A backup power system is only as good as the batteries inside it. Panels, inverters and generators get all the attention, but when the lights go out and something does not come on, the batteries are almost always the reason.
This guide explains why batteries fail, how to test them properly, and when replacement is the right call versus when it can wait.
How batteries degrade
All battery chemistries degrade. The question is how fast, and what accelerates it.
- Cycle count: every charge and discharge cycle uses up a portion of the battery's life. Lead-acid batteries typically manage a few hundred to a couple of thousand cycles depending on depth of discharge. Lithium (LiFePO4) usually manages several thousand.
- Depth of discharge: draining a lead-acid battery below 50% regularly shortens its life significantly. Lithium tolerates deeper discharge but still ages faster when pushed hard.
- Temperature: high temperatures age batteries faster. A battery room that regularly exceeds 30°C will kill batteries years earlier than necessary.
- Charging regime: undercharging leads to sulfation on lead-acid. Overcharging boils off electrolyte. Both shorten life.
- Age: even a battery sitting on a shelf ages. A "new" battery that has been in storage for two years is not new.
Signs a battery bank is failing
Common warnings, in the order they tend to appear:
- Runtime drops noticeably — a system that used to give 6 hours now gives 2.
- The inverter throws a low-voltage fault more often.
- Batteries charge faster than they used to — because they are holding less.
- One or more cells get noticeably hotter than the rest during charging.
- Visible swelling on any cell.
- Electrolyte level dropping unusually fast (flooded lead-acid).
Any of these means a test is overdue, not optional.
How to test a battery bank properly
There are three levels of testing. Each answers a different question.
1. Voltage check (quick, limited)
Measuring open-circuit voltage tells you roughly what state of charge a battery is in. It does not tell you about capacity. A battery can show 12.6 V and still collapse under load.
2. Load test (the real answer)
A load test draws a known current from the battery for a period and measures how the voltage responds. This is what actually reveals whether a battery can deliver.
For a battery bank, testing individual blocks or cells under load is essential, because a bank is only as strong as its weakest cell. A single degraded block can drag the whole bank's runtime down.
3. Capacity test (the complete answer)
A full capacity test discharges the bank at a controlled rate until it reaches end-of-discharge voltage, and measures the total energy delivered. This is the most accurate test and the one most often skipped because it means taking the system offline.
For critical installations, capacity testing on a scheduled basis (annually, or every six months for high-value sites) is what separates a reliable backup from a hope.
When to replace, and when to wait
Guidelines, not rules:
- If a load test shows the bank delivering less than 80% of rated capacity, plan replacement within the year.
- If it shows less than 60%, replace now — the system is no longer meeting its design requirement.
- If a single block is dramatically weaker than the rest, replace that block (matching type, age and chemistry where possible).
- If the bank is visibly swollen, leaking or overheating, take it offline and replace immediately. This is a safety issue, not a performance issue.
Age alone is not a reason to replace. A well-maintained bank at year eight may outperform a neglected bank at year three.
Making batteries last longer
Things that genuinely extend battery life:
- Keeping the battery room ventilated and below 30°C wherever practical.
- Programming the charger correctly for the chemistry and bank size.
- Avoiding regular deep discharge on lead-acid — set the inverter cut-off appropriately.
- Testing on a schedule, not only when something breaks.
- Keeping terminals clean and torqued correctly.
- Replacing failed cells promptly instead of letting them drag the rest down.
Talk to us about battery health
We test, service and replace battery banks for UPS, solar and standby systems. If your backup has not been tested in the last twelve months, it is worth a visit.