A rechargeable battery is one of the most complex parts of our electrical formula. The maximum use of energy stored in hundreds of battery cells is ensured by an electronic control system of its own design. The electronics designed to control the battery has several specifics, especially the emphasis on the reliability and safety of the entire system. It should be noted that the battery for an electric car is a very hard source of high voltage (in our case 600 V), this voltage is constantly present on the battery and in the event of a fault, the battery can become life-threatening. With this in mind, we try to design all parts of our traction battery as safely as possible.
According to FS rules, each accumulator must contain two high-voltage relays (so-called AIR - Accumulator Insulation Relay) separating the positive and negative poles of the battery from the output connector, as well as overcurrent protection in the form of a fuse. The electronics themselves can be divided into three parts: BMS (Battery Management System), DC / DC converter and AMS (Accumulator Management System) control unit.
The Battery Management System must continuously monitor the voltage of all battery cells (or all parallel n-tics) and the temperature of at least 30% of all cells. When measuring the critical voltage or temperature value on the cells (maximum 60 ° C according to FS rules), the high-voltage drive system must be switched off immediately. Another necessary part of the BMS is the function of balancing the voltage of cells with an accuracy of mV units, only with a well-balanced battery it is possible to use the maximum of its energy. The entire battery is divided into several series-connected battery segments with a maximum voltage of 120 V, which can be disconnected if necessary - work or maintenance on the battery is therefore safer. Each of these segments is equipped with a BMS, which, depending on the type of batteries and their mechanical installation, consists of 2 to 3 printed circuit boards connected directly to the batteries.
There are several options for powering low-voltage formula systems - separate batteries with sufficient capacity, a combination of a smaller battery and a DC / DC converter (the battery only supplies the car until the converter starts up and takes over power) or only a DC / DC converter. We decided to implement only a DC / DC converter, the main motivation was the weight saving for a low-voltage battery, and this system has already proven itself on our first autonomous monopost DV.01. The DC / DC converter stored in the traction battery box converts high voltage (600 V) to 24 V to supply all low voltage systems of the formula. The main parameters we emphasize are sufficient power (at least 500 W), high efficiency (over 90%) and small dimensions.
The AMS unit is the main control unit of the formula traction accumulator. Its task is to communicate with the BMS and evaluate information about temperatures and voltages of battery cells, it also measures the current taken or supplied from the battery and its voltage on the outside and inside of the AIRs. The unit directly controls the main battery relays (AIRs), implements the PRECHARGE circuit (controlled charging of the input capacities of traction converters before closing the AIRs), also independently detects the presence of high voltage at the output connector of the battery box and performs several other safety functions. It communicates with the rest of the control units in the formula via two CAN buses. Due to the fact that the AMS unit combines functions on high and low voltage systems, it is necessary to pay careful attention to the separation of these two parts and their galvanic separation when designing the printed circuit board.
The last electronic component is the so-called IMD (Insulation Monitoring Device) - a device for continuous measurement of the insulation state between low and high voltage system, if the insulation resistance falls below a safe limit, the AMS responds by disconnecting the AIRs. We would like to thank the company GHV Trading for the supply of quality components for IMD, which we can rely on to withstand the conditions imposed by them in our racing formula.
A well-functioning battery management electronics system is the basis for the successful operation of our monopost. Thanks to it, we are able to make the most of the potential of the batteries and thus drive our formula towards success on the race tracks.
Figure 4: AMS PCB