Each block of an EV battery needs its own voltage sensor. Right? Maybe not. Cell balancing may be changing that in the near future.

Engineers at Ruhr-Universität Bochum, a university in Germany, have been working on current and voltages sensors that would help reduce the price and weight of EVs.

An example of the sensor circuitry attached to EV batteries for monitoring purposes.

How Electric Vehicle Batteries Work

In case you already know how EV batteries work, you can skip ahead. Battery-powered electric vehicles utilize electricity that is stored in a battery pack to power an electric motor which then drives the wheels forward. Once the onboard batteries are drained, they can be recharged from a wall socket or a charging station.

There are three main components to an EV: electric motor, controller, and battery. When the car is turned on, current flows from the batteries. From there, the controller receives power from the battery and the power flows to the electric motor. However, prior to reaching the motor, the controller converts the batteries 300V DC to 240V AC (maximum), a two-phase power which is more suitable for a motor. The electric motor can now convert this electrical energy to mechanical energy and turn the wheels.

In between the accelerator and the controller are variable potentiometers. These will tell the controller exactly how much power to deliver. If the accelerator is released, it will deliver 0V and, if it is fully pressed, it will deliver the maximum output.

Each battery in the EV consists of an individual block and each block may contain up to 12 cells. In many cases for the EVs that are on the road today, each cell requires its own voltage sensor. Looking at Tesla's Model S, there are 16 modules with 6 groups in series, resulting in 7104 Li-ion 18650 cells. If you know anything about EVs, you might say that this is an extremely large number, and it is. By comparison, the Nissan Leaf has 48 modules with four cells, resulting in only 192 cells.

The benefit to Tesla's design is a much lower price, higher energy density, as well as an increased range of driving. Below is an image of the undercarriage of a Model S, which is also the storage for its batteries.

The base of the Tesla Model S, where its batteries are housed. Image courtesy of Oleg Alexandrov (own work) [CC BY-SA 3.0]

Sensors and Cell Balancing

Lithium ion batteries are quite a fire hazard; due to lithium's instability, the cells are prone to overheating and catching fire. A dramatic illustration of this fact was Samsung's Galaxy Note 7 smartphones this year. To prevent incidences like this, EVs must constantly monitor the cells.

In order to monitor properly, a current sensor and numerous voltage sensors are used. The lead researcher in the development of RUB's new technology is Philip Dost, an EE professor at RUB.

Testing the new battery. Image © Philip Dost.

Dost has reduced the number of current and voltage sensors needed for proper monitoring, regardless the number of cells in the battery. This is done by requiring each sensor—current and voltage—to take on another function: cell balancing. This is a method of extending battery run time and battery life. Cell balancing ensures that the electrical energy is evenly distributed amongst the modules of individual cells.

While each cell might be the same upon its creation, under different conditions, each cell might behave slightly different than others. For example, when one cell is charged completely, other cells are no longer able to charge. Likewise, if a cell is drained completely, the motor isn't able to extract electrical energy from any other cells. This is a problem that cell balancing eliminates by providing for the maximum amount of electrical energy to be extracted and converted into mechanical power.

In short, you might not see another flat EV battery for a long time to come.

Philip Dost (right) and the Department Head, Professor Dr. Constantinos Sourkounis (left).Credit: © RUB, Marquard

Another major result is that future battery systems designed with cell balancing will require fewer sensors, reducing overall fabrication costs, as well as weight.

Dost's innovation is scalable, which ultimately means that it can be utilized in a system of two cells or a system like in the Model S, with 7000 cells. This would prove beneficial in power supply systems that are required to stay operational 24/7, such as those in hospitals. Other systems that would benefit are laptops, tablets, power tools, and mainly EVs. Dost plans on evaluating his prototype in heavy detail.