In short
- Start from a genuine full charge, with the cells balanced at the top.
- Discharge steadily down to your usual cut-off and count the energy delivered by the battery itself.
- Compare with the usable energy you expect, not the gross label figure, and repeat once a year.
A capacity test answers one question: how many kilowatt-hours does the battery deliver between full and your cut-off today. It needs no special equipment if you can read the energy that leaves the battery.
1A real full charge
Charge until the battery reports full and the charge current has tapered off. Leave it there an hour or two so balancing can finish. A pack that starts at an estimated 100 % with one cell lagging will give a smaller number.
2A steady discharge
Use the house in the evening, or a known load, and let the battery discharge without solar input. Steady loads give cleaner results than a few huge peaks. Stop at the cut-off you normally use, for example the inverter low limit.
3Count the energy out
Use the energy measured on the battery side, not the inverter output, which includes conversion losses. Battery current multiplied by voltage, added up over time, is what the battery delivered.
4Read the result honestly
Compare with the usable energy you expect between full and your cut-off, not the gross figure on the label. Temperature matters: a cold battery delivers less. A test at 10 °C and one at 25 °C will not match.
Common traps
Starting from a charge level estimated by the BMS rather than a real full charge, mixing solar input into the test, and reading inverter output instead of battery energy are the three classic ones.
Let the box do the counting
BSLBATT Monitor reads the battery through its CAN or RS485 port and records voltage, current and charge every few seconds. The dashboard shows the energy given back over the night, and the history in the free Cloud lets you repeat the same test next year and compare.
Common questions
Is a lower result than the label a fault?
Not necessarily. Labels give gross capacity in ideal conditions. Compare with the usable energy at your cut-off and temperature.
How often should I test?
Once a year is plenty, ideally in similar weather each time so the results compare.
Does BSLBATT Monitor run the test for me?
It does not change any setting or switch loads. It measures and records the energy, so you only have to start from a full charge and let the night run.
