Power equipment · Batteries
UPS batteries: the consumable in the power chain
The battery is the part of a UPS with a service life measured in years rather than decades, and replacing it is the expected maintenance event rather than a failure. Most of what goes wrong is decided by room temperature, by what was ordered, and by whether anyone measured the string before it was needed.
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What to check before you specify one
- The exact UPS model, because a battery that fits the family may not fit the variant
- String voltage and the number of blocks the unit actually requires, not the quantity in one carton
- Block voltage and amp-hour capacity per cell, which are different figures from the string total
- Terminal type, because the wrong terminal will not land on the existing harness
- Physical dimensions and the route into the cabinet or tray
- Chemistry, and whether the UPS charger supports it: a lithium pack is not a drop-in for VRLA unless the manufacturer says so
- Date of manufacture, so a string is not built from stock of mixed age
- Whether the quoted runtime came from the manufacturer chart at your actual load
Everything else in a UPS is designed to last the life of the installation. The battery is not. It is a consumable with a service life measured in years, it degrades whether or not it is ever called on, and the moment it is called on is the moment its condition stops being theoretical. Planning for that is ordinary asset management. Discovering it during an outage is not.
A string is only as good as its weakest block
A UPS battery is rarely one battery. It is a string of blocks in series, and the string carries the load only as long as every block in it holds up. One failing block pulls down the whole string, which is why a system with plenty of nominal capacity can still drop the load.
This has a practical consequence that surprises people: you cannot judge a battery system by its age alone, and you cannot judge it by a single reading at the terminals. The measurement has to reach every block. IEEE 1188 is the reference for how and how often, and it covers replacement criteria as well as testing, which is the part most maintenance plans leave vague.
Impedance readings tell you where you are; a capacity test tells you what you have
Two kinds of measurement matter and they answer different questions.
Impedance or conductance readings are quick, can be taken with the string in service, and are meaningful mainly as a trend against the baseline taken when the string was commissioned. A block drifting away from its neighbours is the signal. An absolute number without a baseline is much weaker evidence.
A capacity or discharge test is the only measurement that tells you how much runtime the string will actually deliver. It takes the string out of its protective role for the duration, which is why many sites avoid it, and why the ones that do avoid it should be honest that their runtime figure is an expectation rather than a measurement.
Temperature is the variable nobody controls on purpose
VRLA cells are rated at a reference temperature, and their service life falls away as the sustained temperature rises above it. Manufacturers publish derating curves for their own cells, and those curves govern. The important point for a facility is that this is not a small effect and it is usually accidental: a battery cabinet placed in the warmest corner of a plant room, or a room run warmer to save on cooling, will consume battery life faster than the design assumed and the first evidence will be a string that does not make its rated runtime.
If the room is warm, the honest response is to shorten the replacement interval rather than to hope.
Order the string, not the carton
This is where most battery purchases go wrong, and it is worth being blunt about it.
A listing for a replacement battery may describe a single block, a matched pair, or a complete set for a named UPS. The unit may need four, or twenty, or several hundred. The quantity that arrives is whatever the line item said, not whatever the machine requires, and a set that is half a string will not run the load.
So the order needs the exact model of the UPS, the number of blocks that model requires, the block voltage and amp-hour capacity, the terminal type and the physical dimensions. Confirm the quantity against the manufacturer documentation for the installed unit rather than against the product title. Catalogues that list UPS replacement batteries by UPS model, rather than by block size alone, make that cross-check quicker, but the manufacturer documentation still decides. If a seller cannot tell you how many blocks the unit takes, that is the answer to whether they know what they are selling.
Match the blocks in a string
Blocks in one string should be of the same chemistry, the same capacity and, as closely as the supply allows, the same age. Mixing a new block into an aged string does not rescue the string; the old blocks continue to set its limit, and the new block is aged prematurely by the company it keeps. Replacement is normally a whole-string operation for exactly this reason. On a large three-phase system the change is planned as a UPS battery replacement with the UPS on maintenance bypass or a temporary string, so the load is never without protection for longer than the work requires.
Recording the date of manufacture at installation is what makes the next replacement a scheduled event instead of a surprise.
Lithium is a different decision, not a drop-in
Lithium iron phosphate packs are now common in UPS applications and they change the arithmetic: longer service life, better tolerance of warm rooms, lighter, smaller, and a higher price at purchase. What they are not is a direct substitute. The charger, the battery management and the UPS firmware all have to support the chemistry, and the manufacturer has to say so. Fitting a lithium pack to a UPS designed for VRLA because it physically fits is not an upgrade.
Installation and disposal are regulated, not optional
Battery installations fall under the Canadian Electrical Code, Part I, currently CSA C22.1:24, along with whatever the authority having jurisdiction requires for the room. Recent editions have revised requirements around energy storage specifically, so a battery room designed against an older edition is worth rechecking rather than assumed to still comply.
Lead-acid batteries are recyclable and are handled through established recovery channels. The practical point for a facility is to keep the paperwork: a recycling receipt for the removed string is the evidence that the old batteries left the site properly, and it belongs in the same file as the readings taken when the new string went in.
Maintenance
| Task | Interval |
|---|---|
| Visual check: casing distortion, leakage, corrosion at terminals, and the room temperature the string is actually living at | Quarterly, or with any site visit |
| Impedance or conductance reading on every block, compared against the commissioning baseline for that string | Semi-annual, and after any discharge event |
| Connection torque check on the manufacturer schedule, with the readings recorded | Per manufacturer |
| Discharge or capacity test where the site can accept one, planned against a documented procedure | Per IEEE 1188 test schedule and site risk |
| Review of age against expected service life, with the replacement budgeted before the string reaches it | Annual |
Intervals are a starting point for a maintenance plan. The manufacturer documentation for the installed model governs, and a site's operating conditions can shorten any of them.