Battery tender ,and a master cutoff switch,I have them on seven cars ,and never had a problem yet!!

sounds like you may have bigger electrical issues than just bad batteries, I would say you have a draw somewhere in your car that is draining the battery down. Or, is your alternator properly charging your battery when the car is running, if not, your battery never gets a good charging when it is running and operating. First test your alt. output, if thats fine, try looking for typical draw culprits like radios, heaters, cig. lighters, other accessories, etc.