A 12V battery can come in many shapes and sizes. It can be used to start cars or provide power to personal electronics. Typically, a 12V battery is used on outdoor devices that require more power than that provided by a smaller battery. If you aren't sure whether a 12V battery is working, you should test it.
Things You'll Need
Fully charge the 12V battery using the battery charger that came with it. If you are testing the 12V battery in your car, charge it by running the engine for an hour.
Let the fully charged battery rest for 12 hours before testing.
Use the battery for a short period of time in whatever appliance it's used for. If you are testing the battery in your car, start your car, run your high beams for three minutes, then turn off your car.
Test each cell on your 12V battery individually by removing the cap on the first cell with a screwdriver, placing the syringe portion of your hydrometer into the liquid part of the cell and squeezing the hydrometer's top to extract fluid into your tool. Note the position of the bubble on the hydrometer and the battery's gravity reading. Repeat this step for each of the cells of your battery.
Check that the gravity readings of each of the cells don't vary by more than 0.5 from one another. Refer to the table found at BatteryStuff.com to make sure the cell gravities are correct based on the level of charge in your 12V battery.
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