I think this may be bad news, sorry.
With this type of battery it is important that the charger stops charging when it is fully charged, otherwise damage will occur.
In the handbook ( Please see here
, if you don't have a copy), in section 5, it says
"An automatic cutout charger must be used, otherwise irreparable damage will occur and your warranty voided"
If your charger doesn't do this, then this may be the problem, especially if it was left on for some considerable time.
I would suggest you just check that the charger is in fact charger by using a meter on it.
If I can help you further, please just ask me.
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