Monday, December 12, 2016

RV - Solar Charge Controllers

Another long, non-moto post. This, as well as the previous post, are written to help me solidify my thoughts on RV mods. Still too cold for riding (-20°F for most of the week).

My justification to installing solar panels is convenience not dollars. If there is a reasonable amount of sunshine hitting the panels each day, the battery bank will be full every afternoon without running a generator. Less noise, less hassle. Plus, since the RV is considered a second home, I heard that there is a 30% tax credit available until 2019. Then starts decreasing until 2021 where it stays at 10%. I got my information off of the site.

I have been doing a lot of reading on RV solar installations and one of the first sites that I found with really useful information was which is the source of this photo. Ray, the author of the site, has posted a lot of information and reviews including a number of YouTube videos documenting his modifications including the installation of solar power. One of the continuing debates on the solar front seems to be around the two dominant charge controller technologies. Ray defers to other articles such as this one on the Bogart Engineering website which explains why PWM controllers are more cost effective for smaller installations. There has been a lot written about both.
PWM, or Pulse Width Modulation, takes the input current from the solar panel and varies the duty cycle when sending it to the battery. This is shown in the figure on the right. This is an overly simplistic description of how they work. Most of the decent charge controllers provide up to four charging profiles. Bulk, Absorption, Float and Equalization. The last one, equalization, only applicable to flooded batteries. Most PWM controllers are fairly sophisticated and will modify their charge rate based on the type of battery, current charge level and battery temperature so your batteries won't get ruined from overcharging. The panels you select must be designed for the battery voltage you are planning on using. For example, a 100 watt panel for a 12 volt batteries have a operating voltage of around 18 volts. Since the panels are rated at their operating voltage, the peak current for this panel would be 100 watts/18 volts = 5.56 amps. Since the PWM controller just rapidly switches the current on/off based on the charge state of the battery, the current is limited to 5.56 amps.
MPPT, or Maximum Point Power Tracking, takes the full input power from the solar panel (Pmax) and as the output voltage decreases, the current is proportionally increased resulting in almost as much power output to the battery. This is an oversimplification as well. Most MPPT controllers also track battery temperature, battery voltage and panel output voltage. They continuously correct and compensate using a variety of algorithms to stay at Pmax. There is a lot of Internet discussion about the efficiencies of these algorithms but the differences are more applicable to much larger solar installations than you would find on an RV. Except for conversion inefficiencies, the full power of the panel will be sent to the battery. In other words the charge controller will adjust the output current to always deliver almost the full power of the panel to the battery.

PWM: If the battery was deeply discharged, say down to 11.8 VDC, the power going into the battery could be as low as 11.8 V if the battery was still being discharged at a rate greater than the solar charge rate. Using a 100 watt panel rated at 18V
11.8V x  5.56 amps = 66 watts
As the battery charges, the voltage and watts going into the battery increases. But even at 14.4V the current is still at its maximum value of 5.56 amps.
14.4V x 5.56 amps = 80 watts

MPPT: Same situation with an MPPT charge controller. As you can see, as the output voltage decreases, the available current increases. Power-In = Power-Out.
5.56amps x 18v/11.8V = 8.5 amps
8.5 amps * 11.8V = 100 watts
5.56 amp x 18V/14V = 7.15 amps
7.15 amps x 14V = 100 watts

The MPPT controllers cost more. And there isn't a whole lot of difference in real world performance, especially with a modest system. The efficiency drops as the voltage difference between input and output increases plus there is always a conversion inefficiency. Heat would be a byproduct of that inefficiency. Also, the higher the solar panel temperature the lower the output voltage and the smaller the difference in power output between the two controller technologies. So in the example above, output power would be 90 watts assuming a 10% loss in the conversion, In reality, while the batteries are being charged, they would more likely be in the 13V - 14V range so the difference between the two technologies is in the 10% range. On a multi kilowatt system, 10% is significant. On a 400 watt RV mounted system, less so...

This is one of the charge controllers that I'm considering. It is the Renogy 40 Amp Commander MPPT Solar Charge Controller (CMD40) which is rated at 40 amps. On a 12V system, it will handle up to 520 watts of panels. It has an optional remote monitor, the MT-50, that is required for any advanced configuration. If you really want to dig into things, there is an optional USB cable with Windows software that could be used instead of the MT-50. Battery temperature uses an optional probe that you attach to a battery terminal. There were some negative reviews on Amazon but they were from people that didn't realize that you need a specific remote monitor. They bought the cheapest on Amazon which wasn't compatible with this particular controller. The CMD40 has a maximum input of 150 VDC so I am not limited to 12V panels (Vmp = 18V) or having to run the panels in parallel. A decision on which panels has not been made. The conversion efficiency of this controller is claimed to be 96% using 400 watts of 12V panels and drops to 95.5% using 400 watts with 24V panels. I like the flexibility.

The other charge controller being considered is the Morningstar Tristar TS-45 shown here with the optional display. It is a 45 amp PWM charge controller that has been around for a while and has a lot of fabulous reviews. It uses an external battery temperature probe on the battery bank for temperature compensation. It has an optional remote display to show current from the panel and voltage but the remote isn't needed for configuration like the Renogy Commander. There is a RS232 port for custom configuration. Just using the dip switches, you can choose battery profiles and there is one for flooded, lead acid with absorption voltage of 14.8V. This controller will support 800 watts of panels and you are limited to 12V panels only. Battery equalization can be done manually or automatically. Manual equalization is important due to the higher voltages needed at lower temperatures. For example, at 0°C the equalization voltage is over 16V. This is over the limit of the inverter even when powered off. In fact, all of the DC loads should be disconnected during equalization. Since I have the Trimetric 2025, I really don't need a separate charge control monitor.

Under optimal conditions, 4x400 watt panels, 450 amp-hr battery bank discharged 50%. The approximate time to fully recharge the battery bank would be 8 to 10 hrs. The "real life" charge time would be about 20% longer since the battery bank will be flooded lead acid batteries which, according to what I've been reading, needs about 20% more power put in over what was used. 


  1. Getting a tax incentive for the installation of solar power on the RV? Awesome! Never gonna happen in Germany, where you are supposed to live in a "real" home not a mobile home...

    1. RVs have had the "second home" classification here in the U.S. for years. I thought that the solar power tax credit had already expired but it was renewed for five more years until 2021. After that, it'll gradually decrease over the next few years.

  2. I really must look into the second home, 30% tax credit for solar equipment! So glad you're doing all the research on this area RichardM as I plan to leech off your efforts. :)

    We'd discussed the ROI and yeah, it still remains but a dream UNLESS you do a lot of boondocking, I'm talking a lot.

    The present RV'ing paradygm that Martha and I are entertaining doesn't have us in one spot more than 2-3 days before moving on to the next spot.

    1. There was more to the comment but decided it was too much for a reply comment. Maybe a posting is needed.

    2. You're right about ROI. We don't have a built-in generator either that feeds off of the a large gas tank so running a generator is definitely more hassle. I'm including state and regional campgrounds without power as well as Walmart and Cabellas in the "boondocking" category. No noise and less hassle has value.

    3. Yep, I understand, I do believe solar would be best in the walmart/cabella scenario as I believe etiquette demands no generator usage when in such locations. Then again, I only see those locations for one night stays enroute to somewhere else.