"rig ubiquitous car batteries"
Lead-acid isn't lead-acid isn't lead-acid.
Car batteries are designed to be lightweight and provide _enormous_ discharge currents for a short duration (seconds). The lead plate grids on them are relatively fragile and if you deep-discharge them, or use them as leisure batteries (or UPS batteries), they die in short order. (The plates will sulphate up quickly if left discharged below 80%)
Likewise if you use leisure (caravan) batteries to start your car, you'll get one shot at doing so, then start wondering why the plates are fragmented all over the bottom of the case. These are a better choice for UPS use though.
Sealed mat lead acid batteries are a good compromise for most UPS purposes as they don't outgas unless heavily abused.
If you need large capacity, then deep discharge Traction cells are the way to go, but they're not small or light or cheap and you DON'T want them in the same room as the rest of your power electronics (If they boil, every piece of your wiring insulation goes black. I've seen this after a lightning strike blew up a charger on a remote site) and ideally they go in a completely isolated room with good temperature control, nothing that can spark and decent active+passive ventilation.
In all cases, the float voltage is critical to a few millivolts (and different for each design) Get it wrong and you'll eventually kill the cells whilst spreading acid fumes all over the place.
Anyone who tries to sell you a car battery for standby power applications doesn't have the first clue about what they're selling (or is intending to make lots of repeat battery sales).
It's one of the more annoying things I saw whilst spending time in Outer Bumfuckistan, (you could ONLY get car batteries for standby power purposes) along with selling WD40 as "oil" to people who were wondering why everything kept clogging up with gritty shit and needing a constant dousing with more WD40, but still ended up breaking.
Biting the hand that feeds IT
