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farmerkeith / SolarChargerSZBK07ModuleV2

Hardware and software for an MPPT solar charger. It uses an SZBK07 DC converter module, a MOSFET diode. The main software is in the form of an MPPT library header file.
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Evaluating the efficiency of your PWM solution. #2

Open rin67630 opened 3 years ago

rin67630 commented 3 years ago

Hi Farmerkeith,

did you evaluate the efficiency of your PWM solution, measuring the input and output power? I did and had some interesting findings: If you leave the buck-converter alone, i.e. without injection into the feedback of potentiometer, you get about 95% conversion efficiency. The feedback voltage is 1,22V irrespective of which output voltage is set. Let us use 13.8V as an example.

One could imagine, that upon feeding the same voltage into the potentiometer over e.g. a 18K resistor, no change would happen. Astonishing enough, that is not the case.

In order to be voltage-neutral, you need to feed 2,1V. Irrespective of whether feeding raw PWM or smoothing the signal with a 220nF in between capacitor, you need to feed more! But then, my most alarming finding was: the overall efficiency of the buck converter drops below 88%.

I can't explain why, but the conversion is far less efficient when something interferes with the feedback circuit. Currently my best deal with your approach is to refrain completely using PWM and use a tri-state control of the feedback potentiometer: wire a 18k resistor (without a diode) between my ESP8266 3,3V digital IO and the potentiometer, been set to 13,8V, the float battery voltage.

Under normal condition the 3,3V digital IO is set to be an high ohm input and the buck converter regulates to feed everything it can to the battery with 95% efficiency. From time to time, when the weather is fine with the solar panel being able deliver full power and the battery needs an equalization, set the digital IO to output, 0V for two hours, that changes the set-point to 15V. If for any reason, you need to disconnect the charging below 13,8V, set the digital IO to output, 3,3V, that changes the set-point to 9,5V, which is pretty equivalent to stopping any charging process.

But anyhow trying to do an MPPT functionality by injecting PWM (raw or smoothed) into that buck converter is in fact killing more energy, than doing nothing.

Regards Laszlo

rin67630 commented 3 years ago

Finally, I could get a kind of voltage control that does not jeopardize efficiency: 3,2V PWM output to a massive RC low pass filter: 8,2K / 330microfarad, from there 47K to the potentiometer slider. With the voltage potentiometer set to 14,4V (the recommended cycling voltage of my battery) I can get: IO set to input, 14,4V 96% efficiency IO set to output @ 55% PWM 14,4V 95% efficiency IO set to output @ 0% PWM 15,3V 93% efficiency IO set to output @ 72% PWM 13,8V 95% efficiency IO set to output @ 100% PWM 12,9V 95% efficiency

That is enough to control my FLA battery charge as I want to run it. Most of the time I will let it run with IO set to input.

Only when the battery is heavily discharged and need to limit a bit the current, or force an equalization to 15,2V, then I will switch the IO to output and modify the set point.

I will evaluate next summer if I can get an advantage with MPPT, but as far as I have seen with my 100W panel the benefit will be limited.

By the way: I am using a ready made ideal diode module and two INA226 modules to measure the voltage / currents of panel and battery, so i do not need a PCB, just solder a few wires together.

rin67630 commented 3 years ago

Hi,I just have seen that you live in Bathurst NSW, Australia. My daughter and her husband Rev Maxwell Wood, used to live there a couple of years ago, now they are in Sydney.... Greetings from Germany, Keith!