Solar Charge Controller MPPT Calculator query.

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robo asked Mar 11, '22

Solar Charge Controller MPPT Calculator query.

Heya.

I want to have 3 of the 175W 12V Victron Mono solar panels connected in series and don't know what MPPT charge controller to buy.

The panels have the below specs.

Nominal Power (PMPP): 175W
Max-Power Voltage (VMPP): 19.4V
Max-Power Current (IMPP): 9.03A
Open-Circuit Voltage (VOC): 23.7V
Short-Circuit Voltage (ISC): 9.89A

My Batteries are 12V

Based on my calculations the max voltage is 23.7*3 = 71.1

I also need the maximum charge current, as they are wired up in series the current stays the same, but that seems wrong so I'm gonna assume it means the "boost Current" which would be 525/12= 43.75.

Based off these calculations I could use the Smart Solar MPPT 100/50, but when I use the calculator it recommends the 150/35 which seem too high spec. on the voltage and too low on current.

https://mppt.victronenergy.com#djE7U1BNMDQxNzUxMjAwLDMsMSwxNC4yLC0xMCw3MCwxMiw5LjAzLDAuMDQsOS44OSwxNzUsMTkuNCwtMC4zNSwyMy43LDQsNg==

What am I not understanding? I know the temperature can affect the panels but I don't know how to calculate that.

Many thanks

MPPT Controllers

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Answer 1 · 2022-03-12T00:39:38Z

klim8skeptic answered · Mar 12, 2022 at 12:39 AM

Most ppl charge their batteries up to 14 volts. 525 / 14 = 37.5a.

Both controllers are about the same price, so grab the 100-50. You might want to ad another panel later on.

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Answer 2 · 2022-03-12T01:46:26Z

Kevin Windrem answered · Mar 12, 2022 at 01:46 AM

The current remains the same as you series panels. The voltage multiplies.

Open circuit voltage goes UP with a temperature decrease. It's about 0.3-0.5%/degree C depending on panel. Based on the panel and a minimum operating temperature of -10 degrees C, the voltage presented to the controller would be 80 volts.

Proper controller selection is complicated. You must never exceed the controller's PV input voltage spec.

The current spec is the maximum output current so you can calculate maximum power output by considering the battery voltage. A 24 volt battery system would be double the power output of a 12 volt system. For your 3 panels, the theoretical maximum power is 525 watts but unless the lighting is perfect and the panel angle is perpendicular to the sun, you'll never see that much power. If you use a factor of 80%, your 12 volt current would be 35 amps. You can probably use a 100 volt controller for the three panels even though the calculator recommends a 150 volt model. The 150/35 is about the same price as the 100/50. With the 150, you have room to series connect a 4th panel but you'd need to bump up to a 150/45 to net much additional power.

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kevgermany ♦♦ commented · Mar 12, 2022 at 05:58 AM

@Robo

Or the 100/50, but wired 2s+2p

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Answer 3 · 2022-04-27T20:35:33Z

robo answered · Apr 27, 2022 at 08:35 PM

Okay so what I understand from this is that both would work.

The max voltage would be about 80V and current 35ish amps.

If the panels give a larger amperage it will be wasted and if they gave a larger voltage it would break the controller.

So it seems the additional amps on the 100/50 would be wasted and I can't add another panel in the future because I am already near the max voltage. For the 150/35 I could add another panel but wouldn't get much out of it coz of the amp throttle issue. So did the calculator give me the 150/35 option so I would be less likely to break it with a magically higher voltage?

I will just buy whichever controller is cheaper. I have no roof space to add more panels anyway!

Thanks
@klim8skeptic

@Kevin Windrem

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