I have been told when you are running two batteries in sequence, they should be a matched set. I have my second battery running through a continuous duty solenoid so when the key is off, the two are separated, key on they are joined for charging purpose. I also have a small push button connected to the relay powered by the second battery in case the main battery is dead enough that it won't energize the relay to engage the second battery. I added that after I had that exact problem with the first battery dying. GM's biggest fault with these trucks was the pennies they saved using wiring that was too small to handle the loads that we like putting on them. Upgrading wire gauge is essential for the kind of performance I am expecting from my rig.