Linear Technology's LTC3305 is designed for stand-alone operation and does not require any external control circuitry.  All voltage monitoring, gate drive, and fault detection circuitry are integrated.

The LTC3305 employs an auxiliary battery or an alternative storage cell to transfer charge to or from each individual battery in the stack. A mode pin provides two operating modes, timer mode and continuous mode. In timer mode, once the balancing operation is completed, the LTC3305 goes into a low power state for a programmed time and then periodically rebalances the batteries. In continuous mode, the balancing operation continues even after the batteries are balanced to their programmed termination voltage. The LTC3305 is available in a thermally enhanced 38-lead TSSOP package.

Videos
LTC3305 Lead Acid Battery Balancer
LTC3305 Lead Acid Battery Balancer - Video Product Brief