When I was doing some comp crawling, I brought a motorcycle battery and a little cheap multimeter. I have never charged while I walked, but would set it up in a safe place and recharge. I brought the multimeter to check the voltage of the motorcycle battery to make sure it wasn’t under about 11.8v.

With that said, there are tons of Lithium Ion motorcycle batteries out there that weigh about 1/4 of a lead acid battery. Might be a good option. You could carry 2 in a backpack no problem.

with a car battery, if it’s not an AGM battery, or a sealed battery, you will never completely contain the battery acid that’s in the battery, your make a giant acidic mess which doesn’t really hurt, but will kill/decay/ruin every piece of material it leaks on. I have plenty of ruined clothes from this while I was a motorcycle mechanic. If this was your only option I’d pack a ziplock bag full of baking soda to clean this mess up and neutralize it.

Hope this helps.... what voltage are you charging and what is the Mah of your batteries?