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How to check a used UPS
A UPS is a battery in a box with some electronics for company, and on the used market the battery is almost always the part that has already died. Two facts decide the purchase, and neither of them is the price on the listing: how old the cells are, and what shape the waveform is.
The short answer
Price a used UPS as the chassis plus a new set of cells, because that is what you are buying. If the replacement pack costs more than half of a new unit — and on small models it often does — buy the new unit.
Get these first
Network UPS ToolsFreeLinux / Mac / WinTalks to a UPS over USB: battery charge, load, runtime estimate and self-test results.HWiNFO64FreeWindowsEvery sensor on the machine, plus CSV logging. The one tool to install first.PCPartPickerFreeWebCatches clearance and compatibility problems — cooler height, radiator mounts, case and board fit — before you pay.
The five-minute version, then the rest
- Find the date stamp on the cells, not on the unit. Sealed lead-acid batteries last roughly three to five years whether they are cycled or not, and heat shortens that considerably. A five-year-old UPS on original cells is a chassis with a dead weight inside it.
- Price the replacement pack before you make an offer. For small consumer units the cells cost a large fraction of a whole new UPS. That number, not the asking price, is what decides whether this is a good buy — so look it up while you are still standing there.
- Check the waveform against your power supply. Almost every modern PC supply uses active power factor correction, and active-PFC supplies can refuse to run — or drop out, or make an ugly noise — on the stepped “simulated sine wave” output that cheap line-interactive units produce. If the machine has a modern PSU you want pure sine wave. A simulated-sine unit can look perfect on mains and then fail at the exact moment it exists for.
- Read both ratings. UPSes are sold in VA, loads are measured in watts, and the watt figure is the one that constrains you. It is always the lower number, often around 60% of the VA figure. Size it against your measured wall draw rather than the badge on your power supply.
- Run the self-test, then run the real one. Plug in a load you do not mind losing, pull the wall plug, and time how long it genuinely holds. Compare against the runtime the manufacturer publishes for that wattage. Seconds where the chart says minutes means the cells are finished.
- Watch the transfer itself. The machine should not so much as flicker when mains drops. A reboot at the moment of transfer means either a changeover too slow for the power supply, or a waveform it will not accept — and both are reasons this unit will not do the one job you want it for.
- Look for swollen cells, corroded terminals and a bulging case. Lead-acid cells swell as they fail, and a swollen pack can be genuinely difficult to get back out of the chassis. Any smell of rotten eggs means a cell is venting; stop there.
- Test every outlet and the data link. Most units split their sockets into battery-backed and surge-only, and the labels rub off. Confirm which is which, then check the USB or serial cable is included and that the unit still talks to a computer — Network UPS Tools will tell you what it reports.
The one nobody checks
Waveform. A cheap line-interactive UPS outputs a stepped approximation of a sine wave, and a modern active-PFC power supply may simply refuse it — the machine drops out the instant mains does, which is the precise opposite of the point. Worse, it can work on one machine and not on another, so “it worked at my place” proves nothing about yours. If the listing does not say pure sine wave, assume it is not.
Note
VA and watts are not the same rating and the gap is not small. A unit sold as 1000VA may only be rated for around 600W. Measure what your machine actually pulls at the wall under load, then size against the watt figure with headroom — not against the number printed largest on the box.
What to measure
Higher is better▲Higher is betterLower is better▼Lower is betterMust be zero∅Must be zeroMust match the spec=Must match the spec
What a used UPS is really worth
| Metric | Which way | What it tells you | Healthy looks like |
|---|---|---|---|
| Date stamp on the cells | Lower is better | The real age of the only consumable in the box. | Under about two years. Three to five is the whole service life, and a hot room takes the low end of that. |
| Measured runtime under your load | Higher is better | Whether the cells hold any usable charge. | Close to the manufacturer's published figure for that wattage. Far short means the pack is spent. |
| Waveform on battery | Must match the spec | Whether a modern PC power supply will accept it. | Pure sine wave for anything with an active-PFC supply, which in practice means every current PC. |
| Watt rating vs measured draw | Higher is better | The rating that actually limits what you can plug in. | Comfortably above your real wall draw. The VA number on the box is not this number. |
| Behaviour on transfer | Must be zero | What happens at the moment the UPS is for. | No flicker, no reboot, nothing beyond the normal alarm beep. |
| Cell and terminal condition | Must be zero | Swelling, leakage and corrosion. | None. A swollen pack is a cost, a disposal problem, and sometimes stuck in the chassis. |
| Battery-backed vs surge-only outlets | Must match the spec | Which sockets do anything at all in a cut. | Identified, and enough of them for what you actually need to keep alive. This catches people out constantly. |
| Replacement pack cost | Lower is better | The number that decides the whole purchase. | A small fraction of a new unit. At half, buy new and keep the warranty. |
Walk away if
Walk away if
- Any swelling, leaking or corrosion at the cells.
- A load that reboots when the unit transfers to battery.
- A simulated-sine unit being sold for a modern PC.
- Seconds of runtime where the chart promises minutes.
- A replacement pack costing more than half of a new UPS.
- A smell of rotten eggs or of burning — a venting lead-acid cell is not something to live with.
- A unit the seller has never actually run on battery.
What each fault is worth
Bring the measurement, not the opinion. Name the fault, name the fix, name what the fix costs — that is a solvable problem with two acceptable answers, which is what makes it work.
| What you found | What it actually costs | What to ask for |
|---|---|---|
| Cells over three years old | A replacement pack, immediately or very soon. | The full pack cost off, at minimum. Look it up before you offer. |
| Simulated sine wave, modern PC | It may not work for your machine at all. | Only worth chassis money to you — or nothing. |
| Missing data cable or software | No graceful shutdown, no monitoring. | A small amount; most are a standard USB or serial lead. |
| Unknown or untested runtime | You are buying a promise. | Test it there, or price it as if the cells are dead. |
| Swollen pack | Cost, labour, and a disposal problem. | Walk away. This is not a discount situation. |
Ready to go looking?
Take the checklist with you, then browse what is listed. Everything on this page works just as well the other way round when you are the one selling.