[The Jester of Wibbly]

Turns out my C-TEK charger has packed up. 

Can't complain it's lasted hard beatings with daily charging for two batteries for 10 years. 

It was only trickle charging and failed to get off stage 3 with a noticeable humming noise.   Battery tests I did were fine, they were holding charge fine.  But voltage while charging was only 0.3 volts higher which is a lot less than it should.

You can never really be 100% certain it's not a battery gremlin but my instincts were right.   Replaced for a another CTEK charger and I'm back in business.

Bet it was a faulty capacitor.

[Spruce]

Battery issues can be very frustrating.