Battery Management System
The most affordable BMS is a human being, a meter, and a charger! :-)
You just measure each battery's voltage, compare them, and if any are low, charge them a little bit with a small separate charger. Depending on how much you drive, and how good your batteries are, you may have to do this monthly, weekly, or daily! Obviously, this gets to be tedious.
The next step up is to add ways to make this process easier. It could be a single connector or terminal block in a central location, with wires to each battery. It could be a rotary switch and voltmeter. It could be a separate little meter or bargraph per battery, so you can read them without connectors or switches.
Next step up is to automate it somehow. The crudest automatic system is my zener-lamp regulator. It's a couple zener diodes and a light bulb across each 12v battery. If the voltage gets too high, the zeners conduct, the lamp lights, and it bypasses charging current to push the batteries toward balance. These are $5-$10 per battery.
From there, it all depends on how fancy you want to get. Rich Rudman sells regulators for about $40/battery that can bypass more current, provide indications of over- and under-voltage batteries, over-temperature, etc.
My Battery Balancer is an elaborate system that uses a microcomputer to monitor every battery, and charge it as needed to keep them all in balance. It will cost you around $1000, and is open source (see it at http://www.geocities.com/sorefeets/balancerland/intro.htm
-- Lee Hart