Electric Vehicle Battery
The Battery in the e-up!
The electrically driven e-up! is the first high-volume series production electric car by Volkswagen, and solves a whole series of challenges that have been associated with the use of batteries. A high-voltage lithium-ion battery is installed in the e-up! which has a high energy and power density as well as high levels of efficiency. Thanks to its compact construction it fits under the seats and transmission hump, in place where the fuel tank and exhaust system would be fitted in conventional cars.
Cooling is not necessary: in driving mode the battery does not heat up beyond the permitted temperature range, even when temperatures are high outside the car. The battery is controlled by the Battery Management System (BMS) which exchanges information such as the state of charge, output limits and error conditions with the engine control unit and power electronics – this ensures efficient and safe operation while driving.
The BMS controls the battery’s different operating modes while driving or parking, and provides the electric energy for the vehicle via so-called protective relays. If a significant error occurs, these relays are switched off by the BMS software, isolating the battery. The BMS also determines the state of charge of the whole battery as well as for individual cells. This eliminates overcharging and deep discharge of the individual cells during regular driving. This is important because the output limits – the maximum charge and discharge performance as well as the maximum charge and discharge duration – depend strongly on the current charge state and the temperature. As in all batteries, the cells used in the high voltage battery will self-discharge slightly. This leads to differing states of charge in the cells which can impact the car’s range as well as the reliability of the range calculation. The Battery Management System compensates for this problem by continuously comparing the state of charge of all cells and restoring the balance of charge for the whole system by treating the cells individually.
In addition, the high-voltage system of the e-up! is monitored by a main line called the pilot line. This main line acts as a current hoop through all the important high-voltage components. An interruption of this current hoop indicates a fault and results in a red warning light lighting up in the car’s interior, and a preventive shut-off of the battery by the protective relays. The interaction between bodywork and high voltage battery also increases safety: it is set up in such an ideal way that the bodywork protects the battery in many accident scenarios. In addition the protective relays, integrated in the high voltage battery, switch off the current in an accident and deploy airbags or seat belt pre-tensioners.