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Suggest to use a diode in reverse bias to prevent battery discharging into the solar pannel when in the dark. A low voltage silicon diode would be sufficient.
Schottky diodes exhibit a forward voltage drop which is approximately half that of a conventional silicon diodes coupled with very fast reverse recovery. Schottky diodes are thus preferred in switching applications such as switch mode power supplies where very low forward voltage drop and fast switching is a prime consideration.
However, since his solar panel has a marginal charging voltage already, a silicon diode in series would make this voltage -0.5V at the battery.
A Schottky diode can have as little as 0.1V drop at low currents so it would be a much better idea to use that. As a matter of fact, a lot of solar panels do have schottky diode reverse current blocking for this reason.