What should a transmission for an electric vehicle (EV) look like? Engineers at Zelentek, Hillsboro, Oreg., would argue it should be completely electronic. That's why they developed a way to connect cells in a battery pack using semiconductor switches that can give electric vehicles a four-, six-, or eight-speed, non-mechanical transmission.
The scheme works because in an EV's dc motor, torque is proportional to current, while speed is proportional to voltage.
The new electronic transmission (or Reconfiguarble Battery Technology, RBT) uses switches to put the battery cells in the most parallel layout possible for a given motor speed. All batteries (or cells) get connected to a high-current bus from which the propulsion motor draws electricity. Regardless of how it is configured, however, the battery pack is still designed to produce a maximum vehicle speed (voltage) and torque (current).
“It's really not the electric motor that sets the need for a multi-ratio transmission to deliver efficiency and vehicle performance — it's the limitations and compromises inherent in a hardwired battery stack”, says Zelentek CEO Andy Turudic. “There is a significant power loss, and subsequent loss of vehicle range, inside the vehicle's battery stack due to its internal resistance. A mechanical multi-ratio transmission lowers those losses by simply reducing battery current loads. We've taken a different tack by solving the energy loss problem in the battery stack rather than the drivetrain.”
In a six-cell battery pack, for example, all six cells can be switched to be in parallel and generate one-sixth the maximum voltage (speed) and six times the maximum current (torque) with the same amount of power coming from each cell of the hardwired battery stack. At low speeds, in other words, six times the torque is available for the same electricity from the battery stack.
If the cells are reconfigured into three parallel blocks, each with two cells in series, the motor will generate one-third the voltage (speed), with three times the current (torque). And changing the setup to two parallel blocks, each with three cells in series produces half the maximum speed (voltage) and twice the current (torque).
One of the benefits of Zelentek's RBT stems from a significant reduction in power in the battery cells. For example, when the vehicle is running at half speed, reconfiguring the battery stack to two parallel blocks of series cells halves the current drawn from each block. Because power dissipation is proportional to the square of current, the power dissipated in the internal resistance of each cell is reduced by 75%. The semiconductor switches only need to operate at the cell voltage (60 to 100 V), not at the battery stack voltage (600 V). The switches can have lower losses, higher switching efficiencies, and cost less. The RBT should also reduce battery-stack temperatures, extend battery life, and improve vehicle range and performance,
Another advantage is that doing away with the heavy mechanical transmission would let EV designers add up to 12 kW-hr of lithium-ion battery cells, which would extend vehicle range while keeping weight the same.
A six-cell battery pack with a maximum of V volts and T current can be switched into one of four configurations, each providing a motor a range of speeds and torques.