Bill Wright brought next idea :
Given two 12V lead acid gell batteries, both new, both the same type, but
from different sources, how do I balance the charge before fitting them in a
24V appliance?

Bill

Charge each individually using some sort of controlled charger, to
bring each to a full charge after which they should balance.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk