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MPPT battery charge controller

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Simon:
Well I've got me panels and me battery now i need to charge the battery while making the most of me panels. I've wanted to build an MPPT controller for some time and while I initially envisioned a microcontroller supplying the PWM drive for the power swithch and being the complete control module I guess that's not the best idea because I'll need a ridicolously high speed MCU to get a high enough PWM frequency.

So I though: why not use a SMPS but use the MCU to process current sensing information and provide feedback to the SMPS (instead of it's own output) in order to control the voltage. Basically this would use the current output as the basis of feedback control.

Am I nuts ?

Strube09:
Simon,

There is an app note by Microchip that can help you (To an extent).

http://ww1.microchip.com/downloads/en/AppNotes/01211B.pdf

Thanks,
Strube

NiHaoMike:
I'm actually doing just that for part of my senior design project, except it's for an exercise bicycle instead of solar panels.

johnwa:
Have you considered using a standalone SMPS, and regulating on the input voltage? As explained in the app note, the precise optimum operating point varies with light level and temperature. However, setting a fixed operating point (probably around 16-17V for a 12V/36 cell panel) is a much simpler way to go, and should still give you a worthwhile gain over connecting the panels directly to the battery. You might want to try plotting some load curves for the panels you have at different light intensities. It would probably be best to use a diffuser, or a dimmable artificial light source, for these measurements, as uneven shading will muck up the results.

Do you have multiple identical PV panels? If you are going to the trouble of building a buck converter, you can connect the panels in series for more voltage, less current, and therefore less wiring losses.

Simon:
Well I have a mix of 1 50W panel and 2 20W panels so all in parallel, not the best thing but not too bad as they only differ in maximum power voltage by 0.5V.

I didn't know you could get MPPT chips. My idea was to dither the ouput voltage and monitor the current output to the battery so that I regulate for maximum charge current and track the best conditions

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