How to Store a UPS and Spare UPS Batteries

People store UPS units for ordinary reasons: a move, an office closing for a season, a spare unit for emergencies, a cabin used only in summer. The electronics tolerate storage well. The lead-acid battery does not, unless you help it. This page covers why, how fast the clock runs, and how to bring a stored unit back without surprises.

Why storage hurts lead-acid batteries

A lead-acid battery that is not being charged slowly discharges itself through internal chemical reactions. That is harmless at first. The damage starts when the battery spends long periods partly discharged: lead sulfate, a normal product of discharge, grows into larger, harder crystals that the charger later struggles to convert back. Capacity lost this way is largely permanent. This is called sulfation.

So the rules of storage are really about two things: start from full, and do not let the state of charge sink too far before topping it up again.

The self-discharge clock, worked out

VRLA AGM batteries typically self-discharge roughly 2 to 3% of capacity per month at 68 to 77 °F (20 to 25 °C). The rate rises with temperature, approximately doubling for every 10 °C (18 °F) increase. A common guideline is to recharge before the battery loses about a quarter of its charge, which for a 12 V block corresponds to a resting voltage around 12.4 V (the exact voltage-to-charge relationship varies a little between batteries).

The arithmetic for 77 °F: 25 / 3 = 8.3 months. The practical intervals are shorter than the calculated ones on purpose. Self-discharge rates vary by battery and age (older batteries self-discharge faster), and you rarely store a battery that was exactly 100% charged. Battery makers' own storage guidance generally lands in the same range: longer intervals for cool storage, around 3 months for warm.

How to put a UPS into storage

1. Recharge fully. Leave it plugged in and switched on (outputs can be off on many models) for the recharge time in the manual, at least 8 hours and preferably a full day.
2. Shut it down properly. Turn the UPS off with its power button and unplug it from the wall.
3. Disconnect the battery if possible. Many units have an internal battery connector or a battery door that allows disconnection; some ship with it disconnected. This eliminates any small drain from the electronics. Follow the manual and do not open sealed units.
4. Label it. Write the storage date and the next recharge date on a piece of tape on the UPS.
5. Choose the spot. Cool, dry, indoors, off concrete floors only if damp is a concern, away from heaters and sunlight. Ideally between 50 and 77 °F (10 and 25 °C).
6. Store it upright. VRLA batteries can technically operate in many orientations, but upright storage is simplest and what most manuals assume.

Seasonal sites: cabins, boats and summer offices

A UPS at a property used only part of the year faces the worst storage pattern: months unplugged, often in an unheated or unconditioned building. Three approaches work, in order of preference:

• Leave it on utility power. If the building keeps power through the off-season, a UPS left plugged in keeps its battery on float charge. The trade-off is standby consumption, typically a few watts (see UPS efficiency and energy cost), and exposure to summer heat.
• Take it home. A desktop UPS is portable. Storing it in your home's living space and recharging it on your schedule removes the temperature risk.
• Disconnect and accept a refresh visit. If neither works, store it fully charged with the battery disconnected and plan to recharge on each visit. If the gap exceeds about 6 months, expect to replace the battery sooner than you otherwise would.

Whatever you choose, write the plan on the UPS label. Seasonal equipment tends to be handled by whoever arrives first, and "who charged it last" is easily forgotten.

Spare batteries on the shelf

A loose spare battery ages the same way, with two extra concerns. First, its terminals are exposed: store it where nothing metal can bridge them, ideally with terminal covers or tape. Second, you probably have no charger for it other than the UPS itself. A smart charger designed for sealed lead-acid (AGM) batteries can maintain a spare, but it must be the correct type and voltage; a car-battery charger with a high or "desulfation" mode can overcharge a small AGM block.

Before buying a spare, consider the date code. A battery manufactured long before you need it has already consumed part of its life. For most people, buying the replacement when the old battery reaches about 3 to 4 years of age, or when a test says so, gives better value than keeping a spare for years. See when to replace a UPS battery.

Temperature extremes in storage

Bringing a stored UPS back into service

1. Inspect. Look for a bulging battery case, cracks, leaks or white crust at the terminals, and smell for a sour, sulfur-like odor. Any of these: stop and see swollen UPS battery.
2. Acclimate. If it came from a cold place, let it reach room temperature for a few hours so condensation can evaporate before you power it.
3. Reconnect the battery if you disconnected it, following the manual.
4. Charge without load. Plug it in and let it charge fully before connecting equipment. Warmth during charging is normal; heat you cannot comfortably touch is not.
5. Run a self-test. Many units test automatically at startup or on a schedule; you can usually trigger one from the front panel or software. A failed test after a full charge suggests the battery has lost capacity.
6. Verify runtime with a non-critical load if the unit will protect anything important. See self-test and runtime calibration.
7. Reset reminders. Check the UPS date and battery-install settings in software if the model uses them.

If the UPS will not charge or shows a battery fault, UPS not charging walks through the likely causes.

Lithium UPS storage

Lithium iron phosphate batteries self-discharge much more slowly than lead-acid and do not sulfate, so they tolerate storage far better. General lithium-ion guidance is to store at a partial state of charge, often somewhere around 30 to 60%, in a cool place, because sitting at 100% in heat accelerates aging. However, lithium UPS units have their own battery management systems and some include a shipping or storage mode, so follow the manufacturer's instructions, which take precedence over general advice. Do not let any lithium battery sit fully depleted for long, since the BMS itself draws a small current. For more on how lithium UPS units differ, see lithium UPS pros and cons.