This is a solar bike
manifesto primer for anyone who has an ebike and is considering adding solar charging. It’s difficult to write for all levels from novice to expert so this advice is most likely to be helpful to someone who has installed a DIY ebike conversion kit. If your experience is limited to riding a factory, turn-key ebike and you don’t do your own maintenance and repairs then you will want to get help from a friend or family member before trying a solar upgrade. Hint: find someone who has their own multimeter.
I have logged 50,000 miles (80,000 km) on ebikes over the past 13 years. About a quarter of those were touring and road-testing miles with various solar panels for charging the bike. I’ve built several solar ebikes over the years and I’m pleased to report that it is indeed possible to combine these technologies. Whether or not it makes sense to do so will depend on your goals and your budget.
Maybe solar panels are not for you?
If you’re only doing a couple of weekend ebike trips each year, my advice would be to skip the solar panels, borrow a friend’s battery and plug-in charger in addition to your own and find electrical outlets along the way. In fact, anyone who has reliable access to electrical outlets at the end of each day will find that carrying solar panels on the bike is less convenient and more expensive.
But I WANT to put solar panels on my bike!
Ok, I get it. You’ve seen photos and videos of solar bikes and you want to get in on the fun. Maybe you’re curious about solar power and want to extend your range? Perhaps you want to use solar panels as decorative plumage to attract a like-minded mate? It’s certainly a conversation starter. If you don’t enjoy being the center of attention wherever you go, this might not be the path for you.
I’ve come to think of solar upgrades as falling into two broad categories:
- Range extenders: 50-200 watt solar panels to supplement your plug-in charging. Expect to get about 5 miles (8 km) of added range for every hour of charging under ideal conditions with a 100 watt panel. This is a great beginner project because it keeps the cost and complexity low while you learn the basics. You can always upgrade later. Folding panels can be stowed in your panniers and deployed when stopped or strapped to your bike/trailer to collect energy all day. Most trailers will fit a 100-150 watt panel easily.
- Off-grid touring: 200-400 watt solar panels for ultimate roaming freedom away from electrical outlets. Expect to get 50-100 miles (80-160 km) per day. You will need to mount them on the bike so they collect energy all day which presents some challenges due to the large surface area. Recumbents and cargo bikes are popular in this category but I’ve seen some awkward attempts to attach this much to a conventional upright bike.
These range estimates assume you can manage to pack light and always pedal at a moderate effort. Expect to consume about 15 watt-hours per mile (9 Wh/km) while averaging around 14 mph (23 kph). If you’re riding uphill all day, into a headwind, in the rain, without pedaling, then your mileage will vary. These are long-term average values. You’ll get more on a sunny day, less on a cloudy day. If you’re unwilling or physically unable to pedal, cut the daily range estimates in half.
You can use any solar panel you want but “semi flexible” panels made with Sunpower cells for boats and RVs (campers/motorhomes) are your best choice in terms of power per unit weight and ability to withstand rough treatment on a bike. You can find them in all kinds of sizes on your favorite shopping site for around US$2 per watt and up. Traditional glass/aluminum frame rooftop panels are too heavy and should be avoided.
The size of your battery has some bearing on the efficiency of your system but does not determine your range in an off-grid situation. Assuming you’re trying to maximize distance traveled in a day, a bigger battery means you can take longer breaks before your battery is full, at which point you have to choose between getting back on the road or wasting potential solar energy because it has nowhere to go. That’s right, longer charge times are actually a feature. Conversely, an undersized battery (300-400 Wh) coupled with a large solar panel may run into problems with too much charging current for the battery cells or the BMS to handle. In that case, you’ll need to explore getting a bigger battery or using multiple batteries with multiple charge controllers.
Speaking of which, how do you connect your solar panels to your battery? You’ll need a “boost solar charge controller.” Just copy and paste those words into your favorite shopping site. You should find a couple of inexpensive Chinese models with MPPT for around US$30-75. A charge controller is a DC to DC converter which takes the solar panel’s output and converts it to the voltage needed to charge your battery. The “MPPT” business means that it automatically adjusts to finds the “Maximum Power Point” at which the panel’s voltage times current produces the most power. This varies depending on solar irradiance and temperature so we need to have Maximum Power Point Tracking to always extract the most power from our solar panel.
The output can be programmed in 0.1V increments to match your battery voltage. If you have the budget, you should get a Genasun boost controller for US$205. These are not programmable but are available in fixed output voltages. They are fully potted and waterproof instead of having loud cooling fans like some of the cheaper models. The Genasuns respond more quickly to changes in sun/shade which means you’ll get more watt-hours of energy per day on a moving vehicle. They also run cooler, which should (theoretically) make them last longer, and are significantly lighter because they don’t need a big heat sink. Comparison video.
Most solar charge controllers on the market today are made for lead batteries and can only be set to increments of 14.4V, corresponding to nominal 12/24/36/48V lead batteries. You cannot use these safely with your lithium ebike battery. Some of these unsuitable chargers may even state that they support “lithium” but upon closer inspection you may find that they only support multiples of 4 LiFePO4 battery cells in series which happen to like being charged to exactly 14.4V (4*3.6=14.4). Most other lithium cell chemistries need 4.2V per cell so you’ll need 42.0V, 54.6V or 58.8V for your nominal 36/48/52V pack. Your charger will need to be configured to the exact voltage your battery pack needs. If you’re unsure, figure it out before you plug anything into your pack. The labels on the charger that came with your bike and your battery itself are good starting points.
These controllers have PV input ranges which will work with most solar panels — just make sure that the open circuit voltage of your panel (VOC) is less than your battery voltage when empty (around 3.2V per cell) or you may find that you will not be able to charge when the input voltage is higher than the output voltage under some conditions (low battery on a cold day). Understand the specifications of your battery, charge controller and solar panel, keeping in mind that the solar panel voltage is lower than the label value when it gets warm. More on that here.
You may be able to connect two small panels in parallel but with larger panels that will likely exceed the maximum input current so you may need multiple charge controllers. There are trade-offs to be made: for example, a higher input voltage will result in better controller efficiency than a lower input voltage but connecting panels in parallel will give you better partial shading mitigation which matters if any part of your bike or body casts even small shadows on the panel.
If you have a Bosch, Yamaha or Shimano battery… I offer you my condolences. These closed, proprietary systems make it much more difficult to modify and enjoy your bike as you see fit. They’re well-engineered systems, designed to maximize corporate shareholder value and minimize liability and warranty claims. These vendors have no interest in helping you with your wacky solar modification project or supporting inter-operability with equipment from other vendors. If you’re doing pre-purchase research and solar charging is important to you then brands which reject open standards do not deserve your business.
I’ve read that you can trick the Bosch batteries into accepting a charge from a non-Bosch source by applying +5V to the signal pin and keeping charge current at 4A or less. If anyone knows a similar trick that will work with Yamaha or Shimano, please let me know. I know several solar ebike enthusiasts who charge using AC inverters on the bike using the charger that came with their bike and an intermediate 12V battery but these workarounds are heavy and inefficient. They should only be considered as an option of last resort.
I have written extensively about my solar conversions. I mention this as proof of real world experience in this subject matter and not as an example of a low-cost beginner project. Hopefully, my build will provide some inspiration for what is possible. You can do it. Start small and keep it simple. Add more later after you’ve had your first success. It’s not rocket science. If budget is an issue, you may find used solar panels on eBay, craigslist or your local equivalent. Or reach out to local solar installers or RV/boat supply shops and ask if they have any returned, blemished or damaged panels they’re willing to donate to you. Most of all, stop “thinking about it” and get out there and start doing something about it.
My current build has a 315 watt solar array good for around 80 miles (130km) per day. Just for fun, I recently did a 207 mile (333km) single-day ride using 784 Wh from grid-charged batteries and generating 2266 solar Wh so roughly equivalent to carrying six 500 Wh ebike batteries. Here’s a video.
If you want to learn more, I recommend watching the following presentation by someone who is far more knowledgeable than I.
Happy solar biking.