Charge Controller Calc

[DDrdan]

I want to install 2 panels on my motor home and I'm having a little knowledge problem in picking the correct charge controller. I'm not sure if my calc's are correct?

Panels: (2) REC370TP25M72 Mono Panels (Spec Sheet PDF)
Wire gauge: My run (panel to controller) is approx 20 feet. 6 Gauge??
Batteries: Manual switch between (2) 12Vdc 70AH Lithium-ion (Old Spec Sheet - now 70AH) in parallel and (1) 12Vdc 110AH AGM (high amp use audio battery)

The 2 Lithium's will be used the run my motor home 120Vac refrigerator (through a 2000W pure sine inverter) and the Gel will be a backup.

Fridge has a 15A circuit requirement and a 528kw / Year Energy rating so ......... (528,000W ÷ 365 Days) ÷ 24 Hours = 61.42W/H ÷ 120v = 0.52 A/H average. Adding 5% loss @ pure sine Inverter and a 10% factor for start ups = 0.6 A/H. One of my panels would probably be adequate, but I'm looking at expanding the draw to use on counter top appliances for the motor home through a separate inverter.

My existing house batteries will continue to run independent of the solar battery system.

Here's what I'm getting for charge controller requirements:

ELECTRICAL DATA @ STC
Nominal Power - Pmpp (Wp) = 370
Nominal Power Voltage - Vmpp (V) = 39.8
Nominal Power Current - Impp (A) = 9.3
Open Circuit Voltage - Voc (V) = 47
Short Circuit Current - Isc (A) = 10.2

ELECTRICAL DATA @ NMOT
Nominal Power - Pmpp = 276W
Nominal Power Voltage - Vmpp (V) = 37.1
Nominal Power Current - Impp (A) = 7.44
Open Circuit Voltage - Voc (V) = 43.7
Short Circuit Current - Isc (A) = 8.02

I'm traveling so I used a Volt Correction Factor (VCF) of: 13 to 5 degree Fahrenheit ambient = 1.16 VCF

Using 2 panels in series:
47 Voc X 1.16 VCF X 2 = 109.4 V
10.2 Isc A X 1.25 (safety factor) X 1 string = 12.75 A

Panel watt conversion: [740 W ÷ 47V] X 1.25 (safety factor) X 1.16 VCF = 22.83 A
Use: 30amp 150v MPPT Controller


Using 2 panels in parallel:
47 Voc X 1.16 (VCF) = 54.52 V
10.2 Isc A X 1.25 (safety factor) X 2 = 25.5 A

Panel watt conversion: [370 W ÷ 47V] X 1.25 (safety factor) X 1.16 VCF X 2 = 26.88 A

Use: 30amp 100v MPPT Controller

Questions:
1. Do I use 'STC' or 'NMOT' data? I used worst case STC. I'd rather save the money if I can use NMOT data.
2. Are my calc's correct?
3. Is my wire gauge too big too small?

 

[time2roll]

Best to stay with STC ratings. NMOT seems to use reduced light for power testing.

Is the assembled battery 12 volts? 540 watts into 12 volts is closer to 45 amps. OK probably never get that full output but 30 amp controller will max out and limit the power on a day of perfect sun. I would run in series and get the 150v controller. #10 MC4 extension cable is fine to run down to the controller. Almost plug and play.

OK to run in parallel with the 100 volt controller. Need a couple parallel connectors at the panels and an extension cable down to the controller. #10 is still fine but #8 can be found on Amazon to run down to the controller.

 

[DDrdan]

Best to stay with STC ratings. NMOT seems to use reduced light for power testing.

Is the assembled battery 12 volts? 540 watts into 12 volts is closer to 45 amps. OK probably never get that full output but 30 amp controller will max out and limit the power on a day of perfect sun. I would run in series and get the 150v controller. #10 MC4 extension cable is fine to run down to the controller. Almost plug and play.

OK to run in parallel with the 100 volt controller. Need a couple parallel connectors at the panels and an extension cable down to the controller. #10 is still fine but #8 can be found on Amazon to run down to the controller.

Thanks for the response!

"Is the assembled battery 12 volts?"
Yes, the 2 Lithium are in parallel for 12Vdc and each has its own BMS. I'll put the controller output on a 2 way switch to swap the charge to the AGM when I need it. The inverter is 12V.

"probably never get that full output but 30 amp controller will max out and limit the power on a day of perfect sun"
I'm not sure I have enough battery capacity at 240AH to actually use the power it's going to provide on a perfect day? Based on my device usage needs the max my batteries could possibly need would be a 20Amp supply to charge all 3 batteries in less than 4 hours?

"540 watts into 12 volts is closer to 45 amps"

Each panel is 370W - 47 Voc - 10.2 Isc X 2 panels. Shouldn't all my calc's be based on that data as "controller in" volts and amps? Did I make a mistake in my calc's?

Here is what the Victron (Voc & Isc) calculator is giving me:

1S2P

• Solar Array Wattage: 680W
• Temperature Compensated Array Voltage: 53V
• Array Short Circuit Amperage: 20.4A
• Max Series Fuse Rating: 25A

Recommends: MPPT 100|50
*I'll never use 50Amps?

2S1P

• Solar Array Wattage: 680W
• Temperature Compensated Array Voltage: 105V
• Array Short Circuit Amperage: 10.2A
• Max Series Fuse Rating: 25A

Recommends: MPPT 150|50
*Why the same amp over size again? I understand the V rise but not the amp?

Thanks again for your help.
Dan

 

[upnorthandpersonal]

Think: power in == power out. A charge controller amp rating is the output current (battery side). 680W solar --> to 12V battery, means 56A max at battery side.

 

[time2roll]

Yes the controller converts the wattage to the correct battery voltage and increases the amps to keep the wattage equal. Almost like a DC transformer.

 

[MichaelK]

The way you have this planned out, I don't think this is going to work. Let's break things down to outline each part of the system.

First, you want to run an AC refrigerator that consumes 1.44kWh of power per day. Forget about hourly calculations. Focus on 1.44kWh in a 24hr period. A tier-1 inverter might consume ~30W per hour, so you need another 720Wh or 0.72kWh just leaving the inverter on. So, just running the frig, and nothing else, that's ~2.2kWh each and every day. Keep in mind that your typical AiO inverter consumes more like 100W/h, so that's 2.4kWh instead of 0.72kWh. 2.4kWh + .144kWh ~3.6kWh per day.

You have a battery that has 70Ah X 2 parallel strings X 12V = 1680Wh TOTAL. Without solar, that battery is not going to get you through a single 24h period. One day of dark, cloudy weather, and everything is likely to die.

Now, the panels. You plan on laying these flat on top of the motor home? Derate them to only 60% of the STC output for being at a non-optimal angle. If they were pointed directly at the sun, derate to 85%. So what you are likely to get in the middle of summer is.....

370W X 2 panels X 5sunhours X 60% = 2.22kWh of power.

So, with the best quality inverter, consuming the least background power, you might barely squeak by in June. The rest of the year, no. Assuming you get any kind of AiO, then forget it completely. No way you will be able to power just the refrigerator, let alone have lights, and maybe a few hours of TV in the evening.

Stop focusing on little details like wire gauge, and look more at the big picture of total power consumption. Part of the problem here is that you are trying to shoehorn 24V level appliances into a 12V shoe. A 24V system is a better choice for a large size frig like what you want to run. If you just have to stick with 12V, then get a high amperage controller, like an Epever Tracer 6420, or 8420 and three of those 370W panels instead of just two. Focus a lot of attention on the background consumption of whatever inverter you select.

With three panels wired in series, you only need 12 gauge wire to go the short distance you plan to cross.

 

[MisterSandals]

Fridge has a 15A circuit requirement and a 528kw / Year Energy rating so ......... (528,000W ÷ 365 Days) ÷ 24 Hours = 61.42W/H ÷ 120v = 0.52 A/H average. Adding 5% loss @ pure sine Inverter and a 10% factor for start ups = 0.6 A/H.

This sounds like the lions share of your planned needs. If you could monitor this for a day with a Kill-o-watt meter or similar to get a daily watt hour requirement you will be able to plan more accurately. You could be off by quite a bit, i do not know, but i suspect.

The suggestions and insights given so far have been good.

 

[Rocketman]

I full-time in a MotorHome. Last Year I installed a residential fridge (took out the “Never Cold”, at the same time I added 400w of Solar to my 800w. That cover all my loads fine in the summer. Spring & fall I need to either tilt or have really good sun. Winter I need to tilt and have good sun. I will be adding another 200w of solar (because I added Starlink).

Series or Parallel - doesn’t matter - except in the SCC purchased. (Since the 100/50 is cheaper than the 150/50 - I would go parallel). Just make sure the wires between the panels and controller are 10awg.

Can you fit one more panel on the roof if needed? Then they would need to be in parallel? Having your solar panels “overpanelled” will help then the sun is not great.

My concern is I am not sure your battery bank is large enough for the residential fridge. (If you have a poor solar day).

But with a lot of things the best thing to do is install and test it - if you need more battery bank - add an extra then. If you need more solar - add an extra then. Just keep the plans so you can add if needed.

Good Luck

 

[MisterSandals]

if you need more battery bank - add an extra then. If you need more solar - add an extra then.

This is a GREAT thought but its a LOT easier on paper than in an RV.