A simulation that can be performed is represented in the graphs in Figure 4 (simplified). The charging of a Li-ion battery (controlled by an IC) starts with three different voltages:
4.2 V fully charged;
3.6 V 50% charged;
3.0 V empty;
At the start (time 0), the battery’s ambient temperature is 20° C and will ramp up to 70º C, then come back to normal ambient. For the sake of long term reliability, the battery packs used in electric vehicles (EV) are normally operating in a 20% to 85% energy charge range, so they are seldom charged up to the full 4.2 V cell voltage or discharged below a 3.2 V cell voltage.
Figure 4 represents a BMS’ behavior when the temperature reaches different critical thresholds.
As the temperature (represented by voltage source V1) increases, the thermistor follows the change with a delay defined by the response time of the system. For an initial voltage of
4.2 V (green curve)
when the temperature reaches the different successive rising thresholds the battery voltage is automatically reduced in steps by applying short current discharges. For an initial voltage of
3.0 V (red curve)
the charging stops when the rising temperature reaches the first threshold and restarts later when the temperature falls below a certain level.
To measure the battery temperature with the best accuracy and reproducibility, Vishay offers several NTC thermistor packages. The NTCALUG01T guarantees a long lifetime of up to 10,000 hours at 150 oC and exposure to a 2.7 kV dielectric voltage to sense temperature on high voltage/power connecting terminals and bars, which can be at different voltage levels than the controller circuits. Another option for metal surface temperature sensing is the NTCALUG02 thermistor with a low thermal gradient of less than 0.05 K/K.
Within BMS for EV/HEV vehicles, different temperature sensing strategies are possible and are mainly dictated by the battery characteristics, the assembly design, and by control IC algorithms. It’s a whole hybrid science in itself, which is in constant evolution. In this boiling matter, Vishay as a manufacturer makes its contributions by working out diversified mechanical executions and electrical simulation models, and will continue to do so in the future.
About the authors:
Bruno Van Beneden, Senior Marketing Manager for Non-Linear Resistors at Vishay
Bruno Van Beneden currently serves as Senior Application and Design-in Manager at Vishay Intertechnology, specializing in non-linear resistors. He previously served as Development Manager and Product Development Engineer for the Vishay BCcomponents brand, in addition to Product Development Engineer at Philips Components and MBLE. An IEC TC 40 expert, Mr. Van Beneden holds two patents and a master’s in electrical and electronics engineering from KU Leuven Technology Campus De Nayer.
Alain Stas currently serves as Product Marketing Engineer, Non-Linear Resistors, at Vishay. Previously, he worked on the mathematical modeling of biochemical processes at Université libre de Bruxelles (ULB). Stas holds a Master of Science Degree in Civil Engineering, Physics, from ULB, with a specialization in solid-state electronics.