DIY MPPT questions

Hi All,

First time poster to this forum. I am looking for information concerning MPPT's.

A little background: I'm a low budget kinda guy who like to build things himself. Not just because it's fun and rewarding, but if I built it, I can troubleshoot and fix it. I have no formal training in electronics, but have fooled around with electrons for a while. My objective with this project is to build a fairly simple and robust MPPT controller and battery charger. It is being built around a Propeller uC, a very neat multi-core device. I am willing to trade ultimate effeciency for simplicity to a point. I currently have 2 Hoxan 48 watt panels with a Vpmax of 15.8 to play with.

So far, I have the uC PWMing a FET and dumping the output into a resistive or inductive (12 v pump). I have an input current sensor and voltage sensor, so I can calculate the watts being produced. For starters I had the duty cycle tied to a pot, so I could sit there and play with it and find the MPP. Then I programmed it so that I was simply setting an avearge MPP, ie half way between the highest and lowest.

1) As stated above, I am PWM'ing a FET and dumping the output into either a resistive or inductive load. The resistive load is easier for me to use, as the pump can only run with an MPPT in bright sun from the panels I have. I compare how many watts I can pull using my MPPT with how many watts are produced if I just connect the resistive load directly to the panel. I get from 5 times as many watts in indirect light and about 1.5 - 2 times as much in brighter sunlight. I haven't had any really sunny days since I set this all up in the last few days. Is this a reasonable way to measure the relative effeciency of the project?

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.

Well, that's just for starters. Does anyone know of a forums directed towards this kind of tinkering?

Many thanks!