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Buck-boost solar charge controllers

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I am currently doing a project on a solar charge controller for a Lead-acid battery. The project specification stated that the circuit should be able to boost the output of the panel under low light conditions so as to still maintain a charge to the battery. However from looking at PV V/I curves I don't see how this is possible, as the level of solar radiation determines the amount of current that the panel is able to provide, hence if you try to draw to much current the voltage reduces. With low levels of light, trying to boost the current will give an unusable voltage? Does anyone here have experience of building a buck boost converter charger? I was planning on using an LTC3780 for the buck boost circuit and using a PIC to control it. Was planning to use around a 100W solar panel. Any help greatly appreciated.



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boost the output
Not boost the current, boost the voltage.

The job of the charger is to transfer the maximum power! Not current or voltage.
1>There needs to be a way of keeping the battery from overcharging.
2>You need to look for the 'sweet spot' in the PV VI curve, and try to keep the panel there. How to do that?

The LTC3780, or some PWM, needs to not regulate current or voltage but regulate power. The PIC's job is to keep the panel at its sweet spot.

Get some power from the panel
measure power I*V=P (usually input but output should work)
delay 100mS
Start of loop
Increase the current a little, (OR decrease a little)
delay 100mS
measure the power.
If the power increases you are going the right direction (up OR down)
If the power decreases you are going the wrong direction, reverse (up/down)
go to loop
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