Battery load test is done to check the performance and capacity of a battery.
Why do a battery load test?
As batteries age, they loose capacity gradually. The amount of current that they can supply reduce. The ones that are used as power backup batteries have lesser backup time.
One way to check the state of a battery is by measuring the terminal voltage. Unfortunately, measuring the voltage at the terminals of a battery is not sufficient to conclude on the health of the battery. The reason is because a battery that is not loaded and has a good terminal voltage might have a significant voltage drop when a load is connected to the battery. In other words, a bad battery might test good if we are to rely solely on open circuit voltage.
Principle behind battery load test
As mentioned earlier, to do a battery load test, a resistive load is placed across the battery and for some moments, the terminal voltage is monitored. A good battery will not drop below a certain minimum threshold voltage, depending on the installed capacity of the battery.
A battery that is weak will not be able to maintain minimum threshold voltage when a load is connected to it.
How battery load testers work
It is in a similar way described above that off-the-shelf battery load testers work. The tester has a resistive element that serves as a load to the battery. The tester also has a voltmeter to monitor the voltage. Smart electronic types have a function to input the battery specifications, for example, the battery capacity.
In order to load test a battery, the heater element is turn on to draw current from the battery and the tester readout tells us whether the battery is good or bad based on the amount of current drawn and voltage drop of the battery.