Influence of environmental conditions on the sensing accuracy of Li-Ion battery management systems with passive charge balancing

Due to their high storage capacity, low self-discharge rate and long lifetime Lithium-Ion batteries are key elements for the emerging electro-mobility and for stationary “Battery Electrical Storage Systems” (BESS). However, Li-Ion batteries are still very expensive and must be handled very carefully to exploit their features optimally. Hence, “Battery Management Systems” (BMS) have been developed to monitor precisely the voltage, temperature and current of the cells and the battery. These physical quantities are the basic variables to estimate the “State of Charge” (SOC) of the battery. The exact knowledge of the SOC is essential to deploy a battery efficiently in an electric vehicle or a BESS. Thus, the effects of changing environmental conditions and their long-time impact on both, the Li-Ion cells and the measurement electronics have to be understood. This paper investigates the measurement electronics, so we designed a BMS in a master-slave configuration comprising a voltage monitor chip LTC6803 on a slave-board. The specified datasheet precision of the LTC6803-chip was 0.25%, but the overall system accuracy is also influenced by other factors as the PCB design. The BMS system was completed by a custom U/I-slave-board for monitoring the battery currents, featuring a HAIS-200P hall current transducer. The effect of changing climatic conditions on the BMS was evaluated by extensive stress tests. Thermal cycles spanning a range from -30 °C to +70 °C and hot humidity storage tests at 85 °C and “Relative Humidity” (RH) levels of 20 %RH and 85 %RH were performed. Even after these accelerated tests the accuracy of the LTC slave board stayed within its specifications. The accuracy of the U/I-slave board depends on static temperature dependent offsets of the hall sensor. Especially for low currents of 20 A the error exceeds 35%, as the offset is based on the nominal current value of 200 A. However, by continuous temperature monitoring and calibration the precision target of 5% can be reached, even for low currents. While the measurement precision is well suited to estimate the SOC of Li-Ion batteries containing “Nickel-Manganese-Cobalt-Oxide” (NMC), it must be improved for LiFePO₄-cells.