Probably just a case of replacing the batteries, but if you want to think out of the box: there are some hybrid inverters for solar systems that offer a UPS mode. So you could combine a home solar system with modern lithium batteries with your homelab needs for a better UPS.
I second this idea, if it’s feasible. As noted elsewhere in this thread, the lead-acid batteries in UPS units have a limited lifespan, even if not regularly drained. Solar and off-grid enthusiasts have determined that parity between overall lifetime cost of lead-acid versus lithium batteries was reached years ago, and now it’s firmly in lithium’s favor, mostly due to the greater number of recharge cycles.
Contraindications for lithium batteries would include:
high local costs for lithium battery packs
lack of space for the hybrid inverter, as they’re usually not rack-mountable
the homelab drops below 0 C (32 F), in the specific case of LiFePO4 cells
That said, breathing life into old equipment is usually more environmentally friendly than acquiring new equipment.
The primary concern is switchover time. A purpose-built UPS can switch in about 10 microseconds. Those large "solar generator" battery packs switch in about 30 microseconds. That might be fast enough, but it's not guaranteed to be fast enough like a UPS is.
Did y’all mean to say milliseconds, and not microseconds? Sub-millisecond power loss would be less time than one AC cycle, whether 50 or 60 Hz.
Anyway, I do recall seeing some enterprise gear specifying operation through a drop in AC power lasting two cycles, precisely to cover the switch to UPS power, at least for 60 Hz power. So up to 33 milliseconds. A cursory search for hybrid inverters online shows a GroWatt with “<20ms” switchover, so this may be fine for servers and switches, when the inverter is operated without any solar panels.
For consumer grade equipment, all bets are off; some cheaper switch-mode power supplies do very weird things under transient conditions.