Chevy Auxillery Battery

Grit dog wrote:
Not needed. See how it's hooked up and figure out how to ditch it if you don't need the xtra juice.
Sounds like a cool factory ready rv setup though to get an extra battery in the bank if it's isolated from the start battery.

I thought about doing that with my diesel, as in the summer time, under normal conditions it only needs one good high capacity battery.
The just parallel them back up in the truck when not on a camper trip.

Wouldn't you have the issue of it being an automotive battery doing a deep cycle battery job? Suppose you could run an dual purpose marine /rv but now you are asking it to participate in cranking over the diesel in winter. Marine /rv are made to crank but not sure if being the partner to cranking a diesel would be good. Let's ask Mex.....

Not needed. See how it's hooked up and figure out how to ditch it if you don't need the xtra juice.
Sounds like a cool factory ready rv setup though to get an extra battery in the bank if it's isolated from the start battery.

I thought about doing that with my diesel, as in the summer time, under normal conditions it only needs one good high capacity battery.
The just parallel them back up in the truck when not on a camper trip.

colliehauler wrote:
I guess I never seen a gas truck with two batteries. If they are hooked up in parallel you need to replace both.

Mkirsch in the post above explains why.

on gm with gas truck, the second battery is connected to the trailer pigtail through a relay. It charges from the alternator when the truck is running, but the 12V ONLY connects to the trailer pigtail.

at least it was in 2004 and up through 2015 on the 2500/3500 series.

On the diesel the two batteries are connected directly to the pigtail, no relay involved. Not good IMHO.

the second battery on gas vehicles is a factory option.

For the last 15 years, the truck was used for towing a TT. When not towing, the truck sat a majority of the time. Did not originally replace the 2nd battery until the starting battery died 7 years ago. Replaced both then. Now the starting battery has died and I have replaced both again. Moved from a TT to a MH last fall. Now truck is going to be used for work commute. Have always been curious about the need of the 2nd battery since we never used a TC.
Thanks for the insight, and sorry for not mentioning Gas or Diesel to start with.

If you search other sites you will find posts about that system with gas motors. They use the second battery for an rv and charge through a battery isolator. The second battery doesn't contribute to starting the truck motor. On one of those other sites one guy said that his battery didn't last long so he installed a dual purpose marine rv battery. When you run an automotive battery down to 11.x volts it isn't going to last long at all. OP is on an rv site so I'm assuming he has an rv. If he hasn't been sitting camped out on the second battery he may never have had battery trouble. If he's unplugged from the tow vehicle and running on shore power or the rv battery he's never discharging second batt.

Without looking at a schematic or researching more I'm not sure what to say about not using a second battery. My guess is that if you removed the second battery, because of the isolator when you park you would not see power from a battery to the 7 way. If the truck was running it would seem that the isolator would pass power to the second battery circuit and on to the 7 way. The cables would obviously need to be covered properly so no arching and fire hazard occur.

This is just speculation so dont do anything on what I'm saying. If it was mine I'd probably install a dual purpose battery in spot 2. Someone else may show up who has this vehicle and done it. It all seems like a waste if it was actually used for the rv. Who at GM thought that an auto battery should be used in this manner? Maybe I'm missing something and someone will help out here.

I also seem to have read somewhere that some isolators wont pass power to battery two until it sees battery one fully charged.

I guess I never seen a gas truck with two batteries. If they are hooked up in parallel you need to replace both.

Mkirsch in the post above explains why.

It is a Gas truck, both batteries are the same.

Yes, you should replace both. If one goes bad, it will constantly put a strain on the other battery, damaging the "good" battery. You replace the "bad" battery only, and the "good" battery becomes the "bad" battery, in a vicious cycle of ruined batteries that will only be stopped by replacing both at once.

If it's a diesel both batteries work together to start the truck. Both are necessary and both should be replaced at the same time. If this is a gas truck then it truly is an auxillary battery and can be treated separately from the starting battery. Or, even removed. I don't know what it powers in this configurations. It may power the radio, 12v outlets, etc. You may want to keep if if you often use those circuits with the engine off.

You didn't say whether you owned a Diesel or gas truck?