I just thought I would create a post for those who need directions on making sure they have no losses of power from their batteries. My truck has two batteries and I had a power drain. I did not know if it was just a bad battery or something in the system that stayed on all the time.

I imagined I was deep in the winter somewhere and could not afford to not have the power to start my engine on demand, even if sitting for days. Obviously, the auxiliary battery would help here, but not if they were both draining.

Since anything might happen in the future, it seemed wise to put a stop to any possibility of any sort of power loss no matter what.

Since the batteries were both testing as bad, had to be replaced and I did not want to buy new ones too soon, I thought up a solution to protect the new ones.

The concept was simple. I added a contactor on each battery positive output cable. They are each activate by a toggle switch on the dash. They receive the contactor power from the battery, thru the switch and back to the contactor. When the switch is off, no current flows.

(Kind of an anti-theft device; if one does not know to flip the switches, it won't go.)

I used a contactor instead of a relay so as to have enough current flow to start the vehicle. Just needed to specify big enough to temporarily supply the total of both the batteries' CCA to the starter. It is okay if the contactor is rated at less for continuous use.

Surprisingly, this was relatively inexpensive and creating the cut in the cables, installing proper connectors, using small AWG wired thru a fuse holder to the dash and back was no big deal.

It has worked very well indeed.

I should point out that the manual states that there is a relay for the auxiliary battery, but it does not exist on my model. (Don't know why.) But having it on just one without enough thru-put we be unacceptable anyway.

Perhaps my story can help someone else.

 

Interesting way of avoiding a battery drain - and you are right. Contactor is perfect for this application.

Unfortunately, while this probably works fine on our older GMT400 trucks, on a newer vehicle, even the GMT800 trucks (1999-2006), I think disconnecting the battery (or batteries) in this manner would ultimately lead to triggering some of the anti-theft mechanisms, which seem to be problematic if you don't disable them prior to disconnecting the battery for an extended period of time.

 

You are correct. This is easy on older vehicles. Newer vehicles have problems with computerization, and we don't want to forget the problem of lost settings when power is turned off.

It sure is a more useful application when building a custom ride. Even if a person has to have electronic controls and modules for more modern drive trains which reply upon battery backup, one could isolate those items with their own separate battery, and have a main one or two for starting.

Just a thought.