Most multi-stage charge controllers are Pulse Width Modulation (PWM) types. I would recommend using one of at least this design. The newer Maximum Power Point Tracking (MPPT) controllers are even better. They match the output of the solar panels to the battery voltage to insure maximum charge (amps). For example: even though your solar panel is rated at 100 watts, you won't get the full 100 watts unless the battery is at optimum voltage. The Power/Watts is always equal to Volts times Amps or P=E*I (see Ohm's law for more info). With a regular charge controller, if your batteries are low at say 12.4 volts, then your 100 watt solar panel rated at 6 amps at 16.5 volts (6 amps times 16.5 volts = 100 watts) will only charge at 6 amps times 12.4 volts or just 75 watts. You just lost 25% of your capacity! The MPPT controller compensates for the lower battery voltage by delivering closer to 8 amps into the 12.4 volt battery maintaining the full power of the 100 watt solar panel! 100 watts = 12.4 volts times 8 amps = 100 (P=E*I).
The Charge Controller is installed between the Solar Panel array and the Batteries where it automatically maintains the charge on the batteries using the 3 stage charge cycle just decribed. The Power Inverter can also charge the batteries if it is connected to the AC utility grid or in the case of a stand alone system, your own AC Generator.
Summary
If you are using four 75 to 80 Watt solar panels, your charge controller should be rated up to 40 amps. Even though the solar panels don't normally produce that much current, there is an 'edge of cloud effect'. Due to this phenomenon I have seen my four 6 amp panels (4*6=24) pump out over 32 amps. This is well over their rated 24 amps maximum. A good 3 stage 40 amp Charge Controller will run about $140 to $225 depending on features like LCD displays. For eight 75 to 80 watt solar panels you would need two 40 amp Charge Controllers to handle the power or you could increase your system voltage to 24 volts and still use just one 40 amp Charge Controller. Check out Battery Wiring Diagrams for details on how to set-up your system voltage and see the actual wiring diagrams you need with our Battery Bank Designer which will display the required wiring with just 4 clicks!".