It was instructive to take apart a broken light switch to see why it failed. An unexpected side bonus of replacing this switch is I also learned the sealed-lead acid (SLA) batteries in my uninterruptible power supply (UPS) units are no good. I had shut down electricity to the entire house to swap the switch, a project I expected to take 15-20 minutes. This is well under the estimated run time of my UPS. However, less than ten minutes into my project, I started hearing low battery alarms followed by UPS going dark.

I suspected these batteries might be weak, as the usual recommendation is to replace them every two or three years and some of these are coming up on four years old. Doing the switch swap project has confirmed those batteries are long gone. They are still good enough to handle brief flickering blinks of power outage (common when a neighbor's air conditioning kicks in) but not for an extended outage.

The last time I needed new UPS batteries, I bought APC-branded replacement battery cartridges and then took apart the old cartridges. Finding it was built around two SLA batteries in a commodity form factor, I thought next time I should try replacing the batteries with generics instead of buying a whole APC-branded cartridge. Now I will put that idea into practice.

I went online to shop for generic "7AH" SLA batteries. They're not necessarily all seven amp-hours in capacity, but that is typical and it became a way to refer to the form factor as well. Dictating a compatible enclosure size as well as the location and shape of positive and negative terminals. Among listings for "7AH" SLA batteries, I saw some lithium-iron phosphate (or LFP or LiFePO4) batteries packaged into the same form factor and advertised to be drop-in replacements. Hmm, interesting.

On paper, lithium iron phosphate batteries will have a longer useful life. They have lower energy density than the NMC types of lithium-ion batteries popular in our portable electronics. So in electronics context LFP batteries usually meant bigger and heavier battery packs. But in the lead-acid replacement scenario, LFP batteries are smaller and lighter than equivalent lead-acid. 7AH (or 9AH, or 10AH, or 12AH) worth of LFP cells fit comfortably within a commodity 7AH SLA shape, with plenty of room left inside to integrate a battery management system to guard against battery abuse.

Four LFP cells in series have almost the same 12-ish to 14-ish voltage operating range as six lead-acid cells in series, close enough the integrated BMS should prevent any major issues. The biggest disclaimer I saw repeated from several vendors was about battery capacity. While these batteries are compatible with systems designed for lead-acid batteries, a LFP-aware charger is required to access their full capacity. Lead-acid systems typically maintain a standby voltage of 13.8V, and that would only keep these LFP batteries at about 75%-80% full.

I saw that "only 75%-80% full" warning and thought: that's not a bug, that's a feature! Limiting lithium chemistry battery state of charge to about 80% significantly prolongs their useful life relative to keeping them at 100%. And longevity is what I want for UPS batteries. I can accept only getting ~5AH out of 7AH capacity as a tradeoff if it means I still have 5AH after four or more years. I would have to pay a premium for LFP batteries over SLA batteries, but their price difference is much smaller than they used to be. And that's before considering the fact if I don't have to replace them as frequently, I might even come out ahead! This all sounds interesting enough I will give them a try.