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Use an inexpensive digital voltmeter to measure battery voltage. Fully charged, it should be very near 12.6 volts. If that's what you find, check the battery cables to be sure they're clean and tight. During charging, the voltage can be as high as 15.0 volts. Measure on the cable clamps or battery posts, not on the charger clamps. If you still find near 12 volts, the clamps aren't making a good connection.

If you find the battery voltage is closer to 11.0 volts, suspect a shorted cell in the battery.

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