Help with panel and charge controller selection/design

Ok... A little math here (yes, there will be math).

Let's make some assumptions. 90 AH per day @ 12 volts. And 6 amp heater 12 hours per night, at 50% duty cycle. With a 12 volt 260 AH battery bank.

Typically, we use 2 days of storage and 50% maximum discharge from the battery bank for Off Grid Cabins and Homes--That is 4x the daily usage of the storage battery. Gives you long(er) battery cycle life

For RVs, you may want to design for 1 days storage and 50% discharge... RV batteries tend to "age out" before they "cycle out", and given the limited about of space and weight limits, RVs cannot just "throw more lead" and solar panels at the problem. Generally, if you have poor weather or heavy loads from the day before, you may just choose to fire up the genset when needed to charge/run that day's loads.

For an RV system (weekend/few weeks a year usage), planning on cycling the FLA (flooded cell lead acid batteries) between 50% and 80% state of charge is a "reasonable" design goal (charging from 80% to 100% state of charge can easily take 4-8 hours by itself--May not be enough hours of sun in a day, and you may not want to run the genset that long lightly loaded (noise, fumes, fuel).

Your battery bank:

• 260 AH * (0.80 - 0.50 state of charge cycle) = 78 AH of typical cycling capacity.

And if you "stress the battery bank" for an 80% to 20% State of charge:

• 260 AH * (0.80-0.20 SoC cycling) = 156 AH

So, your 60-90 AH over 24 hours is not a bad expectation. And:

• 6 amps * 12 hours overnight heating * 0.50 duty cycle = 18 AH overnight cycling

Again, not bad overall.

And what would you expect from your 400 Watts of Solar Panels (fixed array optimally tilted by season)--Pick Denver, you need to find major city near your camping site (10,000 feet, probably much better harvest unless you are on the north flank of a mountain):
http://www.solarelectricityhandbook.com/solar-irradiance.html

Denver
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel where the angle is adjusted each month to get optimum sunlight.

Jan	Feb	Mar	Apr	May	Jun
4.58	4.99	5.58	5.78	6.24	6.73
Jul	Aug	Sep	Oct	Nov	Dec
6.21	5.89	5.87	5.71	4.79	4.56

A 400 Watt panel tilted to optimum angle (more vertical in winter, more horizontal in summer)... Pick December as typical winter month:

• 400 Watts * 0.61 DC battery based solar system eff * 4.56 hours of sun (ave Dec) = 1,113 WH per average Dec Day
• 1,113 WH per day / 13.6 volts float voltage =~ 82 AH per day harvest
  

While I would be suggesting that you would only "plan" on 50% to 65% of daily predicted harvest (to minimize genset use), certainly in good weather you are pretty close to breaking even in good December weather with your planned setup.

If you where looking for the "typical maximum" solar array for your present battery bank--Assuming ~13% rate of charge max:

• 260 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = ~636 Watt "cost effective" maximum array

All in all, not bad for an RV system.

Your thoughts?

-Bill

We try to "set expectations".... It is very easy to overestimate what a solar power system can supply vs the relatively high power our modern loads consume.

The 13% rate of charge is a good bookmark for the maximum "cost effective" charging current (i.e., 260 AH (at 20 hour rating) * 0.13 rate of charge = 33.8 Amps) for flooded cell deep cycle lead acid batteries. You can charge faster, but then you need to monitor the battery bank temperature (most middle to high end solar charge controllers do have a remote battery temperature sensor). As the battery get hotter, the charging voltage drops (typically -0.005 volts per C per Cell @ 25C standard temperature). It is possible to get into a runaway battery charging condition (high charging current, batteries get hot, battery charging voltage falls, charge controller sees as battery needing more charging, battery gets hotter, etc.).

Very roughly, lead acid batteries will accept high charging current with relatively low thermal rise below ~80% state of charge... As the battery bank gets over ~80% SoC, the battery charger should hold say 14.75 volts, and the battery will accept less and less charging current. Above 90% to 100% SoC, the battery acceptance will fall to ~1% (older, FLA batteries) to even 0.01% or less (AGM and newer batteries).

Regarding PWM and MPPT controllers... Pulse Width Modulated controllers are simply (more or less) just an "on/off" switch that cycles 10's to 100's of times a second (more on, more charging, more off, less charging current).

MPPT Maximum Power Point Tracking controllers. Are (usually) Buck Mode (down converting) switching power supplies. A switching power supply can take high(er) voltage and low(er) current from the solar array and efficiently down convert to high(er) current and low(er) voltage to charge the battery bank.

More or less, because of various reasons, MPPT and PWM controllers end up being similar efficiency... PWM controllers are much less expensive (and usually require more expensive "12 volt" panels--for 12 volt battery bank). MPPT controllers are much more expensive but can use a "high voltage" solar array that can use (usually) less expensive "Grid Tied" solar panels.

More or less, smaller solar array (400 Watts or less) and PWM controller can be built for less money.

And >800 Watt arrays + MPPT controllers can be cheaper/easier to install (cheap panels + expensive MPPT controller).

MPPT controllers do have an advantage in cold weather--In subfreezing weather, solar array Vmp-array voltage rises and MPPT controllers can take that 10-15% or so voltage increase and turn it into more charging power for the battery bank (and running your day time loads).

From my point of view, MPPT controllers excel at running high voltage arrays that can be some distance from the charge controller and battery bank (100's of feet, if needed). PWM controllers need the wire runs from the array to the controller to be pretty short and heavy gauge copper wiring.

Other MPPT controller advantages include larger/more power microprocessors and networking. More optional charging and control algorithms. And the ability to better charge a battery bank in very hot climates (when Vmp-array falls, it approaches the charging voltage of the battery bank.

Regarding predictions of system performance and comparisons--Predictions are good to ~10% (i.e., two measurements/predictions that are within 10%, are pretty much dead nuts the same).

I would not worry about 14.5% rate of charge--Get a charge controller with a remote battery temperature sensor and you should be OK (PWM). Or, with MPPT controllers, many can be reprogrammed to limit their output current (if needed/desired).

Before you purchase hardware, I highly suggest that you do a few back of the envelope designs and see which works best for you (i.e., a PWM vs MPPT design). Look at the installation issues and your system needs/costs.

Solar panels need to be "matched" to PWM controllers... I.e., you need Vmp~17.5-19.0 volt rated panels. With MPPT controllers, you can buy other panels (like 2x 200 Watt Vmp~30+ volts) and run them in parallel (or series) to the MPPT controller... MPPT controllers are much more "forgiving" regarding array and panel choices/configuration (note: this is a complex subject, and you need to do the paper design and make sure the panels you are looking at will work for your needs--There are matching and min/max voltages for the array that need to be matched to the controller input limitations).

What are the 200 Watt panels Imp and Vmp specifications? For "normal" solar panels, Vmp~18 volts is for "12 volt" battery systems. There are portable panels that may not be "standard" match for PWM on 12 volt battery bank. Also, if flexible panels (plastic faced vs glass, or amorphous cells vs crystalline cells) they typically have a shorter life than the normal "glass+aluminum framed crystalline panels.

There are a lot of choices out there. Use the first pass paper design (as we went through your electrical needs vs hardware needed to generate the power--I.e., sizing battery bank, array, charge controller) and read about some of the controllers and see which ones you like/are worth the money to you.

-Bill

Back at my computer:

As an example, 400 Watt array with 20 Volt Vmp:

• 400 watt * 1/20 volt Vmp * 1.25 NEC derating (always check manual, not sure 1.25 is needed) = 25 Amp minimum rated PWM controller
• 400 Watt array * 0.77 panel+controller deratings * 1/14.5 nominal battery charging voltage = 21.2 Amp "minimum" MPPT controller suggested

So, at a bare minimum you are looking at smaller solar charge controllers if you plan on present 400 Watt array.

Here is a modern/fairly high end 30 amp MPPT controller that will accept Vmp-array~21-100 VDC (Standard Rating--marketing spec.).

https://www.solar-electric.com/midnite-solar-kid-mppt-solar-charge-controller-black.html (MPPT from Midnite)

And relatively simple (no bells and whistles) PWM from MorningStar:

https://www.solar-electric.com/morningstar-prostar-ps-30-solar-charge-controller.html (PWM from MorningStar)

This are both good USA companies with phone/online support. Given USA companies, you will be able find much less expensive units if you cast a wider net.

Whatever you end up picking, both of these are well documented and you can see what the range of options are.

I do suggest you get controllers that have a remote battery temperature sensor (or controller is mounted in same space as batteries for ambient temperature measurement).

If you like lots of data, there are Battery Monitor systems. There is an integrated option with the Midnite Kid... Or you can go with completely separate Battery Monitor... A few options:

https://www.solar-electric.com/midnite-solar-whiz-bang-jr-current-sense-module.html (remote shunt for BMS on Kid Controller)

Other battery monitor options:

https://www.solar-electric.com/search/?q=battery+monitor

Trimetric RV 2030 is full monitor (and not easiest to program). (shunt based)
Xantrex and Victron are very nice units (some with programmable remote alarm contacts). (shunt based)

And voltage monitoring BMS based:

MidNite Solar MNBCMS  (easier to wire--May be "good enough" for a simple system)

Again, these are just suggestions to start your search... There are lots of other good products on our host's website, and other places (cough, cough EBay).

Your charge controller should be mounted very close to the battery bank (short heavy wiring, around 0.05 to 0.10 volt maximum drop for Charge to Battery Bus wiring drop).

If you are going to have fairly long distance from the array to the trailer (park RV in shade, stake the panels in full sun, use an "extension" cord from panels/array to charge controller--And MPPT type usually works much better (Vmp-array higher, Imp-array lower, can use much smaller AWG cable from array to controller). Make sure ground mount array is secured against wind/tripping, and cannot easily grow legs and walk from camp.

Note--Some folks like to use twist lock plugs for this cord to remote array connection. Downside is if somebody drives away and forgets the array, twist lock plugs will drag your array behind the RV (learned the hardway decades ago at local fire station, people kept tripping on AC charging cable for fire engine--Somebody got the great idea to use twist lock--And, sometime later, left the engine with the AC cord still connected...).

-Bill