I really haven't paid much attention to this before, but how many different sizes are there of 12 volt batteries -- specifically the ones that are used in the surge/ups units for computer, monitors, etc.? I ordered on because a unit my wife was using didn't have a battery (I don't remember taking it out, but I must have) and the battery I ordered doesn't fit. I usually get them from Batteries Plus, but this one came from Amazon and was about $15 cheaper. I'm not sure if I got a battery for another application.

So what are applications for 12v other than these backup units? I know they are also used in garage door openers with battery backups. I just put a new one in there as well, and both the new one and one it replaced was too big for the surge/ups.

Our ADT system had a 12v battery that was roughly 4x6x2. They come in all sizes. I bought the replacement at Altex. The experience the OP described is why I avoid buying these things from Amazon. Plus a lot of the Chinese stuff on Amazon is junk.

There is usually a relatively standard size based on voltage and Ah capacity and terminal type. Sometimes with the smaller UPS, they'll use the same case for 2 different capacities and I've seen situations where you had to cut back plastic to fit a larger battery into the case.

I haven't had to replace batteries without having the battery itself though. I was always able to crosscheck the size of the ones I was ordering against what I was replacing, but I've always seen them listed by voltage, nominal capacity and terminal rather than dimensions.

APC seems to have about 3 standard sizes for consumer models.
Cyberpower seems to have about 3 too.

You can use different sizes of 12v batteries, but the door might not close, or the battery might rattle around. It will still work same.