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Why a Charge Controller is necessary
TriStar brand charge controller Since the brighter the sunlight, the more voltage the solar cells produce, the excessive voltage could damage the batteries. A charge controller is used to maintain the proper charging voltage on the batteries. As the input voltage from the solar array rises, the charge controller regulates the charge to the batteries preventing any over charging.
Modern multi-stage charge controllers
Most quality charge controller units have what is known as a 3 stage charge cycle that goes like this :
1) BULK :  During the Bulk phase of the charge cycle, the voltage gradually rises to the Bulk level (usually 14.4 to 14.6 volts) while the batteries draw maximum current. When Bulk level voltage is reached the absorption stage begins.
2) ABSORPTION :  During this phase the voltage is maintained at Bulk voltage level for a specified time (usually an hour) while the current gradually tapers off as the batteries charge up.
3) FLOAT :  After the absorption time passes the voltage is lowered to float level (usually 13.4 to 13.7 volts) and the batteries draw a small maintenance current until the next cycle.
The relationship between the current and the voltage during the 3 phases of the charge cycle can be shown visually by the graph below.
This chart shows the three charging stages: Bulk, Absorption, and Float
MPPT Maximum Power Point Tracking
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.
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