I have a Schauer battery charger and starter. It's the lunch box size. Had mine since the mid 80's. Very happy with it. I also use a Battery Tender. Keep in mind, the battery tender is not designed to charge a near dead battery.
It's just intended to keep the battery up when not in use. Going back to the Schauer; I have used it through the years and has never failed me. Car batteries , jet ski batteries, aircraft batterys, lawn mower batteries etc have all been charged or
jumped with my Schumacher charger.