Time to replace your UPS batteries? (Updated 4-May-2023)

I bought a CyberPower CP1500PFCLCD UPS roughly 6.5 years ago. The manufacturer claims it will power 900 watts of equipment for 2.5 minutes at full load (10 minutes at half load). While on battery power, it outputs a true sine wave, whereas most cheaper UPSes output only an approximated sine wave, which can cause issues for more delicate equipment.


After years protecting my NAS, I recently added 3 small form-factor desktops running ESXi, and the reported load is now ±19% (170W).

Last night at 11pm we had a power outage lasting 10 seconds and my lab went dark.



There was no decline in time remaining or percent charge - the UPS gave up immediately.

UPS batteries typically last 3-6 years. I'm still running the original set from 2016, so it's safe to assume the batteries are done.

I usually buy batteries locally to save on shipping. A nearby shop sells 12V 7.5Ah batteries for $22. The original set are 9Ah but, at nearly twice the cost, I'm comfortable with the lower run time of the cheaper ones.

I'll report back once I've changed them out.

Update 4-May-2023: We had another couple of power blips this week and as expected, the UPS died immediately. I changed the batteries today, tested with a small load, and it does seem to hold on.

Right until I shut down, the UPS was reporting 100% charge and an estimated run time of 70min:



With the new batteries, the estimated run time is 48 minutes. This is not unexpected, since the replacement batteries are only rated 7.5Ah compared to the original 9Ah, but still seems optimistic.

We can calculate an upper bound for run time based on load and rated capacity. At last check, the load was 17% or 150 watts. A 12 volt, 7.5 amp-hour battery holds 90 watt-hours. Dividing the capacity by the load yields a 36-minute run time, 25% less than the controller's estimate. The real run time will be even worse due to inefficiencies in the UPS. That being said, most outages here last less than 5 minutes.

It's important to note the rated capacities are for new batteries - older batteries may not hold as much (or any) charge, so you should question uncalibrated UPS run time estimates. Smaller UPSes like mine rarely account for changing battery chemistry, whereas more sophisticated ones regularly switch off of mains and update their capacity model as time goes by. 

Since I bought this UPS, its retail price has risen from $175 to $315, and I don't see 1500VA units from any manufacturer below $260. For $44, I've extended its life by 3-6 years and have $200+ left over to buy RAM 👿

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