Practicality of DIY MPPT controller

[madlabs]

Hi All,

I'm a low budget DIY kinda guy and recently bought an off the grid property. While I am still living on the grid, I am getting a handle on using alternative power. This summer I converted my hot tub over to solar, and have also just added a loop to my wood burning stove to assist heating it during the winter.

I have been playing around with and reading a lot about MPPT controllers. I have set up a rudimentary uC based MPPT, and am getting to work on a buck converter to charge batteries.

However, I see very little in the way of complete DIY mppt controllers/chargers out there. It makes me wonder why. Is decent efficiency the reason?

Why do I want to build my own?

1. Fun
   
2. Because if it craps out, I can fix or replace it.
   
3. Cost
   
4. As I am destined to be poor when old, I want to set myself up to be as independent as possible.
   
5. As I expand my system, I can add to my controller system.
   
   

So, my big question is this: Is it reasonable for a shmuck like myself to make a MPPT controller/charger that achieves 80% or better of what a commercial controller does? I realize that 20% is a lot, but for the reasons above I find it worth it.

[bob g]

ymmv, but

it would seem to me that reaching 80% or better on a diy mppt would be attainable, maybe even better especially on lower power units.

i would think it possible to get over 90% efficiency on a unit of perhaps 30amps output,

when you get into the higher outputs, it seems tht cost goes up and efficiency gets a bit harder?

so maybe you build a few smaller ones, and have higher efficiency and redundancy?

will be interesting what others thinking is on this subject

bob g

[madlabs]

Bob,

A 30A controller that attains 90% of a commercial controller would make me perfectly happy. I wouldn't mind pushing it to 50A, but would settle for 30A. I actually like the idea of several smaller controllers, it allows me to add as I grow and prevents the entire system from going down at once.

I just ordered some samples of the  LM5104M FET driver to start playing with a buck converter. I have a simple MPPT going with a uC and a FET.

BTW, is this the Bob that reccomended this forums to me? If so, thanks!

Jonathan

[boB]

>>BTW, is this the Bob that reccomended this forums to me? If so, thanks!

I think that might have been me, from over at the NAWS forum maybe ?

If you have the time available, then this would make a great and useful project.

If one has more money than time available, I would say go buy an MPPT controller.

Sounds from what you have already done, you are capable of the task.

Time = Money  is so true.

boB

[scottsAI]

madlabs,

Check out:

http://www.fieldlines.com/story/2008/8/26/223841/588

Been discussing how to build a boosting MPPT.

Simpler than a buck.

Use a MPPT module for every panel or two, looks cheap to build.

I have an assortment of panels, can't connect in series for the higher voltage battery pack. Separate MPPT done DIY may be cheap enough to make it worth while doing! Expect efficiency to be on par with any unit out there.

Have fun,

Scott.

[commanda]

Power electronics, once you get above the several watts stage, starts to become challenging. Fet drivers, board layout, stray capacitance and inductance, all become critical. But it certainly is doable. Get a good book on power electronics/switch mode power design as a first step.

Also, many chip manufacturers publish excellent Application Notes. These often include a complete circuit and pcb layout. I did see one on the Microchip site recently (about building an mppt controller) while searching for other stuff.

Amanda

[madlabs]

Scott,

I had alreasy found that thread and had downloaded the schematic. However, with the MPP of the panels I have, I need a buck converter.

Amanda, I am aware that power circuits above a few watts get tricky. Not that that means I know what to do! I spent part of yesterday reviewing app. notes for the FET driver I ordered. For starters I am going to lay out the power traces short and fat, with the chip driver as close as possible to the gate. Any other reccomendations/considerations?

I am still mystified as to why there aren't more DIY controllers out there. It seems as if there would be many. Am I missing something?

Here is the outline of my plan: I am using a 12bit ADC, with a current sensor and voltage divider to calculate the input watts. I am PWM'ing a FET to set the MPP. I then plan to dump that power into a buck converter, again PWM'ing to set the output voltage, and charge batteries with that. I have another current sensor and voltage divider for the charging output, so I'll easily be able to track my system losses.

A few questions:

1. I found that my MPP varies about 2V or so. In more diffuse/lower light situations, the MPP is higher, in brighter conditions it is lower. Is this typical, meaning have I done it right?
   
2. I found that the frequency was fairly unimportant on a resistive load, but played a somewhat larger role on inductive loads. Still, I could vary the frequency fairly widley, say from 1kHz to 23kHz on an inductive load and from 5kHz to 23kHz on  an inductive load. Duty cycle is lower with resistive loads and much higher with inductive loads. Does this sound right?
   
3. How often is it really worth it to track and adjust the MPP? Once a second? 1000 time a second? Especially in low light situations the wattage varies pretty widely so it seems that some averaging will be needed or the controller will spend too much time "hunting" the MPP. Again, I am favoring simplicity over effeciency.
   
4. I would like to make the power switching sections of the circuit on their own board, meaning not on the same board as the uC. So, the PWM would have to be brought to it via a cable. AM I asking for way to much RF or circuit instability like this? The FET driver would be on the board with the FETs, just the PWM would be piped over.
   
   

Thanks for all the input! Is there anyone out there that wants to help with design, especially of the power sections?

Jonathan