In short. Battery voltage is not battery capacity. A bank can read correctly and still fail under load. Reliable UPS support needs the right battery type, sizing for the required runtime, temperature control and regular testing and trending.
Battery types
| Type | Notes |
|---|---|
| VRLA (sealed lead-acid) | Common, low maintenance, sensitive to heat |
| Flooded lead-acid | Long life, needs room and maintenance |
| Lithium-ion | High energy density, long life, needs proper management systems |
Sizing for runtime
Runtime depends on load, battery capacity, efficiency and end voltage. Discharge rates change capacity. Size with the maker’s discharge tables and add margin for ageing and temperature. Choose runtime to cover generator start and transfer plus a margin.
Why voltage misleads
Open-circuit voltage stays near normal until a battery is nearly spent. Capacity shows only under load.
Monitoring chain
Reading → trend → deviation → diagnosis → intervention → verification.
- Record float voltage, temperature and string current.
- Measure internal resistance or conductance per cell or block and trend it.
- Perform load or discharge tests as the maker advises.
- Inspect for swelling, leaks and terminal corrosion.
- Replace weak blocks or strings before failure.
Temperature
Heat shortens life significantly for lead-acid batteries. Keep the room cool, away from UPS exhaust. See battery systems service and preventive maintenance.
Illustrative example
A UPS reports fine battery voltage, but a load test shows runtime of two minutes instead of ten. Internal resistance trends showed the decline months earlier. A planned replacement before the monsoon outages avoids a failure.
Runtime sizing example
Suppose a load of 20 kW must be supported for 10 minutes while a generator starts and transfers, with a UPS efficiency of 94 percent and a battery end-of-discharge voltage defined by the maker.
Battery power required ≈ Load ÷ Inverter efficiency
20 ÷ 0.94 ≈ 21.3 kW from the battery. The battery maker’s table gives the power per cell to a given end voltage at 10 minutes. Divide total power by the number of cells to find the per-cell power, then choose a battery size that meets it with margin for ageing and temperature. Capacity shown at long discharge times does not represent short-time capability, so use the correct table.
Ageing and replacement criteria
| Indicator | Meaning |
|---|---|
| Capacity measured at load test falling below a set percentage of rated | End of useful life, per maker guidance |
| Internal resistance rising markedly from baseline | Degradation |
| Swelling, leaks or high temperature | Immediate inspection |
| Wide spread between block voltages in a string | Weak blocks |
Battery room and safety
- Ventilation as required for the battery type.
- Controlled temperature.
- Spill containment where flooded cells are used.
- Correct PPE and tools; insulated tools for live DC work.
- Clear labelling and emergency isolation.
Monitoring options
Basic monitoring logs string voltage, current and temperature. More advanced systems measure each block’s voltage and internal resistance, and alarm when they deviate. For critical sites the investment often pays back through early warning.
Records
Keep installation date, commissioning test result, every periodic test, and replacements by serial number. These records support warranty claims and replacement planning.
The master engineering question
If the utility fails now, will this battery actually deliver the runtime we assume?
What to send us
Send battery details and test history. Send your application details.
Questions engineers ask
How often to test?
Follow the maker; trend continuously where possible.
Replace the whole string?
Often yes; mixing old and new blocks harms performance.
Does room temperature matter?
Yes, strongly.
Is lithium better?
Different trade-offs; depends on cost, space and management.
Values are illustrative. Follow maker guidance.