On Tuesday, December 13, 2016 at 6:14:13 PM UTC, 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?



This is the typical situation in narrowboats, i.e. two or more batteries strapped in parallel and charged that way both by the alternator (when engine running) or by battery charger.

Take care with solar trickle charging over long periods. I have seen batteries wrecked by this. Once the batteries are charged the trickle goes on electrolysing the water (gassing) and eventually the plates get exposed and damaged.

Robert