Fault-finding
The battery runs flat faster than it used to
Compare the load against the bank rather than assuming the battery failed. Load added to the backed-up circuit, a bank sized against an average outage instead of the worst one, heat ageing the cells, or a charge setting that never fills it completely — in that order of likelihood.
Last reviewed
Work out what the circuit actually draws
Meter the backed-up circuit rather than adding up nameplate ratings. Nameplates overstate substantially and a real measurement usually surprises people. Divide the bank's usable capacity by that measured draw and you have the honest runtime — which is often exactly what you are experiencing, meaning nothing is broken.
Look for load that was added later
A pump, a second fridge, an air conditioner, or a cluster of chargers that quietly joined the essential circuit. This is the single most common cause and it rarely happens deliberately — an electrician adds a socket to the nearest available circuit, and the backed-up side gains a load nobody accounted for.
Check whether the bank is being filled
A bank that never reaches full charge behaves exactly like a smaller bank. Causes are a charge current limit set low, insufficient generation on short days, or absorption voltage below what the cells need. Watch a full sunny day and confirm the bank actually reaches its target voltage and holds it.
Then measure real capacity
With load and charging both verified, a controlled full discharge measured against the commissioning figure tells you how much of the original bank is left. Heat is what usually took it — a bank in a hot, sealed position ages years faster than the datasheet implies, and no amount of configuration recovers that.
Why does my solar battery drain so quickly?
How long should my battery actually last through an outage?
Usable capacity divided by measured load, and that is a number specific to your installation rather than a general figure. A typical essential circuit of a few hundred watts runs comfortably through a normal evening cut on a modest lithium bank. Put an air conditioner on the same circuit and the same bank lasts minutes.
Does deep discharging damage the battery?
Yes, and it is cumulative. Lead-acid is damaged sharply by repeated deep discharge; lithium tolerates it better but still loses cycle life. This is why the cut-off setting exists and why raising it to gain runtime is a false economy — you are trading years of bank life for hours of evening.
Would a bigger battery fix it?
Only if the diagnosis says the bank is genuinely undersized for the load you want backed up. If load crept onto the circuit, moving it off is free and a bigger bank is not. Measure first: buying capacity to cover a load that should never have been on the backed-up side is the most expensive way to solve that problem.
Rather have it done?
The inverter and battery bank are where a solar system is won or lost: sizing, ventilation, cabling and protection decide whether the bank lasts eight years or three. A lithium bank with a hybrid inverter installed properly starts near PKR 320,000 for a household essential load.
Solar inverter and battery bank setup in Islamabad — prices and scope