Ross Herbert wrote:
One possible problem I see with this simple circuit is that it has no inbuilt
hysteresis or maximum voltage charge time. A battery will charge to its preset
maximum voltage and because there is no time limit placed on the charge current
at the maximum voltage, the relay will operate and stop the charge before it has
time for the battery charge capacity to be maximised. This will lead to
"hunting" where the relay will operate and release rapidly, ad-infinitum, as the
battery voltage fluctuates above and below the preset maximum.


The difference between the relay pull in and drop out current
provides the hysteresis. Typical general purpose relays will
pull it at roughly 90% of nominal rated current, and drop out
at roughly 10% of nominal rated current.

Ed