[sirmoveon]

No, don't do it. I understand his thought process because they are both 12v batteries with more capacity, but car batteries are made for high burst of energy which a car engine ignition requires, whereas UPS batteries are made for slow drains. Also, these UPS are made for charging battery cells in a certain way, if you start to stack a bank of batteries of the same model in parallel hoping for more capacity, even then its a problem for the UPS's charger, they won't charge evenly and eventually becoming a problem.

[zdragnar]

Marine deep cycle batteries might work better, but at some point I'm pretty sure lithium would be price competitive.

[chatmasta]

I like to keep my hardware competence sufficiently low so that I’m never cursed with the false confidence to even consider “drilling a hole in a UPS,” nevermind wiring it to a car battery in my closet…

[nancyminusone]

You seem like the kind of guy who doesn't enjoy a nice sulfuric acid spill on the floor, haha

[chatmasta]

I will mess with all kinds of hardware, especially mini PCs and routers.. I once had a few hundred iPhones in my closet… but I draw the line at anything that uses batteries or electricity in a non-standard way. If the wire can’t carry data, I’m not touching it.

Maybe it’s because when I was a kid, I fancied myself an experimenter, and I had a wire ripped off a lamp, and touched the two ends together…

[bluGill]

It isn't quite that bad. the batteries are close enough that it will work.

the real worry is these are already a fire hazzard and so something goes wrong insurance will blame the mod even if not at fault