Why talk about it now?

Solar Charge Controller

Posted by aonomus on August 10, 2009

So recently I put together a small 3W solar array of cheap amorphous solar panels, and I have since given the small charge controller to my parents who have 3x 15W solar panels (also amorphous). Currently I’m running a test, but from the looks of it there was a modest amount of unharnessed energy (ie: the panels are there, but they weren’t being used to their full potential without a proper charge controller for automation.

My current charge controller schematic is here, but I have a future plan to add a extra stage which cuts out when the battery voltage drops below a set level (so that the load will be disconnected when the battery voltage drops too low).

Essentially what happens is that the opamp compares the battery voltage (through a divider) against a voltage reference, and as the voltage of the battery approaches the reference, the output rises (which in turn increases the current flow through the shunt, decreasing voltage that reaches the battery). The equilibrium is reached fairly quickly in fact once the battery reaches the set voltage.

One point to note is that with this charge controller, most of the power is actually dissipated in the transistor, and while I may have just used a TIP122, choosing a transistor over a darlington, and using a schottky diode will decrease the heatsink requirements.  With a 3W array, no heating occurs (in fact probably no heatsink would be needed), and with a 15W panel, the small heatsink actually gets quite warm to the touch, but with either panel, almost no heating can be registered from the resistors.

The charge controller in all its glory. Note the omitted second transistor.

While this charge controller is good for keeping batteries at float charge, it is not the most efficient. When a battery is very ‘hungry’, the solar panel voltage drops to that of the battery, typically out of the maximum efficiency range of the panel. A better type of charge controller for very large solar arrays is called a MPPT or maximum power point tracker charge controller that acts as a dc-dc converter (buck boost I would imagine) to get the most power out of your panels in varying sunlight inputs.

Some ideas I have for a better version:

  • Low Rds(on) MOSFET to disable load (or inversely to connect load) to prevent over-discharge
  • LED indicators to display charge or ‘full’ status
  • LM3915 based voltage gauge
  • Modular approach to charge multiple independent batteries (fleet maintainance  of 12V lanterns, flashlights, radios, or other devices)

Hopefully I will get around to building a nice regulator for a 45W solar panel array for my parents as a nice little gift, assuming I can keep a 60AH battery topped off, one could use maybe 20-30AH of power from the battery per day, maybe something like a nice fan.

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