How to check battery charger

If it charges the battery, it's working. If it doesn't, it's not...

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Adding to SkyDancer's answer, if the battery doesn't charge, that doesn't necessarily mean that the charger isn't working. Batteries can become defective so they will not hold a charge. In that case you toss the battery, not the charger.

All a battery charger does is apply a direct current (DC) voltage to the battery with enough current to activate the cells. The voltage has to be the same as the rated voltage of the battery. You can check the voltage with a voltmeter. Never put a charger on a battery outside the range the charger is intended to be used for. They can explode.