The Challenge

 

Connecting Li-ion batteries in series used to be the only way to get high enough voltage to a battery electric power train. This approach brought with it certain problems that we set out to solve;

  • If even a single cell is overcharged or over-discharged, it will be destroyed and may even catch fire
  • Full capacity of a battery pack is dependent on the properties of the weakest individual cell
  • Expensive, unreliable and complex Battery Management System (BMS) required to keep the pack in balance

Modern electric motor drives are very efficient. However, they are most efficient on full power, which is not used very often, especially in vehicles. Basically, the traditional way of constructing a battery powertrain requires us to raise the battery voltage, creating cell balance problems, and then needs the motor driver to lower the voltage in most driving situations. We thought this doesn’t make much sense.

The Solution

 

L7 Drive is a complete one device solution that turns the whole setup the other way around. We start with one large Li-ion battery cell, or a set of parallel connected cells, which behave like one big cell. Our novel patented technology then raises the voltage to the motor, but only when needed, and only as much as needed. That way we can get the best efficiency on partial load, which is used most of the time.

Unbeatable Benefits

  • No need for BMS. One cell is always in balance with itself. Parallel connections balance the voltage automatically.
  • Full capacity is always usable. One weak cell will not stop the vehicle and a faulty cell will be isolated by a fuse.
  • Only one device is needed. L7 Drive takes care of the battery control, motor control and also controls the charging.
  • Voltage conversion works both ways, so L7 device can recuperate the motor braking energy back to the battery.
  • A simple AC-DC converter at voltage anywhere between the battery voltage and device’s maximum voltage can be used for charging.
  • L7 Drive unit can also charge the battery from unstable voltage sources, such as solar panels or a small wind generator. No separate inverter needed.
  • L7 Drive unit can produce the maximum power independent of the motor speed, so the launch power is always more than adequate.
  • L7 Drive technology is suitable for all Li-ion chemistries, so you can choose the chemistry and cell format according to your needs.

Scalability

 

L7 Drive technology is scalable from 250W to 15kW and upwards. Our first commercially available models are 250W unit for electric assisted bicycles and 2kW unit for electric scooters, e-rickshaws and other light electric vehicles. They can be used with any Li-ion battery type available today or in the future, with one big cell, or any parallel connected pack. A bigger unit becoming available late 2019 with 8kW power and integrated battery pack is designed for autorickshaws and other L5e or L7e class vehicles. L7 Drive technology is compatible with all available electric motor types. The first models will be made for BLDC.