How to test the capacity of your battery
To test the storage capacity of a rechargeable battery like NiMH you need a battery charger / Analyzer like the La Crosee BC 900 / 700 Series or the Maha BC9009 and they have a mode to test the capacity of the batteries.
How do you measure the capacity of the battery
The only way to find out the true capacity is to fully discharge the cell at a fixed current, and measure the time it took and the capacity of discharge in (mAh) that is why the test modes in the charger take a lot of time.
For La Crosse BC 900 / 9009 use the “test mode” to determine the capacity of your battery, During a ‘Test’ operation in BC-9009, the battery is first fully charged and then the discharge process start, the capacity (mAh) reading is continuously updated during the discharge cycle. Once the discharge cycle is done, the capacity reading is frozen and shown on the LCD display.
Do note that using the Test mode to test the batteries can take a long time as the charger will first charger the battery and then start to discharge the same until its capacity is depleted and this will show you the capacity of the battery.
Can I instantly determine the capacity of a battery?
No battery testers in the world can instantly determine the amount of charge stored in a rechargeable battery. The best they can do is to measure the battery voltage and ESTIMATE the charge.
Estimate the battery status by checking the voltage
This is a very quick way to estimate the status of a rechargeable cell “AA” / “AAA” 1.2v battery by using a simple multimeter and checking the terminal voltage of the battery, do note that this is not an accurate way to measure the battery capacity as it just to give you a rough idea about status for a NiMH rechargeable battery.
The voltage testing with multimeter should be done when the battery is in resting stage and its voltage has stabilized that is at least after 36 hrs or more of charging a battery and not using the battery under any load.
If the voltage in Volts (V) is around
1.36 - 1.38 – Consider the battery as fully charged.
around 1.30 – It has around 80% of the capacity
around 1.2 or lower the cell is almost fully discharged.
Capacity of a NiMH cell decreases as it ages
As a battery ages its capacity to hold power gradually decreases though the outer shell of the battery remains the same, the capacity holding decreases. For example a 2100 mAh battery might hold only 1400 mAh power after a couple of years of usage (NiMH batteries should easily last over 5-10 years or around 500 charge cycles if they are not abused, when the battery capacity falls below 80% you should retire those cells or avoid using them in high power drain devices.