Running Second Battery

dj_bosse · April 20, 2020

Hey everyone. I'm looking to run a second battery so I don't have to run new accessories off the original battery. I also want the second battery to be able to start the truck if/when the main battery dies, but don't want the main battery to be able to drain the second battery. I did quite a bit of research and thought I had it all figured out, and started running my wiring. Here's my wiring to allow the second battery to start/run the truck in case the main battery dies: positive cable running off the second battery into a 200A fuse, which runs into a battery isolator. The isolator has a ground wire and a wire running to the ignition fuse (with the understanding that when the key is turned to accessories, the isolator will allow the batteries to connect). The other side of the isolator connects to another 200A fuse to my alternator (which has been upgraded to accommodate the additional demand).

Here's my question/confusion. With the key off the batteries have 0.2V difference in voltage. With the key on (but not running) the difference drops to 0.1V. With the truck running they're the same voltage +/-. With the key on and off (immediately after key off) the batteries have the same amperage draw. From this, it makes me concerned that they're not being separated/connected how I want them to. Now assuming the wiring is correct, I'm also confused on how the isolator can be commanded to allow the batteries to connect in the case that the main battery dies. If the isolator is told to allow them to connect when the key is turned, but the main battery is completely dead, there wouldn't be any power being delivered the ignition fuse until the key is turned. So if that's the case, there wouldn't be anything to tell the isolator to allow the batteries to connect, so the second battery wouldn't be able to start truck.

RoarinRow · June 30, 2020

Probably already too late, but did you see this Painless Dual Battery setup - https://www.painlessperformance.com/wc/40102

It has indicator lights to tell you what mode of charging is happening or not happening, and I believe can jump start your main battery in an emergency. I had this similar kit in my Chevy Avalanche and had to jump start my main battery on more than one occasion.

dj_bosse · July 2, 2020

Appreciate the reply. I ended up doing multiple voltage tests on the batteries during start and it appears everything is functioning as desired with the setup I have. I ended up putting in a battery with zero charge to simulate a dead primary battery condition, and the truck started without hesitation.