode does it draw off one of the batteries or both equally?

Thanks

On the DuraMax the batteries are hardwired in parallel without any isolation (I inspected mine today). The starter gets 12V, and in the ACC position the draw is from both batteries.

Note that there is no option for "Auxiliary Battery" on a DuraMax. The "Auxiliary Battery" option on other models may be more than just a second battery (as other posters have described).

CMNTMXR81 · October 1, 2003

On the diesel trucks you need to unhook both.

I'm assuming the 8.1 is the same way, unless they use the same setup Bob G is referring to.

Regardless, unhook both grounds and you can't go wrong.

8.1's don't have dual batteries unless you specifically order it.

SoMnDMAX · October 1, 2003

8.1's don't have dual batteries unless you specifically order it.

Yep knew that. Didn't know exactly how they were wired when equipped.

bigdogs · October 1, 2003

Thanks BlackSilver. I wasn't sure. I know some tow trucks used to jump at 24v to make the starter spin faster

CMNTMXR81 · October 1, 2003

They're wired the same way any other truck with dual batteries would be.

bigdogs · October 1, 2003

This is my first truck with dual batteries. I have seen isolators and dual battery switches on other people's but never had one myself.

Bob G. · October 1, 2003

Alan,

I just checked my batteries connections again on my 1500HD so I could give you accurate info. Mine is a 2002 and they may have made changes for 2003.

The main battery + cable goes to a terminal block with a red housing. The alternator is connected to the main battery there, then the + cable continues to the starter.

The aux battery is connected to a megafuse behind the battery, then is connected to a relay. The output side of the relay goes to the starter and is connected to the main battery cable there.

When the ignition is on, the relay is a closed circuit and the aux battery and main battery are connected in parallel and are both being charged by the alternator.

When the ignition is off, the relay is an open circuit and the aux and main batteries are isolated from each other.

The aux battery has smaller cables than the main battery and is limited to 30amps for accessories. My manual says to use the top side of the relay for accessories (cable from megafuse). This way you can always start your truck in case you leave something on.

I have the light for the cap hooked up to the aux battery. I may add aux backup lights next spring and will hook them up there as well.

Bob G.

AlanTDickey · October 3, 2003

Thanks Bob for the info!!!

To clarify (I'm fairly ignorant about circuits):

What is the "top side of the relay" and the "cable from megafuse"?

When the ignition is off, are all of the electrical devices (including the power adapter plugs) run off (1) the main batt, (2) aux batt, (3) combination??

I'm pretty sure that my 2003 manual does not say a word about the aux batt - unless I'm blind. I thought that was strange.

Bob G. · October 7, 2003

Alan,

The relay is mounted on the firewall next to the aux battery. My manual says to use the top post of the relay for additional accessories. The megafuse is mounted between the aux battery and the firewall. I removed the aux battery right after I bought my truck so I could check it out. There was nothing connected to the aux battery from the factory except the relay that connects it to the starter. The truck camper option would have its power run from the aux battery. Like I said before, I'm not sure why my truck was ordered with the aux battery option.

Bob G.