Pi-pop, Ebike without Battery: Are Ultracapacitors a Viable Alternative?

French Pi-pop is selling e-bikes exclusively powered by supercapacitors, instead of batteries. Discover their pros and cons vs. battery e-bikes.

• What is a supercapacitor?
• How ultracapacitors work?
• Advantages of supercapacitors compared to lithium-ion batteries
• Advantages of lithium-ion batteries compared to supercapacitors
• Why supercapacitors are scarcely adopted on ebikes? Yet, they’re performant!
• Pi-pop supercapacitor ebike, quick overview
• Which one is better? The two together!

What is a supercapacitor?

Supercapacitors are basically devices to store energy, just like lithium-ion-batteries. They use an insulating material between their plates to separate the collection of positive and negative charges building on each side’s plates. Supercapacitors absorb static electricity for future use.

How ultracapacitors work?

Batteries work well as a continuous source of low power. Nevertheless, they are weak at handling peak power demands or recapturing energy in modern applications, since they supply their power and recharge slowly.

ULTRACAPACITORS deliver quick amounts of energy during peak power demands, then quickly store energy and capture excess power that is otherwise lost. They efficiently complement a battery because they discharge and recharge quickly.

Primary energy source can be pedalling, a battery, or regenerative braking.

Advantages of supercapacitors compared to lithium-ion batteries

Let’s illustrate this matter referring to batteries for electric bikes, taking as a benchmark a 500 Wh quality battery, like a Bosch or a Yamaha, costing about 430 € VAT excluded in Europe. Ultracapacitors:

1. Have a much higher power density, called also specific power, which means that they can deliver up to 10,000 W per kilogram, versus up to 3,000 W per kilogram for batteries. This translates, for an ebike, into prompt acceleration when starting, at traffic lights and even on climbs.
2. They wear off slowlier than batteries, lasting on average 20 years against 10 years on average for lithium-ion batteries, provided that the latter are well cared for.
3. Whereas batteries lose some 3% of their capacity every year even when they are stored, and more when they are used, supercapacitors keep the same energy density over the years.
4. They handle extreme temperatures better. They can charge and work (i.e. output) from -40 Celsius to 65 Celsius (-45 Fahrenheit to 149 Fahrenheit), versus 0 Celsius to 45 Celsius (32 F to 113 F) charging and -20 Celsius to 60 Celsius (-4 F to 140 F) working for batteries. You may have noticed that charging your battery at 7 ° C, say in a garage without heating, takes about 20-30% longer than with 20 ° C. Consequently, extremely cold or extremely hot weather are less penalizing with a supercapacitor e-bike than with a battery one.
5. They are more ecological, although batteries are not very polluting. Lithium extraction for our 500 Wh sample battery would emit about 35 kg of carbon dioxide, with another 15 to 25 kg for its production, depending on the energy production mix of the manufacturing country (7 kg for France, barely a kg for Norway, and 30 kg for Poland). This brings the total of carbon dioxide emitted to manufacture our battery to an average 55 kg, which is what a small car emits in a 400 km trip. And yes, batteries can be recycled.
6. Charging much faster than batteries, they don’t need to be charged on the electrical network, being suitable for absorbing electric energy from regenerative braking or dynamos while pedalling. Battery ebikes energy consumption amounts to roughly 0.2 cents of dollar per km.
7. They can’t catch fire nor explode like batteries. Hence they can be transported by trains, airplanes, buses, ships, etc. without any restriction, since they are not considered dangerous goods.

Consequently, supercapacitors seem quite useful for city e-bikes that must stop and start again frequently, and ride short distances. They can thus be charged by braking and promptly deliver their energy at the start, at traffic lights, or when re-accelerating at round-abouts.

Advantages of lithium-ion batteries compared to ultracapacitors

1. Lithium-ion batteries have got a much higher energy density, also called specific energy, that is the capacity of storing energy. You can reckon 180 Wh per kilogram for batteries, versus a mere 20 Wh per kilogram for supercapacitors. Indeed, our sample battery stores 500 Wh in 2.8 kg, whereas a super supercapacitor weighing the same stores barely 56 Wh, which is nine times less. On a ride that is longer than 3 km, batteries really make a difference, steadily delivering strong power to the ebike motor, up to 100 km.
2. Batteries are capable of delivering power even at very small output for a very long time, unlike supercapacitors that discharge very quickly.
3. Batteries tap electricity from the network, whereas capacitors tap it from regenerative braking, which supplies little, and from pedalling, with dynamos, which make it harder.
4. An idle or stored battery loses about 6% of its capacity every month, whereas a supercapacitor loses it all in a few minutes.
5. Batteries are cheaper, our sample costing less 0.8 euro per Wh VAT excluded, versus at least €20 per Wh for a supercapacitor.
6. Batteries don’t burden your pedalling with dynamos, like supercapacitors ebikes do.
7. They can supply energy for a long time to lightings, clacsons, smartphone chargers, ebike computers. Supercapacitor discharge too fast to enable the same.

Why are supercapacitors scarcely adopted on ebikes? Yet, they’re performant!

In the automotive sector, supercapacitors are increasingly adopted. We’ve experienced their efficiency during a journey with a diesel Peugeot. At every traffic light, two seconds after stopping, the engine would automatically turn off, saving fuel and avoiding harmful overheating. As soon as one touched the accelerator, it would start so quickly that we wondered how the battery could handle such an energy depletion, since starting a diesel requires a lot of electric energy. The reason is that many Peugeot deploy supercapacitors to capture energy while braking. Supercapacitors are also frequently adopted for regenerative braking, by Toyota and other manufacturers. The reason they are very rare in the ebike industry, is that ebikes produce little energy braking, being much slower and lighter than cars. We used to ride a Matra Tidal Force electric bike. Its regenerative braking, on a daily 300 m downhill road, added about 5% charge to the 300 Wh battery, whereas riding back uphill the same road consumed roughly 20% of the battery. Therefore the supercapacitors were not worth neither their cost nor their weight.

The other way for ebikes to charge supercapacitors are dynamos thrusted by pedalling. Needless to say, they require more effort to one’s legs.

Pi-pop supercapacitor ebike, quick overview

Pi-pop is a French ebike that has no battery, providing its assistance with supercapacitors, that are charged by braking and pedalling. They plan to multiply their sales by 10 next year, from 1,200 to 12,000. At €2,450, it’s a little more expensive than a comparable battery e-bike. Indeed, it looks and features components similar to a €1,600-2,000 battery ebike. By its weight is comparable to the latter.

Which one is better? The two together!

Combining battery and supecapacitors would be great, although Pi-pop is betting on supercapacitors alone for their ebikes. Nevertheless, as Maxwell Technologies, the world leading manufacturer of supercapacitor, clearly states on its website, supercapacitors are a complement for primary energy sources, be they batteries or combustion engines. On the other hand, pedalling is a primary energy source, so Pi-pop may be very successful.

Pi-pop ebike energy consumption – notice the red (= lack of energy) above the climb after a short re-charge on a short descent

Our overall impression

Pi-pop is a very interesting ebike that could pave the way for a new generation of this fantastic mobility means, especially for short trips in the city. What remains to be seen is the performance of supercapacitor ebikes when it comes to mastering climbs or deliver sufficient support for medium and long distances without stops. Pi-pop very honestly illustrates this weakness in the above image.

Pi-Pop eBike at a glance

• Frame: aluminium
• Frame size: one-size-fits-all frame
• Suspension fork: Zoom 141D
• Motor/Generator: Aikema Electric Drive Systems AKM100SX 250W, 45Nm
• Energy storage system: supercapacitors
• Control unit: Pi-pop
• Drivetrain: Shimano Tourney RD-TY300D
• Brakes: Tektro MD-M280
• Weight: 21.7 kg
• Maximum permitted total weight: 120 kg
• Colour: white
• Price: 2,450 euros

Pictures: SAS Solutechnic Engineery Electronic; Maxwell Technologies.