Why your UPS backup keeps getting shorter
On the first day of load-shedding your UPS runs for hours. A few days later the same fans go off within the hour. Your battery is probably not broken. It is not getting enough time on the grid to refill.
The recharge gap
Every cut takes energy out of the battery. Every hour of grid puts some back. If the cuts take out more than the grid puts back, the battery starts each cut a little emptier than the last, until it settles at a low level. That shortfall is the recharge gap.
An example
A typical home: a 1 kVA UPS with a 200 Ah tubular battery, running fans, lights, a router, phones and a fridge, with load-shedding of 6 h a day, 1 h every 4 h.
- A fully charged battery would give 3.9 h.
- But the UPS charger puts back only about 144 W, so in the 3 h between cuts the battery cannot refill.
- After 4 days it settles: each cut starts at about 38% charge, and the backup is 47 min.
- Everything stays on for only 4.5 h of the 6 h of daily cuts.
The box promised hours. The real, daily answer is 47 min.
Why a bigger battery may not help
A bigger battery holds more, but it refills at the same speed: the charger and the battery's safe charging rate set that. In this example a bigger battery still gives 47 min once settled. It only helps if the grid is on long enough to fill it.
What does help
- Solar panels. They charge the battery during the day, cuts or not. Adding 1 kW here gives 3.1 h per cut and covers 6 h of 6 h.
- A lithium battery. It loses less energy when charging and can be charged faster. Here it gives 3.8 h per cut. Your UPS or inverter must support lithium; many older UPS chargers are made only for lead-acid batteries.
- A faster charger. More charging current refills more between cuts, if the battery can safely take it.
- Less load during cuts. Every appliance you switch off leaves more for the next cut.
Figures are from LoadReserve's calculation with typical values; see how it works.
Last updated .