Batteries: Enough to Get You Started!
Every electric ride will be powered by a battery, and it is the single most costly component of the product. It is also the part of the ride that will allow you to drive the distance you need to go and provide the power to accelerate and climb hills. Electric vehicle manufacturers have two options when deciding on the battery: Will I go with an off-the-shelf solution or will I develop my own batteries? Both options have their pros and cons for the rider and for the manufacturer.
The first step is to define the performance criteria. The three most critical parameters are Voltage, Current, and Capacity. Voltage will set the top speed; Current will set the acceleration; and Capacity relates to how far you can go on a charge. On an electric vehicle, Voltage and Current will impact component selection of the drivetrain. The motor and controller choice must have the capability of handling these battery specs. Often, the motor controller is electronically “tuned” to limit the power delivered to the motor from the battery.
Historically, battery voltages have been in multiples of 12V (volts), because early electric vehicles (EVs) strung batteries in series (adding voltage by 12V) and stacking them in parallel (adding their current and capacity). With the introduction of lithium cells, battery manufacturers and DIY’ers have begun to make batteries with a variety of pack voltages, current ratings, and capacities.
Within the umbrella of Lithium batteries, there are many different chemistries, each with their own strengths and weaknesses. For example, Lithium Iron Phosphate is a very durable and stable chemistry, known for its ability to deliver bursts of very high current levels. These are often found in cordless power tools. Other chemistries are known for their high capacity, but cannot deliver the same power, while still other chemistries may lie in the middle of the road for capacity and current, but can be recharged several thousand times before getting “inefficient” (for EVs that is 80% efficency).
There is no right or wrong answer to the choice of battery, but in general, a higher voltage will require lower current levels to deliver the same power. College courses in Electrical Engineering teach us that Power = Current x Voltage. For example, a 5kW (kilowatt) output coming from a 72V battery only requires about 70A (amp) of current, whereas it requires 140A from a 36V system. Keeping current down increases efficiency and allows for thinner and more flexible cables (not to mention less expensive), which comes in particularly handy when cleanly routing cables throughout the EV.
Unlike their lead acid predecessors, Lithium battery cells never want to be over-charged or over-discharged. They have a “sweet spot” (or range) of voltages in which they like to spend their time. A Battery Management System (BMS) is the electronic circuitry that helps monitor cell voltages and protect the battery from many types of potential failure.
Once the performance criteria are set, the question becomes whether to use an off-the-shelf solution or to develop in-house.
Using Existing Battery Manufacturer
An off-the-shelf solution generally reduces development time and cost, and leverages any regulatory or certifications the manufacturer has already performed, but control over form factor (shape), performance characteristics, and system communication are greatly limited.
Developing an in-house solution
Bringing the design in-house allows complete control of the performance characteristics of the battery pack and form factor. This, of course, takes time to develop, and comes with the financial and time burden of regulatory testing.
Another very strong benefit of developing an in-house solution is that the communication between the battery and the rest of the system lies in the hands of the developer. It is not sufficient enough to simply measure the voltage of a battery to determine its health. Information gathered by the BMS can be formatted and then communicated to the vehicle’s other subsystems as a way to monitor the health of and prevent possible damage to the battery.
Monday Motorbikes solution
The Monday Motorbikes GEN7 battery is a custom solution designed specifically for our motorbikes. It is a 60V system capable of 150A of continuous current and has a capacity of 2,580 Wh (or 2.58 kWh). To put that into perspective, a typical e-bicycle may run a 36 volt system, capable of 20 amps, and has a capacity of 800 Wh. While our motorbikes meet the same regulatory requirements of a typical e-bicycle, our performance capability is several cuts above.
Fun and easy to ride!