MPPT/constant voltage circuit

[casper.bang]

I ordered a cheap China MPPT solar charger on Ebay, not really believing it did true MPPT, but also not really scared of ending up with a simple PWM charger for just $7. I learned from Dave to take things apart, and so I did, not seeing any sign of inductors for the DC-DC step. I think I see a quad op-amp (battery level monitoring?), some Schmitt triggers (hysteresis?), 3 noname power FET's (chargers), a few transistors and a bunch of discrete components - but definitely nothing to convince me this is a MPPT (or even odd hybrid) charger, would you agree?

Now, my amorph panels are the most optimal at 17.8V and i testing my setup the panels got pulled down to 12V by the battery so I guess that's ultimate evidence. 

Question is, how much would it involve, to do a MPPT frontend which would improve the effeciency by 25% or so, by basically holding the voltage at 17.8V (if possible) and adjusting the current accordingly? Could I use some COTS buck step-down component or make one myself using a 555, FET and inductor?

[DaveW]

MPPT circuits aren't that hard to make; I did one using a chipkit dev board (a pic version of an arduino) and that was more complicated than it needed to be as we were avoiding using a current sensor.
There are loads of algorithms around, but a simple one consists of picking a duty cycle for the boost/buck converter, then increasing it slightly. If that makes the efficiency drop, then reduce it instead. Doing this continuously keeps up with dynamic loads pretty well and should be quick to get working

[bilko]

Silicon Chip article here

http://archive.siliconchip.com.au/cms/A_112794/article.html

[casper.bang]

Thanks for the feedback Dave. I wasn't actually going for a MCU solution sweeping for the optimal setting (I already know from my panel's specs, and have verified manually, that it's 18V). My main problem is that I don't know how to make the "constant 18V bridge" connecting the 21V floating solar panel(s) with the 12V charge input.

[DaveW]

Assuming your load is constant then, you just need a fixed duty cycle. Either a controller IC from Linear, Texas et al. or a 555 tuned to the right duty ratio and a good frequency should do the job nicely. The actual boost/buck circuit is pretty simple, but make sure to choose good parts; FETs that switch quickly, quick Schottkey diodes etc.

[DaveW]

Just to add; one thing you will find with PV cells is that the internal resistance of the cells varies dramatically over temperature and with irradiance, this means that the maximum power point does tend to move fairly dramatically hour by hour. We found we got good results by adding a light sensor input to our control algorithm, maybe for your setup this isn't as much of an issue though

[casper.bang]

Assuming your load is constant then, you just need a fixed duty cycle. Either a controller IC from Linear, Texas et al. or a 555 tuned to the right duty ratio and a good frequency should do the job nicely. The actual boost/buck circuit is pretty simple, but make sure to choose good parts; FETs that switch quickly, quick Schottkey diodes etc.

I know it's cheating, and no longer a tracking solution, but I was looking at another cheapo ebay item to perhaps improve my setup, namely this 3$ DC-DC board. All I would have to do to boost effeciency, is tweak for a nominal range, 18V drain and 12V source. If $3 gets me 15-20% more out of my small 2x10W panels, I'm all for it. What is unclear to me, is whether the LM2596 will boost current as it drops the voltage?!

Just to add; one thing you will find with PV cells is that the internal resistance of the cells varies dramatically over temperature and with irradiance, this means that the maximum power point does tend to move fairly dramatically hour by hour.

Fair point. My panels are amorph ones though, so temperature and brightness reaction tends to be much more nominal, but not as optimal, as mono/poly panels.

We found we got good results by adding a light sensor input to our control algorithm, maybe for your setup this isn't as much of an issue though

Interesting! However, surely, regardless of what the light sensor may suggest for your heuristic based control algorithm, nothing beats a sweep across the 12V - 22V range, multiplying current and voltage output? Are you using the change in brightness to trigger this sweep?

[DaveW]

I know it's cheating, but I was looking at another cheapo ebay item to perhaps improve my setup, namely this 3$ DC-DC board. All I would have to do to boost effeciency, is tweak for a nominal range, 18V drain and 12V source. If $3 gets me 15-20% more out of my small 2x10W panels, I'm all for it. What is unclear to me, is whether the LM2596 will boost current as it drops the voltage?!

That's not cheating, that's good COTS based engineering!

We found we got good results by adding a light sensor input to our control algorithm, maybe for your setup this isn't as much of an issue though

Interesting! However, surely, regardless of what the light sensor may suggest for your heuristic based control algorithm, nothing beats a sweep across the 12V - 22V range, multiplying current and voltage output? Are you using the change in brightness to trigger this sweep?

Ours was a bit different insomuch as we didn't want to use a current sensor as these tend to be one of the more expensive parts on the board, this does make everything a bit trickier (details here if you're interested, https://dl.dropboxusercontent.com/u/5453644/MPPT%20Report.pdf