I had this. The charger thought the battery was charged but it was actually open circuit. Try a known good battery.
My battery had corroded inside and one interconnecting strap between cells had failed and another wasn't far behind. Managed to fix the strap with wire and a soldering iron but that battery is on borrowed time.
Its also possible there is a cable or joint fault inside the tool itself.
Qualcast was a once proud quality make but is now largely labels stuck on cheap Chinese tat with zero support and sold through Argos.
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