Solar Charging for Nerds

This is part 2 of a 3 part series about our plans on how to get across Canada with a 100% electric car towing a camping trailer. Part 1 was all about the charging plans for our trip. Since our solar panels are an important part of our trip, this blog post will give you tons more information!

Let’s get down to the nitty-gritty! But first a warning: Do not read this post while operating machinery, side effects might be severe drowsiness. At least for some, like Silke, but not myself. 😉

I hope you all red my previous blog post “100% Electric across Canada”. So, you should now know all about our plans on how to NOT get stranded with our 100% electric car and camping trailer. Let’s get into some more details here.

The fun thing about this trip is that I get to experiment with ideas and play with technical toys. As an ex-engineer, my fingers are twitching to do this kind of stuff!  We settled on the Alto R1723 camping trailer by Safari Condo, Quebec, for three reasons:

  1. Low weight, about 1,900 pounds empty.
  2. Retractable roof to reduce drag while driving.
  3. Simple, rounded roof shape gives ample space to install semi-flexible solar panels.

That means, I get to play with batteries, solar panels and electrical gadgets! Yeah!

Over the past 4 years, I have equipped several boats with solar panels and Lithium Iron Phosphate (LiFePo) batteries (some of our friends are adventurous and very trusting ;-)…). Some of these boats operate totally independent of any external charging sources for months at a time and rely entirely on the installed solar panels. What could be better than to try this on a ‘land yacht’, or should I say ‘small camping trailer’?

A typical North American house uses about 11 kWh /day averaged over a one year period. (numbers www.eia.gov) So, running a boat on solar with 7 kWh daily generation, which is easily produced on a small catamaran, makes for very comfortable living. We’ve got some hands-on experience with that.

Why Lithium Iron Phosphate (LiFePo) Batteries?

Lithium batteries are infamous for going up in flames. However, not all lithium batteries are created equal! For example Lithium-Ion, used in laptops and cell phones, are the ones most prone to overheating, but they also have the most energy per weight. Boeing made headlines a few years ago with their Dreamliner Lithium-Cobaltoxide  batteries going up in flames. One important aspect to consider with regard to the inherent safety of batteries is how much energy can be released if the batteries are compromised. The more energy is released, the easier it is for the process (fire) to get going and continue. Think of a campfire with damp wood. You may get it started with ample newspaper, but then it just dies, as the burning process cannot be maintained by the minimal energy released from the damp wood. It’s similar with Lithium Iron Phosphate (LiFePo) batteries.

The graph below shows AT WHAT temperature a lithium-type battery would release energy (horizontal axis) and HOW MUCH energy it would then release (vertical axis). The green curve shows that for LiFePo batteries, the energy release only happens at very high temperatures (300ºC/ 570ºF) and that the energy release is very small. Thus, LiFePo batteries are inherently safe.

Another advantage of LiFePo batteries is their longevity – when properly treated. Under the conditions we will experience with our camping trailer, they should last for more than 10 years. That means that under consideration of that kind of life span, LiFePo batteries are not more expensive than regular Lead-Acid batteries.

Here is what I have decided to do to keep my LiFePo batteries happy and healthy:

  • No Thermal Management – the rates of charge and discharge for this kind of application are quite low. That means it takes the equipment several hours for charging or discharging. There will be no substantial heating due to electrical loads. You just need to make sure you do not charge the LiFePo battery if it is freezing, they do not like that at all. Ah well, so no winter camping for us. 😉
  • No expensive Automatic Balancing System – I do not plan to charge my batteries past 90% state-of-charge (SOC) or discharge below 10% SOC. At the high voltage end, an unbalanced battery may cause over-charge of one or two cells. Instead, I use an inexpensive Battery Cell Monitor, which allows me to read the individual cells’ voltages. Should they drift apart, which they have not done in three years of operation (we’re reusing our boat’s batteries, hence the experience), I can balance the high cell manually by draining individual cells using a 12V automotive light bulb.
  • No expensive, special Lithium Charger – LiFePos like to be charged to a specific voltage, about 3.5V/cell, and then stop charging. To achieve this, I installed a Programmable Voltmeter with relays and solenoids to disconnect the chargers from the battery at 13.8V, or 3.45V/cell. All charging stops at that point. (See diagrams later in this article)
  • Low Voltage Protection – Just like Lead-Acid batteries, LiFePo batteries do not like to be discharged too low. However, while Lead-Acid batteries like to stay above 50% state-of-charge (SOC), for LiFePo, about 10% SOC is the magic number. The same programmable voltmeter that senses overcharging also senses when the battery is going too low. In this case, it will open another solenoid and disconnect everything that draws power from the battery, which means that all lights would go out in our trailer…

What did I do with the Solar Panels?

I installed 800Wp (Watt peak) semi-flexible solar panels to charge the battery of our camping trailer while driving. These panels can be bent a little bit. They are glued to the roof using double-sided Scotch Tape. I connected the panels electrically in two groups of four. This is a good compromise between minimizing losses due to different sun angles for each panel and the amount of cables I would have to install if the panels were all individually connected to the charger.

Solar Charging for Nerds4

For that reason, I chose the following arrangement of solar panels: Panels with the same number (1 or 2, see diagram below) are connected together in series. Electrical cables for the #1 panels and the #2 panels are then led individually to the charger.  Hence, I can use thinner and lighter cables!

Solar Charging for Nerds5

For the Solar Charger, I chose the Outback Power Flexmax 60 maximum power point tracking (MPPT) charge controller. I know this controller well from my boat installations and like its quality and performance.

Why using an MPPT Controller?

There are two styles of solar controllers:

  1. PWM = Pulse Width Modulation and
  2. MPPT= Maximum Power Point Tracking.

A PWM charge controller operates by reducing the voltage that the battery ‘sees’ from the solar panel. The data sheet below shows that our typical 12V solar panels provide maximum power at 17.8V. However, the battery is charged at about 13.5V. That means that the PWM controller will ‘throw away’ about 4.3V or 25% of the energy the panels provide. That’s not very efficient! Not what we need when in a pinch!

An MPPT controller has a buck transformer, which it uses to adjust the load on the solar panels, such that they will operate at the maximum power point. This power point adjusted every second or so for the ever changing sun conditions. The MPPT controller converts the maximum power 17.8V and 5.62A at the solar panel to 13.5V and 7.4A at the battery. Thus, I am gaining about 1.8A per panel. For eight panels, this results in an additional 14.4A . Below is typical graph from a manufacturer of an MPPT controller comparing it to a PWM controller.

Feeling ‘over-powered’ yet? 😉 Still not enough input? Good news: There’s even more information…Stay tuned for blog post 3 of 3 in this little series… and don’t miss it: sign up for automatic email notification on our homepage. Thank you!

 

8 thoughts on “Solar Charging for Nerds

  1. Thank you for all that detail, very interesting! (Caveat: I am an Env engineer; I have an excuse 😉

    I bet our Leaf could even tow that trailer weight, but we would be stopping pretty frequently to take advantage of the solar setup!

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    • Thank you! It’ll be very interesting for us to see what impact the camping trailer has on the car’s range. Rolf will test it soon and write about it.

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  2. I cant wait to follow your trip! I have a Smart EV now and awaiting the Model 3.
    If you decide to hit Niagara Falls on your tour, e mail me if you need anything. I’m so into all of this with 10kw solar up and another 3 to go to feed the Tesla when it arrives.
    Have a great time and I will be waiting to see your blogs..
    Tim

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  3. Pingback: Battery Power for Electric Car & Camping Trailer Charging Set-up | Promoting Sustainable Energy

  4. Excellent article Rolf. I’ll be picking your brain in the future when I finalize and purchase my a teardrop trailer + solar panels to go with my Model X too. You should submit these articles to Home Power Magazine for publication! It’s unfortunate my step dad passed away 3.5 years ago (he was a subscriber) – he worked for the State Highway Department in Oregon, USA as an electrician. He would have been ecstatic on what you and Silke are doing across Canada and the advances you and Tesla have made to promote sustainable energy. Wish you the best on your TeslaXCanada trip. Cheers!

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    • Thank you, Ed! If you like, we’d be happy if you shared the article with Home Power Magazine, linking to our blog post. We will have less and less time to publish anywhere but our own pages, but would be very grateful if others help us spread the word. Especially sharing with magazines or organizations we didn’t even think of! Thanks a lot, Ed! Much appreciated!

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  5. Hi
    Great adventure just beginning.
    We have the same Alto trailer in yellow, towing with an Outhback. We love it, it’s been from Ont to California already. Got the factory solar panels, replaced the factory battery with a 100ah AGM inside the trailer, less weight on the hitch plus the battery stays warmer.

    Have been building a home solar system over the last 12 years, tried to use battery power daily but that eats the lead acid battery’s in 3 to 4 years. Started looking into super caps because the have >500k cycle lives. Found some 2800F one on eBay at a good price, so bought 221 of them, built a 3600F 32V capacitor, which on sunny days provides power during peek power times an beyond. The battery voltage is just in float mode, providing power during outages, had power for a 4 day outage in the spring, with still 50%left in the battery’s.

    Once you get to Ontario we can bring our Alto ,camp and meet at you camp ground, would be great to discuss renewable ideas and methods.

    Leo

    P.S have a M3 reservation in on March 31

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