Bill Wright wrote
newshound wrote

With this arrangement, isn't there a danger that if one battery
deteriorates (e.g. a cell shorting out) it will knacker the good one?

Yes. But if they're the same age and type you wouldn't worry about which
one had failed, you'd simply change both.

I wonder if the batteries are knackered anyway, or at least well past
their best. After all, a decent battery of the correct size should start
the engine without further help.

It isnt a car, it’s a piece of farm machinery that doesn’t get used a lot.