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Originally Posted by FTD
How does one test a sealed battery?
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Most batteries are more than 12 volts. They have 6 cells, each producing around 2.1 - 2.2 volts. Wired in series, these cells combine to produce about 12.6 - 13.2 volts.
To test the battery, you want to disconnect one terminal and then using a multimeter (DMM), touch a lead on their respective battery post (- or +). If the battery is healthy, you should be seeing voltage in the 12.6 - 13.2 volt range.
Anything under 12.6 or over 13.6 says there's something not right, either low electrolyte, sulphur crytals built up on the plates, a cracked plate or cell. The battery may still work, but is on it's way out and is likely to make the alternator work harder, placing more strain on it's service life and reducing MPG slightly too.
It's also a good idea to test the alternator function while you at it. To do this, replace the battery terminal and start the car. With just the car running, placing the DMM leads as before, you should see somewhere between 13.5 - 14.5 volts (for the oem Bosche alternator) - this is the alternator voltage. Now, turn on accessories such as the headlights, radio, turn signals, fan (but not the AC), this is called a load test. The minimum voltage reading should now should be no less than 13.0 - 13.2 volts. If under this voltage (and assuming the battery tested healthy), or over 14.0 volts, suspect a problem with the alternator, usually the diode pack (one or more diodes may be faulty).