Solar Powered Electric Bike

Solar Powered Electric Bike

I happen to be a strong proponent of cycling and energy conservation initiatives. Although I usually ride a traditional or “regular” bicycle, I have also recently constructed an electric bicycle. My intention was to consider using the electric bicycle for trips where the time spent commuting needed to be minimized. In other words, for those days that I felt lazy, how late could I sleep in each morning, yet still arrive at work on time if traveling by bicycle?

Once of my first concerns with electric bicycles is the environmental impact of purchasing and using lithium batteries. Eventually they will wear out and need replacement, whereas the parts on a traditional bicycle may last considerably longer and generate less waste. However, with the recent improvements in lithium ion batteries, and the use of an appropriate charger that will charge them to only 80% capacity to maximize their life, it becomes reasonably practical to use an ebike for certain rides. I also recently came across a term paper posted by Justin Lemire-Elmore, from April 2014, in which he makes the case that electric bikes consume fewer resources than regular bikes, once the transportation and production costs of food for the human rider are factored into the equation. Although I think this concept is interesting, I thought I could take it one step further.

Thus, a solar-powered electric bicycle concept was conceived. I figured that with a solar panel and large battery, I could charge the battery all day, and then use that power to ultimately charge my bicycle battery. This means I had the full power of the sun all day long, rather than trying to attach a solar panel directly to my bicycle. The downside to this approach is that it introduces additional losses in the storage and conversion of the energy.

First, I decided to reuse a 50 watt solar panel, battery, and solar charger controller that I had previously used for another project. Using the 20 Ah lithium battery (actually, a LiFeMnPO4 Prismatic Battery), I was able to connect a standard 300-watt inverter to convert the 12.8 volts DC from the battery into 115 volts AC.
Solar Battery I then connected the 52-volt Luna Cycle battery charger to the inverter, changing the 115 volts AC into 52 volts DC. Inefficient? Yes, most likely. But it was easy and available. This charger worked perfectly from the 115 volts AC output from the inverter.
Charger The charger is nice because it gives you the ability to select the charging current from 1-5 amps, and to select the amount of charge you want to put into your bicycle battery (from 80-100%). This allowed me to choose a lower current setting to make sure I wasn’t trying to draw more power than the inverter was capable of putting out. When charging my bicycle battery at 3 amps, I found the charger was only consuming approximately 200 watts from the inverter.

This setup is not ideal, and has a serious limitation. Although the 20 Ah lithium battery is being charged by the sun through a solar panel, it has a capacity of only 256 Watt-hours. To arrive at this number, you simply need to know the battery’s nominal voltage (12.8 V) and its capacity in ampere-hours (20 Ah). Then by multiplying (Ah x V = Wh), you arrive at 256 Watt-hours. Unfortunately, the bicycle battery that I chose is rated at 702 Watt-hours. This means I will only be able to charge my bicycle battery about a third of the way before running out of power, and this is assuming no losses through the inverter and charger, which is not correct. However, for small depletions of my bicycle battery (i.e, 20-30%), I should be able to replenish the charge solely through solar power! It may not be perfect yet, but it is progress!