I just went thru this with my L555, I had a bad battery which in turn killed my alternator which in turn drained my batteries. I bought a new battery which was also bad (my luck) and it took me a bit to figure out why. I finally disconnected both batteries and charged them up and found the new was fully charged then in 2 days it was putting out 8.6 volts. My rectifier was bad on the alternator which made the alternator pull 12 volts and drain the batteries when the tractor was off. what I would do in your case is remove the neg of one battery left side and remove the positive from the other battery(right side), then take a volt meter and measure from the positive terminal to the positive cable (you just removed) it should show about 2 volts, if it shows 12 volts then you have a problem / drain. Also dont assume a battery is good, I had a brand now one that took a full charge and still was bad.