Article

How To Make A Solar Powered Electric Bike


by ⋅ Founder and Editor-in-Chief, OutdoorGearLab
Thursday June 21, 2012 7:26pm
 
Click to enlarge
Solar Powered Electric Bike with Goal Zero Boulder 30 panel (battery and inverter are hidden in panniers).
Credit: Chris McNamara

All electric vehicles, bikes, cars, and scooters suffer the same flaw: short range. Of course range is all relative. If you bike two miles a day to the store and back, most electric bikes are just fine. But if, like me, you are trying to make your electric bike replace more and more car trips, you always want more range. With that in mind, I set out to see if I could extend the range of my electric bike using a solar panel.

The Bike
My Trek Valencia+ is the best electric bike I have used (I have tested four so far). It has a range of 7-30 miles depending on how much pedal power you put in. I generally get 15 miles between charges. I can get to work and back no problem (14 mile round trip). But to go to the San Francisco and back is a 30 mile round trip. To do this, I would barely make it one way, then have to recharge.

NOTE: if you are new to electric bikes, range numbers are very subjective. They depend entirely on how you ride the bike and the terrain. It's like a Toyota Prius. I can get 65+ mpg if I hyper-mile on flat terrain. I have a friend who rarely gets more than 40mpg. But in general, I get around 50 in normal driving. The range of electric bikes swings even more.

The Panel
Click to enlarge
Goal Zero Sherpa 120 Explorer Kit charges a peddle-assist electric bike so that the gear review can get home, West Marin.
Credit: Chris McNamara

I have used the Goal Zero Sherpa 120 Kit to charge my bike before as seen in the photo to the right. It works, but the flexible Nomad 27 Panel panel that comes in the kit is too floppy to ride and charge at the same time. So I opted for the Goal Zero Boulder 30 panel. This panel uses less surface area than the Nomad 27 and puts out slightly more power (30 watts vs 27 watts). The downside is that the Boulder 30 is heavier (6.6 lb vs. 3.4 lb). I secured the panel with bungee cords so that I can still reach into the panniers and access the battery and inverter.

Click to enlarge
Solar Powered Electric Bike with Goal Zero Boulder 30, Sherpa 120 and inverter.
Credit: Chris McNamara

The Battery and Inverter
In a perfect world the panel would plug directly to my bike's battery. I have not found a way to easily do this (send me an email if you know how I can). So I have to plug the panel into an inverter that then goes to an inverter which then goes to the bike's inverter and finally to the bike's battery. The power is going from DC to AC to DC. This likely results in a 20 percent-plus efficiency loss. Also, the battery and inverter and bike charger add another six pounds and a lot of cables to the system. Not ideal, but it works.

Did it work?
Yes. The extra Goal Zero Sherpa 120 battery alone (which starts out each day full) gave me another full charge. And in the course of a day, with my bike in the sun five-plus hours, I get another full charge. So I essentially tripled the range of my electric bike! I used to get 15 miles and now I get 45.

That said, there are some major downsides. The Trek Valencia+, which is already very back heavy, is now even worse. The whole system weighs 12.7 pounds. It is also expensive. Here are the costs and weight breakdowns:
Bike charger: 1.5 lb.
Boulder 30 Panel: 6.6 lb. $240-300
Sherpa 120 Battery 3.6 lb. $320-400
Sherpa Inverter 0.8 lb. $43-80


Bottom Line: Should you make a solar electric bike?
This was a fun experiment but the cost will not be worth it to most people. Better to just bring your charger with you everywhere and find a friendly cafe that will let you plug in for a java break.

If there was a way to plug the panel directly into the the bike battery, it would eliminate most of the weight and cost.

More Solar
  • I did a complete Portable Solar Review. The Sherpa 120 kit that was used here won Editors' Choice in 2012.

  Article Views: 24,594
Chris McNamara
About the Author
Climbing Magazine once computed that three percent of Chris McNamara's life on earth has been spent on the face of El Capitan—an accomplishment that has left friends and family pondering Chris' sanity. He's climbed El Capitan over 70 times and holds nine big wall speed climbing records. In 1998 Chris did the first Girdle Traverse of El Capitan, an epic 75-pitch route that begs the question, "Why?" Outside Magazine has called Chris one of "the world's finest aid climbers." He's the winner of the 1999 Bates Award from the American Alpine Club and founder of the American Safe Climbing Association, a nonprofit group that has replaced over 5000 dangerous anchor bolts.

Chris is a graduate of UC Berkeley and serves on the board of the ASCA, and Rowell Legacy Committee. You can follow him on Facebook, Twitter or ChrisMcNamara.com. He also runs a Lake Tahoe Vacation Rental.

Comments
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Matt M · Climber · Alamo City  Jun 18, 2012
10:24am PT
Chris - Couple of notes on this:

The boulder 30 is certainly a nice panel but it's cost per WATT is WAY higher than other panels out there. You could save a TON of money buying another brand and just DIY a GoalZero connector.

In my testing the nomad 27 doesn't get anywhere CLOSE to 27W of output. I've never seen much more than 11W. I suspect you'll see a large jump in performance using that Boulder 30 (assuming Goal Zero didn't grossly overrate that one too).
Vladimir · Hunter · wenatchee, wa  Aug 5, 2012
11:26pm PT
I have had the same frustrations with solar charging my electric Trek bike.

Here on my blog is my experiment with solar charging:
http://usbackroads.blogspot.com/2012/04/trek-electric-bicycles-part-two.html

It is towards the end of the article. I used a jumper battery as a buffer and the charging the Trek even with a 125-watt solar panel quickly depleted the battery.

I would be careful about hooking a lithium-ion battery directly (through an inverter) to the battery. The lithium-ion batteries have to been charged fairly carefully and quite frankly I am not sure how good the Trek charger is at tapering a charge and how it responds to the fluctuations built into direct solar charging.

I would like to see an electrical engineer evaluate the system. In the perfect world I would like a lithium-ion charge controller that is NOT the Trek supplied charger.

I did contact several electric bike manufactuers to ask about solar chargers, but there was little interest.

I do have an off-grid solar house and here is my success with a solar-powered fishing boat: http://usbackroads.blogspot.com/2012/05/solar-powered-fishing-boat.html

It would be nice to get the electric bicycles work as well as my solar electric pontoon boat.

Vladimir
Chris McNamara · Founder and Editor-in-Chief, OutdoorGearLab  Aug 25, 2012
09:52am PT
Matt, true that the Goal Zero is expensive per watt. But what I love about it is few parts and plug and play. There are thousands of solar options out there that would be better and costs less… BUT they require, as you mention DIY electrical work. That is great for the DIY'ers, but there is a massive audience out there that has little to zero electrical knowledge that just wants to buy stuff, plug it in and see it work. Hopefully more solar companies will create solar solutions that require little to 0 electrical knowledge. (I know DIY'ers are scoffing right now and saying "geeze its really not that hard to understand watt/volt/amp/charge controller relationships" which may be true… but it is just complicated enough that is is keeping probably 95% of the potential audience for more varied solar applications away).

We currently do not have a set up to evaluate panel performance (and publish them with any authority), but it would be great to get an expert to test all these panels independently to see the actual wattage. I have solar panels on my house and in shopping for those panels, which I imagine go through more stringent "wattage certification", I was told by installers that the actual wattage can very 6% from what the manufacturer claims.


Vladimir, great blog posts! Your experience with the panel actually draining the battery highlights my comment above to Matt. It will be great when there are more products that make it very clear what you can and can't charge with a solar panel set up. I did not do any long term testing on my bike and it could be I was screwing up the battery in a way that would compound over time. And I agree the Trek charger just doesn't feel as precise as it should.

You have a great line in your Trek review "We have joined the flat earth society as all the hills have disappeared due to the bicycles."

Are you going to make up some stickers "FLS" or "Flat Earth Society: member". Would be great to put that on back of the E-bike
Palomarbob · Camper · Southern California  Nov 22, 2012
09:55am PT
Palomarbob math 2007
This will help you
((Mph)^2.51) ((# miles/day)^1.45) ((# seats)^0.305) = (457) (solar panel watts)

10 mph
50 miles/day
1 seat
= 206 Solar panel watts
Varoum · Backpacker · Montréal, Canada  Jan 4, 2013
10:45pm PT
Things could be made easier. If one could fit, on the bike, a Goal Zero battery with a matching solar panel. Then went off the bike. You could use the battery to power something else and also charge it.

OK! I am dreaming just a bit. But it was fun in the process. ;)
Terry Hope   Oct 3, 2013
05:10pm PT
I also built a solar bike, except my will work on solar power alone without any battery needed…

project: SolarCross
http://www.solarcross.thekpv.com
bmelendez   Jan 2, 2014
01:05pm PT
I have an electric bike with lithium ion battery. It is a yukon trail brand. Would I be able to just get inverter and solar panel for use with existing battery?
Rusdy · Mountain Biker · Australia  May 5, 2014
03:31am PT
but there is a massive audience out there that has little to zero electrical knowledge that just wants to buy stuff, plug it in and see it work

How true. Even after 2 years since the original article, I haven't seen any system that fits your criteria above (i.e. for bigger market audience). Solar charging for electric bike is definitely still in a niche market.

At the premium end, this website offers flexible panel to charge directly to battery (specific to 13 lithium in series tho'). However, at USD1200 for the 120W flexible panel? It has to be the end of the world scenario for me to get this one.

To date, best combo I've found is if you have any panel (which usually 12 or 24V panel) with Genasun boost MPPT charger. If you have 48V battery system, you're in luck, as that is much cheaper than the custom voltage one (if you have 36V battery system). There are more and more bargains out there for this small panels.

For myself, I have solar charging system (components only) that cost me less than USD200, even including the 12V 80W solar panel (I struck a bargain with the panel). DIY charger tho' (link here).
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