After you identify any issue at the time to charge the item, the easiest way to verify if the issue is related to the battery or the charger is plugging the AC charger into the unit and wait about 30 minutes.

If the charger is warm to the touch, the charger is moving current to the unit, then the battery would need to be replaced, but if the charger temperature hasn't changed, the charger would need to be replaced.

The battery and charger can also be taken to a BLACK+DECKER Service Agent who can inspect them. Details can be found on this link https://service.blackanddecker.co.uk/BD/GB/en-GB/AgentLocator/SearchAgentLocators