Why dont I get 50W from my Renogy 50W mono panel?

[SPL Tech]

I have a single 50w mono 12v panel that is used to charge a 35ah battery in the back of my van off a Renogy 12/24v 30A PWM controller. The system's sole function is to run a really small fridge and small lighting system.

Anyway, I know that PWM controllers are unable to provide the full power output of the solar panel, unlike MPPT, because voltage above 14.4v is basically lost energy. I am questioning the accuracy of that statement though, and here is why.

I did some testing. In full sun at high noon with the panel pointed directly to the sun, the maximum output I could achieve from the panel as measured with a multimeter was 3.01A @ 12.4v which is about 37W. This was when the panel was connected to the battery and the battery was over 50% depleted, so the battery would accept as much current as the panel would throw at it. However, and here is the catch, the voltage output of the panel was not ~18v as the sticker on the back would suggest, rather it was the same voltage as the battery, 12.4. This tells me that the PWM solar controller simply runs the solar panel and battery in parallel with no PWM modulation until the voltage of the battery reaches the PWM activation voltage of the controller, which in my case I have it set to 13.8v. I did some testing at other voltages ranging from a completely dead battery, 10.5v, up to fully charged at 14.0v. In all cases, as long as the battery voltage is below about 13.0v, the controller simply parallels the solar panel with the battery.

In other words, if the battery is not fully charged, the solar controller is basically invisible, it's effectively the exact same as if I just hooked the solar panel straight to the battery with no controller at all. So if the PWM controller does NOT use any PWM at voltages below the float voltage set point (13.8v), and it just runs the panel in parallel with the battery, the whole "PWM controllers cant capture excess voltage" shouldent apply, because there is no excess voltage to capture. Thus, why dont I get the full 50W from the panel?

For fun, I actually tried connecting the panel straight to the battery with no PWM controller at all with the battery 50% depleted, and I got the same results, max current was about 3.04A @ 12.4v (38W or so) in full sun, high noon, bright day, pointed perfectly at the sun.

My cable losses are minimal. I measured a voltage drop of only 0.07v @3A across the entire cable network. I measured from the solar panel terminals to the battery terminals.

[Sunking]

You have answered your own question. All PWM in effect is just a switch that is closed and opened real fast. At 100% modulation is the exact same as connecting the battery directly to the panel.

What may be throwing you off is not understanding how batteries charge up. Just because you set the voltage to 14 volts, you are not going to have 14 volts on the battery until it is fully charged up. When a PWM charger is at 100% modulation, the switch is closed 100% of the time. Same as connecting the panel directly to the battery. With any PWM controller Output Current = Input Current. In otherwords whatever the epanel current is.

The voltage of a charging battery is = battery OCV + ( Charge Current x Battery Internal Resistance).

So lets say your battery is 50% SOC (12.1 volts OCV. The open circuit voltage is roughly 12.10 volts. A 35 AH battery internal resistance is only around .05 Ohms. So if you put a 3 amp charge current into the battery the voltage on the battery terminal is only 12.1 + (3 amps x .05 Ohms) = 12.25 volts despite you having set the voltage to 14 volts. (37 watts from a 50 watt panel) As the battery charges the OCV will begin to rise over a period of time.When the battery OCV reaches 13.85 vots, the PWM will now start to modulate less than 100% to regulate the voltage and charge current will taper off toward 0 amps. When th ebattery OCV reaches 14 volts, there will be 0 amps and close to to0% modulation, but never 0%. .

FWIW a MPPT controller Output Current = Panel Wattage / Battery OCV. So instead of 3 amps on the same battery you get 4.1 amps and you would see 12.3 volts on the battery initially.

So your answer is you get exactly what you observed. Perfectly normal.

OCV = Open Circuit Voltage. Disconnected

[SPL Tech]

If the max output of the solar panel is 3A regardless of voltage, and the battery cant really charge higher than 12.7V, then the max I could ever get is 3A @ 12.7v or 39W or so? Obviously we can raise the voltage higher, but the battery cant store voltage above 12.7v so it's mostly pointless power above 12.7v.

I am curious how an MPPT controller could boost the current above what the panel can physically produce on a dead short. When I dead short the panel, I get about 3.2A on my multimeter. I understand that if the voltage output of the panel is higher than what the battery needs, it's possible to design a circuit that converts the voltage to a lower voltage with the benefit of increased current (I assume that's what MPPT does?). Does the MPPT controller actually consume any electricity? I can get one for $20 on eBay, so I wonder if it would be worth it in my application. I had read that MPPT is only beneficial in applications where the solar exceeds 300W, but if MPPT provides more current to the battery, and they are as cheap as PWM controllers nowadays, why not get one for all applications regardless of how small the solar panel is?

[sensij]

Some of the super inexpensive mppt controllers are really PWM in disguise, so be careful bottom fishing. Your basic observation is correct... mppt is more efficient in all applications, but until the system gets large enough, it might not be worth paying for.

[Mike90250]

I'm not sure your panel would support running any size fridge, even a really small one. Generally, the smaller the fridge, the less efficient. Often a full size energy star fridge is more efficient than a small dorm or bar fridge.