Restore a 40v Li battery that holds charge but won't recharge?

Restore a 40v Li battery that holds charge but won't recharge?

I have a 40v, 2 AH, Type 4 Black and Decker lithium battery for a trimmer that is capable of holding a charge but when placed in a charger produces a flashing error signal. I have another of the same battery that charges ok, so the problem isn’t the charger. Also, the flaky battery will run

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