Under full sun conditions the MOSFETS got a bit warm and the whole unit radiated a lot of RFI. This design uses a set of relays - one for each panel. If the controller decides the battery voltage is too low, it engages another relay; if its too high, it drops one out.
This tends to 'float' the voltage at about 14 volts as long as sufficient sunlight
The controller also implements a network style interface (open collector TTL, 9600 baud). This makes it possible to 'tweak' the charge settings using a terminal program and an RS-232 to TTL interface (coming to this web site soon). If you choose to duplicate this device, you may leave out the network stuff and 'hardwire' the charge settings in software.