Best Solar Charge Controller? And Series vs Parallel Wiring

[Scalf77]

I hate when I do a long detailed post, hit send and find out I was logged out. Should have copied it.

Anyway I will try to repeat it. The specs on your panel were Vmp of 17.6 and a Imp of 4,5 or 17.6 X 4.5 = 79.2 or 80 watts. These numbers are based on Standard Test Conditions (STC) that are not real world but they work for comparison sake. The temperature of the panel can have a pretty big effect on the voltage output of the panel, it will drop with higher temps and temps on top of the van can get pretty high. This is why you want some airflow under the panel. While current output is also effected by temp, the real driver is caused by solar irradiance.
We can simplify things because we are using the same panel for the both MPPT and PWM controllers. With a PWM controller we would take that 80 watt panel and again for simplicity we use a 13 volt battery charge set point.. So the best power we can do with a PWM controller is Imp of 4.5 amps X our 13 volt set point. This gives us an output 58.5 watts, so really your 80 watt is putting out only 60 watts . As temperature goes up the Vmp will go down, but the PWM controller is just regulating the output voltage down to the set point. If the Vmp went down to 15 volts it would not affect the output of a PWM controller.

A MPPT controller is designed to use the extra voltage headroom. A simplest way to look at this is we would get the same 58.5 watts out of panel, but we would also look to gain power left , we have Vmp 17.6 minus a the 13 volt set point and get 4.6 volts * 4.5 Amps or 20 watts to gain back. Again for simplicity sake let’s say we can get 90% efficiency out of this conversion. This would give us 72 watts or 5.5 amp of output at the set point of 13 volts. So in this case we gain 1 amp of output over the PWM controller. If we now look at the same 15 volt Vmp do to temperature we get ((15 * 4,5) *90%) we get a current of 4.67 amps at 13 volts. So after taking into account for temperature we really only see a 0.17 amp increase with our MPPT controller.

Now if you are using a grid tie panel with a higher Vmp, it would become a lot easier to see the gain that a MPPT controller was designed for. You can wire two of your panels in series and get a higher Vmp, this would allow you to take advantage of a MPPT controller. The problem with wiring panels in series is that shading affect , specifically if one panel is partially shaded it can have affect on the output of the other panel that is in series with it.

I think it would make sense with your panels to go with a PWM controller.

Anyway I see from your post over at midniteforum that you are generally getting idea on the difference between the two types of controllers. Outside of cost I do not see a negative of going to MPPT. Grape has some pretty good prices on mono-crystalline panels 160 watt output. They have a Vmp of 18.5 volts making it better for MPPT , they are 58 " x 26". Mono-crystalline has less voltage drop due to temperature than Poly-crystalline.

Morningstar makes a couple of comparable controllers a Pro Star 30 and a step up would be the Tristar 45. The tristar brings with it more configurability for your battery charging, and also supports voltage sense line for managing the set point to your battery.. The longer the run from your controller to battery this become more important. Be aware that there pricing usually does not include a monitor (that is usually extra)

-greg