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Types of Charge Controllers

Simple 1 or 2 stage Controller:

These rely on relays or shunt transistors to control the voltage in one or two steps. These essentially just short or disconnect the solar panel when a certain voltage is reached. The only real claim to fame is their reliability - they have so few components, there is not much to break.

3- Stage and/or PWN (Pulse Width Modulation)

These controllers send out a series of short charging pulses to the battery - a very rapid "on-off" switch. The controller constantly checks the state of the battery to determine how fast to send pulses, and how long (wide) the pulses will be. In a fully charged battery with no load, it may just "tick" every few seconds and send a short pulse to the battery. In a discharged battery, the pulses would be very long and almost continuous, or the controller may go into "full on" mode. The controller checks the state of charge on the battery between pulses and adjusts itself each time. These are pretty much the industry standard now.

Maximum Power Point Tracking (MPPT) Controllers:

“These are the ultimate in controllers with prices to match - but with efficiencies in the 93% to 97% range. They can save considerable money on larger systems since they provide 15% to 40% more power to the battery. Actual gain can vary widely depending weather, temperature, battery, state of charge, and other factors.”.

A MPPT is an electronic DC to DC converter that optimizes the match between the solar array (PV panels), and the battery bank or utility grid. To put it simply, they convert a higher voltage DC output from solar panels (and a few wind generators) down to the lower voltage needed to charge batteries.

The charge controller looks at the output of the panels, and compares it to the battery voltage. It then figures out what is the best power that the panel can put out to charge the battery. It takes this and converts it to best voltage to get maximum AMPS into the battery. (It is Amps into the battery that counts). Grid connected systems are becoming more popular as the price of solar drops and electric rates go up.

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