There’s two things I know of that will cause this the first is one of the three tiny surface mounted capacitors to the left of socket on the left hand side. I found the one of them went short circuit which makes the charger think it’s got a battery connected. I took them out one by one till I found which one was the problem if you are lucky then it will be the first one but I would replace them all anyway with normal capacitors. They are there to stop interference terminating the charge prematurely (apparently). I have fixed two this way and they are both going strong (Is yours Geoff?) I think I still have the values at work and will post them tomorrow. Spartan Missile had the same problem and his was traced to the top left (I think) transistor on the rail?

Jason