On 13/12/2016 18:46, jim wrote:

Lead from +ve1 to +ve2 and 2x -ves commoned to the metalwork.


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

Makes sense!
Thanks

I suppose it's important that any leads and connectors that aren't
common to both batteries are perfectly clean, to avoid a situation where
one battery is a 'passenger'.

I mean the battery terminals, mainly.

Bill