The lawnmower's batteries died first.  Fred replaced them, then the charger quit charging them.  He got frustrated and decided to buy a new electric lawnmower.  Later he asked me if I wanted the old one.  I am curious if I can fix it.  The transformer is OK, but the charger is not charging, so like you suggested, one or more diodes may have failed.  I havent had time to work on it anymore.  The other possiblity I was thinking of was the relay.  Not sure what function the relay serves other than to handle heavy current so the solid state parts can be lighter duty.