In short. Battery autonomy is the time a UPS can carry its load after the supply fails. Choose it from what the battery must bridge: generator start and transfer, a failed first start, and, where needed, an orderly shutdown. Quoted runtime is a new-battery figure at specific conditions, so apply allowances for age, temperature, discharge rate and end voltage.
What autonomy must cover
- Time for the generator to start, reach rated voltage and frequency, and transfer.
- Time for a second start attempt if the first fails.
- Time for operators to respond to alarms.
- Time for an orderly shutdown of IT or process equipment if no source returns.
Where a generator is present, minutes are usually enough. Where there is none, autonomy must cover the entire outage or a shutdown.
Basic energy estimate
Example, illustrative only: a 100 kW load, 10 minutes autonomy, inverter efficiency 0.95, usable depth of discharge 0.8 and an ageing and temperature factor of 0.8.
Energy = 100 × (10 ÷ 60) ÷ (0.95 × 0.8 × 0.8) = 16.7 ÷ 0.608 ≈ 27 kWh. Manufacturers size by discharge tables at a constant power rate to an end voltage, which is more accurate than this estimate, so use their data for the final selection.
Why real runtime is shorter than catalogue runtime
| Factor | Effect |
|---|---|
| Battery age | Capacity falls over life; a common end-of-life point is about 80 per cent of rated |
| Temperature | Cold reduces available capacity; hot shortens life |
| High discharge rate | Short autonomy at high power gives less usable energy per kWh |
| End-of-discharge voltage | A higher cutoff cuts the usable energy |
| Load growth | More load, less time |
| Weak cells | One bad cell can reduce a string’s performance |
Battery technology considerations
- Valve-regulated lead-acid: common and low cost; shorter design life, temperature-sensitive.
- Lithium-ion: longer life, higher cost and requires battery management and fire-safety design.
- Vented lead-acid or nickel-cadmium: long life for large installations, with maintenance requirements.
Select by life-cycle cost, space, temperature and the site’s safety rules.
String arrangement
Use more than one string in parallel where the design allows, so a single string fault does not remove all autonomy. Provide isolation so one string can be maintained while others remain online. See battery monitoring.
Verification
- A discharge test at site acceptance to confirm autonomy at actual load.
- Periodic capacity or impedance checks, and online monitoring if available.
- Replacement before measured capacity falls below the design margin.
Runtime is not nameplate times load
Battery capacity in Ah at a 10 hour rate is not available at a 10 minute discharge. High discharge rates reduce usable capacity. Battery voltage is not capacity: a bank can sit at correct float voltage and still fail on load.
Required battery power per cell = (load kW ÷ inverter efficiency) ÷ number of cells, for the specified time to end-voltage
Sizing steps (illustrative)
- Load: 200 kW, inverter efficiency 95 percent: DC power 210.5 kW.
- Autonomy target: 10 minutes.
- Choose battery from the maker’s discharge table (watts per cell at 10 minutes to end voltage).
- Apply ageing factor 1.25 and temperature correction.
- Check recharge time and charger rating.
Factors that shrink runtime
| Factor | Effect |
|---|---|
| Age (end of life at 80 percent capacity) | Reduces runtime by about 20 percent |
| Low temperature | Reduces capacity |
| High temperature | Shortens life, roughly halving life for every 8 to 10 degrees above 25 °C |
| Weak cells in string | One weak cell limits string |
Testing and monitoring
- Periodic load or discharge test against rated autonomy.
- Internal resistance or impedance trending.
- Battery monitoring systems that watch each unit.
Hyderabad’s summer heat shortens battery life if the battery room is not cooled. Follow the manufacturer’s guidance and applicable standards. See battery systems.
Common mistakes
- Sizing from the catalogue minutes without ageing and temperature allowances.
- Choosing autonomy that equals the generator start time with no margin for a failed start.
- Ignoring load growth after commissioning.
- Skipping discharge tests, so true capacity is unknown.
What to send us
Send your load, the generator start time, the temperature range and the autonomy you require. Send your application details. See also UPS autonomy guide.
Questions engineers ask
How many minutes of UPS autonomy do I need?
Enough to cover generator start, transfer and a failed first attempt, plus margin. Where no generator exists, enough to ride through or shut down in order.
Why does my UPS give less time than the catalogue says?
Catalogue times are for new batteries at set conditions. Ageing, temperature, load and cutoff voltage all reduce the real figure.
How often should batteries be replaced?
By measured health and manufacturer guidance. Many VRLA installations replace in a window of several years, depending on temperature and use.
Is lithium-ion better than lead-acid for UPS?
It can offer longer life and smaller size, but cost and safety design differ. Compare life-cycle cost for your site.
Does adding load affect autonomy?
Yes. Autonomy falls faster than linearly at higher discharge rates, so recheck after load changes.
Figures here are illustrative. Use manufacturer discharge data for final sizing.