Just because a battery is giving you 12 volts or more does not indicate that the battery is good. The only way to correctly test your battery is to put a load on it. Once a load is applied to a bad battery you will see the voltage drop and it may drop dramatically. Connecting a VOM or multimeter to your battery is only going to tell you how many volts DC is is producing. It will not tell you the actual condition of the battery.
These batteries are no different the the 18volt battery in your cordless weed whacker. when its new you can whack the whole lawn. Two years later you may only be able to whack half the lawn. the battery is still charging to 18volts and the charger tells you it is charged. When in reality it no longer has the ability to handle the load it could when new.
Charge it and take the battery to your local auto parts store. They will load test it for free.
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