I decided to built this:
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It keeps the battery a little bit active, preventing sulphation by loading the battery with a hefty current (40 A) for a short interval (50 ?s) approximately every two minutes.
But they used a fast zener diode (BZT03 27V) across the FET for protection, as rather large voltage spikes can occur when the FET switches off.
I can not find this diode where I am, so can you suggest a substitute? I guess I can not just put regular 27V zener diode here?