When should UPS batteries be replaced? Three measurements decide
A UPS that drops the load the moment the mains fails is almost always a battery problem, not a dead unit. Whether to replace is not a question of how many years old the battery is — it is a question of three measurements.
Three battery types, three trade-offs
| Type | Design life (ideal) | Advantages | Drawbacks |
|---|---|---|---|
| Valve-regulated sealed lead-acid (VRLA) | 3–5 years | Lowest cost, widest availability | Heavier, needs space, sensitive to heat |
| Gel (OPzV) | 5–8 years | Heat tolerant, long cycle life, maintenance free | Higher price, stricter charging requirements |
| Lithium iron phosphate (LiFePO₄) | 8 years and beyond | Light, compact, longest cycle life | Higher upfront cost, needs BMS protection |
Every one of these chemistries is available through our battery range, and we select the type around the site rather than the other way round.
Heat is the real killer
Battery life and ambient temperature have an exponential relationship: for every 10 °C rise in ambient temperature, the chemical ageing rate of a lead-acid battery roughly doubles.
That means a battery rated for 5 years may genuinely last 5 years in a 25 °C machine room — and little more than 2 years in a Hong Kong room that sits at 32–35 °C in summer, or in a communications cabinet with no air conditioning. Many complaints that "these batteries failed far too soon" trace back to cooling, not to quality.
Deciding whether to replace: three objective indicators
- Cell voltage imbalance — if some cells in a string sit noticeably below the others, individual degradation has already begun.
- Internal resistance — a rising internal resistance is the most sensitive early indicator of capacity loss; it shows up well before the usable capacity visibly drops.
- Measured discharge capacity — the most direct test: once measured capacity falls below 80% of the rated value, the string should be replaced.
Age alone is not a reliable guide — batteries of the same age can be in very different condition given different temperatures and discharge histories. A periodic internal-resistance and capacity test is the cheapest habit you can adopt, because it lets you replace a string before it fails rather than discovering the problem on the night of an outage.
Why the string is replaced as a whole
A battery string works in series, and the usable capacity of the whole string is set by the weakest block. When old and new blocks are mixed, the new ones are forced to carry more of the charge and discharge stress, their internal resistance climbs faster, and the string is typically unbalanced again within months.
That is why we always recommend replacing the whole string, from the same batch, of the same model and of the same age. It is not about selling a few more batteries — it is about not paying twice.
One more thing: old batteries must be recycled
Lead-acid batteries contain heavy metals and electrolyte and must not go out with general waste. We collect and recycle them as part of our battery replacement work.