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33 discharging them too fast can lead to the cells overheating and causing a fire. The cells also have to be discharged equally. One cell discharging too fast can dramatically reduce the performance of a vehicle by reducing the current available, even if the other cells are still perfectly operational. The cells have to be charged evenly as well. Even though lithium-ion cells do not have the same ‘memory’ effect as NiMH cells, which need to be fully charged and discharged each cycle, they do have a limited life of 300 to 500 cycles. Charging and discharging some cells more often than others also leads to an imbalance in the charge in the battery pack, again reducing the vehicle’s performance, so the whole battery system has to be replaced earlier than would otherwise be necessary. As a result, the BMS has to maintain an accuracy of ±1 mV across a 200 V range and support that reliably for a ten-year equipment lifetime. That is a considerable challenge. It is not just a challenge for the initial design though. Over time there is long- term drift with the different chemistries, and the power management controllers have to take this degradation into account over a five- to ten-year period and for different conditions and battery chemistries. Now that electric vehicles have been in the field for ten years or so, designers are starting to see other things happening. For example, all the 80 cells in a battery pack for an electric vehicle are connected via a single negative rail that acts like a bus bar. Developers are finding that there can be a build-up of metal on the connectors that is creating an offset which has to be accounted for in the power management algorithms. This offset is not because the cells are degrading or failing, so the system has to be recalibrated regularly with a negative input (usually -1 V across a large battery pack) to account for that offset. That can be difficult to implement on a power management chip that has been designed to work from 0 to 5 V. The power management IC (PMIC) also has to take into account temperature fluctuations, which can range from 20 to 85 C, while maintaining the utmost accuracy at the higher temperature. These BMS designs may also be needed to actually discharge the cells to generate heat to bring the pack up to its most efficient operating temperature, especially if they are operating in hostile environments such as underwater or on ice – environments where unmanned systems are most applicable of course. Early BMS designs were richly instrumented with lots of monitoring lines so that every cell in the battery pack was looked after by its own management Power management systems | Focus Unmanned Systems Technology | October/November 2017 The operation of a battery pack can be controlled with a stack of power management ICs (Courtesy of Maxim Integrated)