I am not a techie, and am led to believe (so I could be wrong)

The rule of thumb is to have all batteries the same chemistry, the same size (as in amp hour), and the same age

Something to do with different chemistry, requiring different charging requirements

Different age of batteries, could see the old battery, pulling the new battery down

I am sure that the techies, will be kind enough to explain it, better than I can

Tony you are spot on! your explanation is good and clear. Old batteries determine what charger will recognise. The charger will analyse the oldest battery first and if this can only be charged to say 80% then then the others will only reach that point. So new ones will never reach maximum. all batteries must be same type ie: lead acid, agm, calcium ect. because charger has different rates of charge. Voltage + amperage it's not just electricity.

hi
so if a different size new battery has a different capacity that's ok weird !! Because a mix of old and new acts the same way and is not recommended ..

Irrespective if the batteries are glasses of water and we fill them up if u have an xtra larger glass this struggles to get full as the smaller ones are getting overfull .IE the voltage charge rate then averages out some are over full some are under full . Satisfactory but not ideal .

Battery chemistry should not be mixed .

Ideally batteries same type capacity and age . The voltage curve characteristics will then be the same . This is what the chargers sense.