On 08/04/2021 23:05, rick wrote:
On 07/04/2021 10:32, GB wrote:


I think that one issue could be that the two batteries were NOT
exactly the same voltage to begin with. They have low internal
resistance, so even a small voltage difference between them could
drive rather large currents through them.

I think it would be better to charge them separately in future.

That is my thought - Voltages were not going to match
One was a new 170 Ah
The other a 9 yr old 74 Ah

A float charge voltage from such a charger should be independent of
battery capacity or age, unless one of the batteries is taking current,
usually evident by gassing.

It is a good idea to check battery voltages periodically. Float charge
should be ~13.7V Anything over 13V will maintain charge.