That UPS eventually died, and I'm not sure if it was because it was hooked up to a larger battery than it was designed for, but it's still only 12 volts so I don't think the electronics would notice. What they may notice is extended run-time in the event of a power failure.

It was a crude mod. Take the cover off and remove the existing little security alarm battery, use tin snips to cut a hole in the side of the metal UPS cover (this was challenging, it was relatively thick metal, I'd recommend using an angle grinder in an appropriately safe environment far away from the internals of the UPS), and feed the battery cables out through the hole. I probably got some additional cables with appropriately sized terminations to effectively extend the short existing ones (since they were only designed to be used within the device). And then connect it up to a car battery.

Cover any exposed metal on the connectors with that shrink rubber tubing or electrical tape. Be very careful with exposed metal around it anywhere, especially touching the RED POSITIVE pole of the battery. Get a battery box - I got one for the big-ass AGM battery.

Test it out on a laptop that's had it's battery removed or disconnected that, just in case, you don't care too much about losing.

Get a battery charger that can revive a flat battery, and do a full refresh/renew charge on the car battery once a year or after it's had to push through a power outage that may have used more than a few percent of its capacity.

Personally, I think it's safer a less hassle to go for a LiFePo4 (LFP) Power Station style device that has UPS capabilities. LFP batteries have 3,000-ish cycle lifetimes, which could be nearly ten years with daily use.

> use tin snips to cut a hole in the side of the metal UPS cover (this was challenging, it was relatively thick metal, I'd recommend using an angle grinder in an appropriately safe environment far away from the internals of the UPS)

Why not just drill a hole? Drill bits large enough to drill a hole for 120A cables exist.

> Get a battery charger that can revive a flat battery, and do a full refresh/renew charge on the car battery once a year or after it's had to push through a power outage that may have used more than a few percent of its capacity.

If you're going this route I'd recommend a marine battery. Car batteries don't handle deep cycles well, and, TBH, UPS chargers aren't designed for failed car batteries (nor marine batteries) and can possibly cause an explosion in the lead-acid battery has a few dead cells.

This really doesn't seem like something one would want to mess around with if they don't know what they're doing (fire hazards and all...)

My commercial UPS already scares me at the fire potential. No way I would take on the risk of some DIY on something that could burn down the place or electrocute me.

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.

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

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

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…

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

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…

It's a bit trickier than you think. And can be dangerous.

The discharging circuitry is fine, but the _charger_ might overheat because a larger battery can draw more current while charging for longer periods. I discovered that when I tried to attach a "lead-acid compatible" LFP battery to an UPS.

These days, it's just easier to buy a dedicated rack-mountable LFP battery originally meant for solar installations, an inverter/charger controller, and a rectifier. The rectifier output will serve as a "solar panel" input for the battery. You get a double-conversion UPS with days-long holdover time for a fraction of a lead-acid UPS.

There's not much to it, you just take the small 12V sealed lead-acid cell out from the bottom of the UPS, extend the two leads, and connect a larger capacity lead-acid battery of the same voltage.

If you don't recognize the terms "sealed", "lead-acid", "battery", "capacity", or "voltage" then you shouldn't do this.

About the only advantage of it is that it's cheap (free if you find a UPS in the trash with an already dead battery), but those cheap UPSs make really crap quality power, and for some of them the only reason they don't overheat is because their stock battery is so small. It's a bit like how you can cook a whole turkey in the microwave, but you probably don't want to.