Types of battery
The sealed lead-acid battery (VRLA - Value Regulated Lead Acid) is currently dominant battery used in electric bicycles in China where the emphasis is on low cost. However, electric bicycles made in China for the exportmarket are mostly equipped with Lithium Ion (Li-ion) and in some case Nickel Metal Hydride (NiMH) batteries that are both lighter weight and operate up to 2,000 recharge cycles. The Nickel-Metal-Hydride (NiMH) battery is also used in China and in around half the pedelecs sold in the European Union. Their performance however is significantly reduced in cold weather and they need to be fully discharged regularly to maximize battery life.The other half of pedelecs in Europe is equipped with a Li-ion battery. The majority of past safety relevant issues with regards to lithium-ion batteries, which put a question mark over their suitability for electric bike applications, have been for the most part solved. Li-ion batteries have a narrow and defined "window" of operation, if the cell or pack deviates outside this operational window, they can enter an unstable operational condition. Hence, Li-ion batteries of all types must be equipped with the appropriate battery management system (BMS) to maintain the cell and/or pack parameters (voltage, current, temperature) within its stability window. There are numerous Li-ion cell types on the market to choose from. However, in this framework, we will limit commentary to the three most widely used Li-ion varieties in the E-bike sector.
Lithium Ion batteries
The most common Li-ion cell on the market is the lithium nickel manganese cobalt oxide (Li-NMC) with a nominal voltage of 3.6 V per cell. This cell offers a good mixture power to energy. Li-NMC cells perform well at low temperatures, and generally have a good safety record. The most common all round cell type is the 18650 design, which is produced on a scale of hundreds of millions per year, at low cost, and at a high manufacturing quality.The second most commonly produced Li-ion based cell type on the market is the cell Lithium-Polymer (Li-Po) type with a nominal voltage of 3.3 to 3.6 V per cell. This type can consist of a number of chemistries. This cell type offers significant advantages in packing design/form and in high power applications. However, it often has the disadvantage of limited availability and high costs due to limited production. It can be considered a specialist battery.
The third place contender for the most common Li-ion cell type for electric bicycles applications is lithium iron phosphate (LiFePO4 or LFE) with a nominal voltage of 3.3 V per cell. This cell type is considered the safest of Li-ion family. It exhibits considerable electrical and thermal stability if the cell deviates outside its normal operational window. However, at present compared with the Li-NMC and Li-polymer cell types, the LFE cells have considerably lower nominal voltage, energy densities, and higher production cost.
Regardless of the cell type, all Li-ion cells require a minimum level of electronic management and charger safety management. The responsibility for the implementation of cell/pack electronic safety measures and certification lies with the electric bike manufacturer and not the cell manufacturer.
Storage capability of batteries
Batteries for pedelecs are now typically produced at 24 V, 36 V, and with a few exceptions 48 V, whereas most Chinese electric bicycles operate at 12 V (lead-acid batteries). Batteries are rated in two forms: rated capacity Ah (amp-hours) and/or rated power Wh (watt-hours).The electric "rated" power, in watts, as produced by a battery, is a product of battery voltage and amps that flow into the motor to which the battery is connected. Multiplying the battery’s nominal voltage by the battery’s Ah rating yields the number of rated watt-hours (Wh = Vnom x Ah e.g. nominal 24 V x rated 10 Ah = 240 Wh), this is a unit of the energy storage capability of a battery.
The major difference between lead-acid, NiMH and Li-ion batteries is the storage capability measured in Watthours of energy per unit weight (Wh/kg). The Wh/kg for lead-acid is around 30, for NiMH around 90 and Li-ion around 120 or higher. Thus for the same weight, Li-ion will have around four times the energy of a lead-acid battery that means the pedelec will travel four times further with the Li-ion battery. Li-ion would also have less volume.
How much energy should a pedelec battery carry? The average "healthy" cyclist can pedal with a mechanical effort of around 100 watts at 15 km/h. So if one wants to ride 2 hours with a battery one will need 200 watthours of energy to maintain the 15 km/h speed with the battery-motor drive unit. Actually it will take more battery energy since there are losses in the system to overcome.
Most pedelecs carry battery energy capacity (watt-hours/kilogram) of 250 watt-hours (China mainly) to 800 watt-hours or higher for pedelecs in Europe and North America. The range will vary depending on the weight of the rider, the terrain, speed, battery age and how aggressive the rider uses the pedelec. Reliable manufacturers quote ranges from 40 to 60 km (36 V - 500 Wh systems). Some pedelecs are standard equipped with a spare battery that doubles the range of the vehicle.
Cost of battery
Cost is another important element for batteries and that is generally specified as €/Wh, the cost in Euros per unit of energy or rated Wh. Lead-acid batteries are currently priced around 30 €/kWh, NiMH and Li-ion at around 300 to 600 €/kWh, a factor of twenty difference. This explains the big price difference between electric bikes with lead-acid and those that use NiMH or Li-ion. It is expected that the price of Li-ion will decrease as more Li-ion batteries are put into production for four-wheel and light electric vehicles.
Manufacturers will use more automation at all levels of materials and cell production that will provide high quality, reliable and less costly batteries. A replacement battery would cost from two to three times the above mentioned prices; that would include battery pack fabrication, distribution and shipping costs. Furthermore, it is unlikely that the €/Wh cost of Li-ion batteries will ever come on par or fall below the cost of lead acid batteries. This is mainly due to the intrinsic design requirements of Li-ion batteries i.e. BMS, cell manufacturing complexity, and raw material sourcing, … These costs are not present or relevant for lead acid based systems. The most realistic long-term €/Wh cost target for Li-ion systems is closer to 200 – 250 €/Wh.
As pedelecs are usually torque sensor controlled, the sophistication of the software in the controller is also a factor. Note: higher voltages allow the motor to operate with more torque, and at higher RPM. This leads to more battery cells. Hence, there is a trade off in cost versus performance. The higher voltage systems require a more costly battery, but deliver significantly better performance (higher fun factor).
Charging
Most pedelecs have an indicator of the state of battery charge that informs the rider when the battery needs to be recharged. This is called a "state-of-charge" (SOC) meter. Since, the vast majority of battery accidents occur during the charging procedure. It is essential to use a dedicated charger that is both electrically and mechanically coded for the specific battery on the electric bike i.e. two-way charger to battery communication, with the exchange of the essential battery parameters: (1) battery ID, (2) nominal battery voltage, (3) charge end voltage, (5) maximum allowable charge capacity in Ah, (6) charge timeout. If any one or all of these parameters are not fulfilled, then the charger procedure will not be started.
Using the wrong charger can have several consequences. The battery may suffer from overcharge, and overheat, which can result in the battery management system shutting down, perhaps permanently. Or in a hardware Žfire¡ fuse blowing. Or in damage to the cells that will result in a dead battery. Or in a battery that never reaches full charge, or in subtle cell damage that reduces range and performance. In extreme cases, the wrong charger can result in a fire or explosion. There are efforts in the EU to standardize chargers and connectors. More can be learned about this at www.energybus.com. Useful charge of a battery is generally programmed to run from 20% to 90% SOC. Thus when the SOC goes below the 20% mark, the battery indicator will show need to recharge.
Battery location
Battery location is an important style and safety issue. With lead acid batteries, their heavy weight requires a location as low as possible to offer a safe centre of gravity. Lithium Ion cells are quite light, allowing them to be mounted under luggage racks, or inside frame tubes, or in other locations. The most common locations in the EU today are in a plastic box under the rear luggage rack, or inside the main down tube of the bike.
Life span of battery
The life span of a battery of an electric bicycle can be expressed in a number of complete discharge cycles. A real life span for lead batteries is approximately 200 cycles. The lifetime for NiMH batteries and for Li-ion batteries are in the range of 500 complete cycles. Next to the life span expressed in cycles, the battery also has a limited life in absolute time. Typically, the ageing of the battery becomes more and more noticeable after about five years because the useful energy capacity starts to drop significantly (below 80% of its rated capacity) and the self-discharge of the battery increases.
Source: ETRA for PRESTO-project, WP2 deliverable 2.3, p. 41-43
