A Battery Electronics Unit (BEU) for Balancing Lithium-Ion Batteries 2008-01-2907
Lithium-ion batteries have become prominent in many applications, because of their high energy-to-weight ratio. Unlike other types of cells, lithium-ion cells do not exhibit natural cell-to-cell balancing mechanisms. Over time, lithium-ion batteries may become unbalanced, leading to one or more cells becoming overcharged, causing cell damage.
Cell balancing is required to achieve the maximum mission life for a lithium-ion battery, by reducing the possibility of overcharging or deep discharging. A BEU has been developed that uses a high-efficiency autonomous balancing circuit to maintain uniform charge on the series cells in a 24-cell battery. The balancing circuits operate continuously in all modes of operation, including charge, discharge and standby. The cell balancing currents are proportional to the voltage difference between the cells, gradually diminishing to zero as the cells achieve balance.
Each cell balancing circuit is a transformer-coupled forward converter with resonant reset, using planar transformers. A Phase Lock Loop (PLL) circuit provides zero-loss switching over the full range of variation due to changes in temperature and component aging.
In addition to cell balancing, the BEU also provides individual cell voltage monitoring, 1553 telemetry, reconditioning load control and cell bypass relay drivers.
Test data is presented that shows cell balancing and cell voltage monitoring accuracy of better than 10 millivolts, over a temperature range of -35 °C to +75°C.