I built a solar-powered bike. It works, but not quite as well as I’d hoped.

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I started with a 2008 Currie “E-Zip Mountain Trailz” (sic; ugh) electric bike, which works pretty well.

That is powered by a pair of 12v 10AH SLA batteries, in series for 24 volts. I’ve always been afraid to ride for long distances for fear I’d run out of power before getting home. (The thing weighs 70 pounds, and I’m no athlete, so it’s painful to go up hills without power.)

I’d been thinking of putting solar panels on it to charge the batteries while I ride. I didn’t have any appropriately-sized panels around, so I didn’t do it until this summer, when I decided to try using a bike trailer to hold the panels.

The trailer solves the problem of the panels being shaded by my body when I ride, and also avoids problems with the weight of the panel hardware messing up the balance of the bike.

I have some 20 year old Siemens SM55 modules, rated at 55 watts (new, when pointed at the sun). These things are huge and heavy – 51 x 12 inches, and 12 pounds each (1300 x 330 mm, 5.5 kg). And since the eBike runs on 24 volts, I’d need 2 panels in series to charge the battery (at least without a DC-DC boost converter). That’s 24 pounds, not counting mounting hardware.

But, hey, with a trailer, who cares how much it weighs, right? And 110 watts is a lot of power – likely enough to completely power the bike while riding (on average, on a sunny day).

So I got a used bike trailer; $50 on Craigslist:

The bike trailer as I got it

The bike trailer as I got it (nylon roof & child seat removed)

I took off some of the trailer hardware to make it easier to mount the panels, and installed a plywood floor to make a storage compartment under the panels.

Then I used a couple of pieces of lumber to attach two panels together (at the ends), and mounted more lumber on the sides for mounting on the trailer. I cut slots to fit the trailer poles, and attached the panels to the trailer using standoffs and cable ties:

Cable tie straps hold the panels on

Cable tie straps hold the panels on

I also had to use more lumber to reinforce the trailer poles, which were only meant to hold a nylon sunshade (not 30+ pounds of panel hardware).

I was going to get a 24v charge controller to avoid putting too much voltage into the batteries, but I haven’t gotten around to that – I just wired the output of the panels (in series) parallel with the 24V battery; good enough for now.

Solar panel output wired in parallel with the battery

Solar panel output wired in parallel with the battery

"Watts-Up" meter is just used as a voltmeter. The venerable Garmin GPS V is also powered off the 24v system.

“Watts-Up” meter is just used as a voltmeter.
The venerable Garmin GPS V is also powered off the 24v system.

Finally, I made a bumper sticker to go on the back of the panel; it reads “CAUTION – HIGH CURRENT STAND BACK 500 MM” (just visible in the last picture below, if you zoom in). I figured that would keep the ignorant from messing with the bike (and it fits my skewed sense of humor).

So, how does it work?

It works. Mostly, sort of. As long as I’m riding in full sunlight I can go indefinitely if the road is level or not too steep.

I tried a test run riding to work – about 12 miles – on a sunny day.

I weigh nearly 220 pounds, the bike is 70 pounds, and the trailer with the panels is something like another 50 pounds. So the whole setup is maybe 350 pounds. That’s too much for the motor to pull up a steep hill without a lot of help from me.

I got to work OK and not too tired, in about 90 minutes (about 8.2 mph average – not at all impressive by road bicyclist standards). I did have to work my way up hills.

At work I got delayed and ended up going home close to dusk. I started out riding in the shade. Halfway home, I had a dead, dead battery and about 7 miles still to go. And 350 pounds to drag up each hill. After mighty struggles, I gave up and called for a ride home.

A week or so later, I tried the same trip on a light steel street bicycle (about 20 pounds, with narrow smooth tires and no suspension – in other words, efficient).

I got to work again in about the same time, only slightly more tired than with the electric bike. With the vastly lighter and more efficient bike, going up hills with that was no harder than with the solar bike (with the motor helping). And I was able to get home with no problem.

In the end, a simple street bike turned out to be just as fast, not much more effort to ride, and way more practical – it even works in the shade.

But it was fun to try.