I bought a "Ring" brand smart charger: it initially suggested the battery was only 30% charged.
Left it on overnight, it staggered up to 75%. The condition indicator said charge...
Took it down to Halfords, they did a quick test and agreed to replace it.
The technician said that it probably had a failed cell.
I have to say that George at Salisbury Halfords was excellent, helping me chose the best matched charger and then today sorting out the exchange.

The new charger can be used as a trickle charger, he suggested connecting it for a couple of days a week, and not leaving it on all the time.

Peter,
66, 2016 Porsche Boxster S
No longer driving Tarka, the 2014 Plus 8...