Sizing a MPPT for RV system

Bob_Marley007 · October 2019

I have three 370W solar panels that I plan to install on my travel trailer in parallel. Their specs are

Vmp – 38.15V

Imp – 9.7A

Voc – 47V

Isc- 10.15A

My battery set up will be two 180AH 12V LifePO4 batteries.

What is the size MPPT charge controller would I be needing?

I am confused on the calculation. I have read both ways to calculate.

1. Take total watts of solar array divided by battery voltage in my case 1110w / 12V = 92.5Amps so would need a min 100amp MPPT

2. Each panel produces 9.7A so in parallel a total of 29.1A so only need a 30A MPPT

Which is correct?

Also since this is on a travel trailer is there anything I need to do differently when installing the batteries to compensate for the way they charge from the truck alternator when the 7-pin connector is hooked up? I do know that the 7-pin connector hot lead is only live when the truck is running so there is no chance of the TT batteries draining the truck batter, but I am not sure if the alternator which is 80amps can handle the TT battery bank with no ill effects.

Thanks all for your time.

BB. · October 2019

Welcome to the forum Bob,

I presume that you are running a 12 volt @ 360 AH battery bank (not 24 volt @ 180 AH).

Depending on the brand/model of MPPT charge controller, you can run your three MPPT panels in series (Vmp-array=114.45 volts) or run the three panels in parallel. With 3 panels in series, that is a relatively high Vmp-array voltage, and you would probably need an even more expensive MPPT charge controller (higher Vpanel input rating)--That would not make sense for you project (unless you were going to have a "portable" array that would be 100's of feet from your trailer--Trailer in shade with panels in sun).

So, assuming your panels are in parallel. There is the maximum "useful" rated controller:

3*370 Watt panels * 1/12.0 volt battery bank = 92.5 Amp rated @ 12 volt output charge controller

And there is the typical maximum wattage will output to the charge controller... Hot panels (cells) produce lower voltage than cold cells--And the Vmp-std test condition is done with "cool cells" (around 25C). In real operation, the cells are much hotter and the output power is reduced (power=voltage*current, lower voltage, lower power for the MPPT controller to "down convert"). So, the typical derating is around 75% to 77% of solar panel output (you will only reach ~77% of rated output on cool/clear days at/round noon, and will not exceed that value unless you are in subfreezing weather). So, the typical MPPT "cost effective" controller would be rated around:

3 * 370 Watt panels * 0.77 panel+controller * derating * 1/14.5 volts nominal charging voltage = 58.9 Amp @ 12 volt rated controller

MPPT controllers can safely and effectively control their maximum output current (based on actual current, and controller internal temperature). So, it is perfectly fine to have a "larger than rated" solar array.

As always, temperature is the death of electronics (and batteries). You want your system installed in a part the RV with good air circulation so that everything keeps cool.

Also, you you have choice of controller that have active cooling (fans) or passive cooling (big heat sinks). Also, if you will be in the trailer during the day, you might want to avoid fans in the charge controller (and AC inverter)--They can be a bit annoying if you are looking for peace and quiet.

For your project, I would suggest looking for 60-90 amp charge controllers... And 60 amp will work fine for you. There will be rare conditions when the array will output more current/power (subfreezing weather, blanket of reflecting snow, etc.).

-Bill

BB. · October 2019

And to answer the rest of your questions... The rating of MPPT charge controllers are (mostly) based on the maximum Input voltage, and the maximum output current. With your Vmp-array voltage of ~30 volts, and the MPPT controller taking high voltage/low current and "down converting" to lower voltage/higher battery charging current, that is what they do (sort of like the DC version of a transformer).

In my limited experience, your trailer will probably see a maximum of ~10 amps through the trailer connector. The long run of 10-14 AWG vehicle/trailer wiring will not let much more current through. For example, say 15 feet of 14 AWG cable with 2 volts of drop (14 volts at alternator, 12 volts at trailer). Using a voltage drop calculator:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=12&phase=dc&noofconductor=1&distance=15&distanceunit=feet&amperes=10&x=51&y=20

You will get something like 0.76 volt drop in the wiring... And depending on the isolator (relay, solid state), you may get more drop.

At 14.0 alternator voltage - 0.76 volts = 13.24 volts at the battery--Which is not really that high of charging voltage and will not quickly/fully recharge the Li Ion (or even lead acid) battery...

And since your solar power system charges at ~14.5 to 14.8 volts, it may end up trying to "charge" your truck power system as you drive down the road... With newer vehicles, I guess, that this could cause a fault in the vehicle's system (just a guess-I have no idea what new cars do in terms of monitoring these days).

You have to look at the voltage ratings of your LiFePO4 batteries to make sure you charge them correctly. And you need to double check that your MPPT charge controller has temperature compensation turned off (Lead Acid batteries require higher charging voltages when they get cold--Li Ion do not).

And make sure your batteries do not go below freezing when charging... Most Li Ion batteries should not be charged when below ~32 to 25 degrees F or they will be damaged (some BMS systems may shutdown battery in subfreezing conditions). If camping in snow--Then you may want the option to insulate your battery box to keep it warm. And discharge capabilities drop from 20F to 0F... Low temperatures is where Lead Acid batteries are typically "better" than Li Ion types.

If you actively need charging for your trailer from the vehicle while driving--You might need to look at a DC to DC or use an AC inverter to AC battery charger to "properly" use the vehicle to recharge the trailer. The 12 volt line through the trailer plug is just not going to do much.

-Bill

Bob_Marley007 · October 2019

@BB. I am in Florida and the trailer will mostly be used here and up north only in the Summer so don't plan to ever have it in freezing temperatures so don't think I will see any of the occasional current spikes from cold temps. So I think a 60-90A MPPT may be the way I go.

Thanks for your input.

BB. · October 2019

You are very welcome Bob,

-Bill

mcgivor · October 2019

Also since this is on a travel trailer is there anything I need to do differently when installing the batteries to compensate for the way they charge from the truck alternator when the 7-pin connector is hooked up? I do know that the 7-pin connector hot lead is only live when the truck is running so there is no chance of the TT batteries draining the truck batter, but I am not sure if the alternator which is 80amps can handle the TT battery bank with no ill effects.

Do some research into charging Lifepo4 with an alternator ,

Lithium batteries have extremely low resistance which can potentially be harmful to the alternator causing it ro fail, Victron has some videos on YouTube which discuss this and methods to solve the issues. Personally I've not dealt with anything on the subject but have seen said videos so thought I would relay the information so as to prevent possible dissapointment.

https://youtu.be/jgoIocPgOug

mike95490 · October 2019

On a trailer, with 38V panels and a 12v battery system. You could see 85A from the array in good conditions (rare)

I would wire the array in parallel, with a 3 channel circuit breaker combiner box.

The advantage of wireing in Series, is you don't need the combiner, but still need a disconnect breaker

The disadvantage of Series, is if shade falls on one panel (Tree shadow, antenna, whatever) it will throttle the entire array back, activating the bypass diodes in the shaded panel.

The advantage of wiring in parallel, is shade only affects the 1 panel.

The disadvantage is you need to use a bit more wire.

Either way you need a 60-80A controller. The better 60A controllers can easily handle being overpaneled a bit, saving the expense of a 90A controller. You only loose a little bit of charging possibility. Panels flat on a roof, don't generate full power because the angle is bad.

I'd suggest the Morningstar Tristar MPPT-60. It's fanless, and has a built in web interface, and is programmable to different settings

Sizing a MPPT for RV system

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