Small Appliance Repair Questions? Ask a Technician ASAP
You have done all the things that I would have done already, You now have to check the output of the charger to be sure that that is working correctly ie giving out the listed voltage or slightly above the listed value. If that is OK then yes you have a faulty battery.
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If you have a look on the transformer it should state it there, But it is 22.5 volts DC at 1.25 amps