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Lithium batteries are awesome, but I think lead acid just makes a lot more sense for a UPS- they like to be fully charged, last virtually forever when maintained that way, and don't require any complex charge management circuitry.

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> they like to be fully charged, last virtually forever when maintained that way

If that's the case why do the lead acid batteries in consumer UPS's never last more than 3-5 years?

I think cheap batteries, poorly designed starter circuits, and occasional full discharge in use, which all coordinate to damage the batteries early. Also if your power is low quality, it causes the device to switch to battery often, and can put a lot of cycles without actually having a power outage. High quality lead acid batteries in solar systems, car starting, and commercial UPSs often last a decade or longer.

VRLA batteries are commonplace for commercial UPS units. For lithium, I’d explore LiFePO4 batteries. Not all lithium chemistries are alike and LiFePO4 are considered safe. It doesn’t have great capacity to weight ratio but it wasn’t meant for consumer electronics anyway.

I've been a LiFePO4 evangelist for years! I always prefer that chemistry over Li-Ion for IOT devices and recommend it to all my clients. Energy density is rarely the driving constraint except in EV or flight applications

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