Solar panel voltage on connection to PWM - What is happening?

Dear all,

I just finished a small powerwall project that I am going to post soon. Before I write a post I would like to fully understand it. At this moment, I still have a question about the solar panel voltage and I hope you can help me.

I have a 10W (12V) solar panel placed in front of a window on a very cloudy day (no current production expected):
- The measured open voltage in that position is 16-17V approx. (specified at 18V).
- Whenever the panel is connected to a PWM controller that is yet not connected to a battery, the voltage from the panel goes down to 3,5V. The PWM remains off.
- Whenever the panel is connected to a PWM controller that is connected to a 13V battery, the voltage from the panel stays equal to the battery voltage (13V). The PWM is on.

I assume the lower panel voltage happens as result of the solar panel trying to feed the PWM; something that doesn't occur because the panel is not producing current at all. If I place a cloth on the panel and block the light, the voltage goes even lower (1,5V). So this part feels quite understood and clear.

I however fail to understand why is the voltage of the panel matching the voltage of the battery when the battery is also connected to the PWM. PWM should only act as a current valve and not modify panel voltage, right? My theory for this phenomenon is that the panel is trying to pull current to feed the battery but at no current, the voltage is going down. The reason why the voltage doesn't go below the battery voltage is because the consumption of the PWM is transferred to the battery and somehow the PWM is keeping the panel voltage at the battery voltage for (a) maximize current production given the near to zero panel output or (b) simply to keep the current from the battery to go to the panel.

My expectation is also that If the panel would be under the sun, then the panel voltage would go beyond the battery voltage and some current would start flowing into the battery too.

Could somebody help me understand what is happening?

Thanks a lot! Very appreciated.

Greetings.

All of the off grid system (4) that I have built, you always connect the battery first and solar second and load last. And you revise the order when you tear it down. Maybe the PWM is protecting it self?

ElBeau said:

All of the off grid system (4) that I have built, you always connect the battery first and solar second and load last. And you revise the order when you tear it down. Maybe the PWM is protecting it self?

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It is a good reminder. I did follow this order when both battery and panel were connected to the PWM. So that point is ok.

I think your panel does not have reverse flow protection.

There should also be a diode in the panel that prevents back flow during low light conditions. This prevents the panel from discharging at night.

Does your 10 watt panel have a diode? My low wattage panels do not have a diode that I can see, so I don’t know if they have that reverse flow protection or not. Some of them, like my SOLARGIZER car battery charger does not have a built in diode, but that reverse flow protection is built into the circuit box that comes with it.

I guess my next question would be why is the voltage able to make it through the PWM. I’d connect my Multimeter between a lead and measure how the amps change in various conditions as I have it in light and low light.

If there is no reverse protection in the PWM and Panel, then you should see at the panel positive amperage when charging and negative amperage when there is no light.

A PWM controller is just a switch between battery and panel. (plus some control electronics). When battery gets fully charged the PWM switch opens and closes to regulate the average current to the battery keeping battery at proper charge voltage. When illumination drops from PV panel to a low level the PWM controller just disconnects from battery. It typically engages a bleed resistor across the PV panel to know when there is again enough generated current to make it worth while to reconnect to battery.

A PV panel is an illumination based current source. Open circuit, it is clamped in maximum voltage by the inherent diode of solar cells. With open circuit voltage of 16-17v you have about 28 cells in series in the panel.

Your PWM controller electronics may be drawing more current then you provided by your poorly illuminated panel causing panel voltage to drop out. Most PV controller rely on battery to power it so without a battery the controller may not operate.

When battery is below PWM charge voltage regulation its switch connects PV panel directly to battery. Instead of being clamped by inherent PV cells' diodes it is now clamped by battery voltage. This assumes you have enough series connected cells to exceed battery voltage.

Remember PV cells are illuminated based current sources so the amount of PV current output is dependent on illumination and area of each cell. A good cell produces about 35 mA to 45 mA per square cm. A typical 156 x 156 mm cell is 8.5 to 9 amps at full sun level. You can put a current meter directly across PV panel to measure illumination current generated. It is okay to short out a PV panel or have it open circuited.

Real cells have a some parallel shunt resistance and small amount of series resistance. If you leave a PV panel connected directly to battery at night the PV panel shunt resistance will drain some current from battery. Maximum power output from PV is where the panel load is set so the inherent cell's diode just begin to conduct. This results in the maximum voltage and most of the illumination generated current dumped to external loading. A PWM controller just connects PV to battery so PV power will just be illumination current x battery voltage. The inherent cells' diode conduction voltage is in 0.5v to 0.7v range and is dependent on temperature and goes down as temp goes up at about -2.15 mV per deg C per cell.

An MPPT controller has a switching inductor power converter (typically buck switcher) so the load on the panel can be adjusted for a given illumination current level from panel to achieve the loading where the PV inherent cells' diode just begins to conduct. An MPPT controller needs a panel with enough series cells so open circuit PV voltage is 4 to 6 volts above battery voltage. There is a couple volts overhead required for switcher plus a few volts overhead for MPPT algorhym to wiggle around to find MPPT point. Your panel does not have enough overhead voltage to run an MPPT controller nor would the power gain be worth the extra cost of MPPT controller.

RCinFLA said:

When battery is below PWM charge voltage regulation its switch connects PV panel directly to battery. Instead of being clamped by inherent PV cells' diodes it is now clamped by battery voltage. This assumes you have enough series connected cells to exceed battery voltage.

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Wow RCinFLA, amazing answer!! Thanks a lot for it and all the time you've put in it!! So actually the voltage from the battery is leaking to the solar panel when the battery is within charge voltage range because of the open switch; this is why the panel voltage becomes the battery voltage. The fact that I measure 0 Ampere from PWM to panel could just simply be that the panel has a protection diode (as chrisski mentions) or that my cheap ammeter LED display doesn't measure negative current.

Tomorrow I will use a tester to measure current between PWM and panel to see what happens. This is also suggested by chrisski

FInally, also thanks to chrisski for the elaborated answer!!

Jordi said:

Wow RCinFLA, amazing answer!!

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agreed @RCinFLA