Factory Dual Battery setup with a Winch

[TC70C10]

I've got the factory dual battery setup in my 81 K5. I just put on my new front bumper and my Warn 9500ti winch 2 weeks ago. I got stuck in a bog on the way down from hunting cabin last Sunday. Winch is run to spare battery and and with engine running it ran down fast. Is the factory spare battery setup more of a trickle charge for running accessories with truck off like a pickup truck with a camper shell? The battery ran down fast under load. I had to run jumpers from the starting battery and then the winch pulled me right out. Factory setup has small wires (10guage?) running from battery to relay, to junction box etc.