Dual Pro SS4 4 Bank Battery Charger w/3 Bank B.O.S. [SS4/BOS12V3]SS4 4 Bank Battery Charger with 3 Bank B. O. S. This kit includes Dual Pros B. O. S. (Battery Optimization System) 12V 3 Bank (PN: BOS12V3) and Dual Pros Sportsman Series Battery Charger 40A 4 10A Banks 12V 48V (PN: SS4). B. O. S. (Battery Optimization System) 12V 3 Bank B. O. S. actively shuttles energy between batteries to keep the pack balanced, increasing battery strength and longevity. Batteries in a pack will become out of balance and the pack