Unmanned Systems Technology 016 | Hydromea Vertex AUV | Power management systems | Unmanned Space Vehicles | Continental CD-155 turbodiesel | Swift 020 UAV | ECUs | DSEI 2017 Show report

October/November 2017 | Unmanned Systems Technology 34 Focus | Power management systems system to get the most out of every cell and so provide the best possible power capacity. However, there is now a wider diversity in the designs, with some developers focusing more on cost, simplifying the BMS and sacrificing some of the power capacity. This is made easier as the performance of the cells increases, so there is no actual change in the capacity and the range of a vehicle remains the same. This challenge is set to become even greater as electric vehicles move from 12 to 48 V systems. More of the battery cells are connected in series to provide the higher voltage, and this can cause larger voltage spikes (for example if batteries are added or removed in a ‘hot plug’ or ‘hot swap’ architecture) that have to be handled by the power management system. When an electrical load is dumped in a 12 V system, the spikes can reach up to 45 V. Now, with 48 V, there are transients of up to 70 V, which are pushing the input voltages of the PMICs. The move to 48 V will also mean changing from a copper cable harness to an aluminium one, which can handle the higher voltage but with a lower current and so can be three to four times lighter. There will be a need too for power converters that can bridge between 12 to 48 V and can handle these different electrical loads with multiple kilowatts of power. These controllers will also use a technique called envelope tracking, which can provide a 7-10% boost in efficiency in the current management. It is similar to the maximum power point tracking (MPPT) technique used for solar cells, where an algorithm optimises a power converter to track the most efficient conversion points. The power management challenge is further complicated with designs that recover excess energy in the system. Kinetic energy recovery systems (KERS), as used in Formula One racecars and other electric vehicles, do this by harvesting energy from the motors during braking, energy that would otherwise be lost as heat. This power is stored in devices called ultracapacitors, which can charge up quickly and hold the charge for a reasonable length of time (such as a few minutes). The energy in them is then either fed to the motors as the vehicle accelerates, or used to charge the battery system. This involves complex algorithms in a KERS controller that has to link to the BMS. Ultracapacitors are being built with new materials such as three- dimensional carbon sponges that can provide higher energy densities with lower weight, and they are already being fitted to supermarket delivery vehicles in the UK to boost the range of the battery packs. KERS systems can also be used to extend the range of autonomous delivery vehicles. Moving to 48 V allows a lighter wiring harness in a driverless car but puts more pressure on the performance of the power management subsystem (Courtesy of Intersil)