On Fri, 07 Aug 2015 08:55:39 +0100, Harry Bloomfield wrote:

I now just connect the battery every month to 3 months, to bring it
back up.

Needs to be a near "fit 'n forget" solution. Something that needs to
be done evry three months or WHY probably won't be.

If you can find one, a plug in timer which can be set to come on once
per month for a couple of hours, would be ideal.

That is an alternative, if I can find a suitable charger. I think I
have a constant voltage power supply somewhere that could be
"adjusted on test" to give say a 50mA charge rate. Enough to
replenish the self discharge in a few hours but not enough to dry the
battery if the timeswitch fails to on.

Down side is lots of bits to fail. KISS ...

--
Cheers
Dave.