I assume you have an even number of batteries to place them in series. If the charging system is 12 volt, you could build an alternating relay that connected the charger to one set of batteries for 5 minutes and switched to the other set for 5 minutes. This could easily be done with a MC14541 timer chip and a DPDT relay. If solar, this could be run off the panel power so it "sleeps" at night. One thing people don't think about is that relays have a life of about 500K. If you count the number of cycles in a year, that adds up fast. The timer could be set up for periods of hours for less cycles. The charger electronics should keep the batteries balanced.