On 13/12/2016 18:14, jim wrote:

old tractor, 2 x 12v biggish batteries linked in parallel provide
the power to start.

Only used for fetching haylage once a week so batteries often
don't start it the next time....

Is it OK to just charge the pair of batteries by clipping on a
battery charger, without disconnecting the interconnecting
wiring?

Or is it necessary or advisable (and why please) to electrically
isolate each and charge one at a time?

For longer term would a solar trickle charger be worthwhile?

TIA

Assuming that they are in parallel and it isn't a 24V vehicle...

They get charged and discharged together by the vehicle, so you might as
well do the same with an external charger. I suppose you could check
periodically whether there's an imbalance if you were bothered, but
really once you suspect a battery is knackered replace them both. Stands
to reason the new ones should be identical.

Bill