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Several versions of lithium battery management systems (BMS) were developed to address the need for taking care of individual cells in a traction pack of an electric vehicle or in a stationary battery (UPS, stand-by backup power supplies, mobile carts for medical instrumentation, etc). The BMS systems mainly target OEM manufacturers and conversion and custom tailored to the battery specifications. Systems vary in complexity and functionality. There are two main types of the BMS MMC offers: pure hardware based for especially harsh operating environments, and traditional microcontroller driven type.The hardware based systems use modular charging approach, while traditional software based systems consist of main controller and slave nodes.Physically node PCBs are designed to fit particular cells they monitor and control. This eliminates errors caused by remote sensing or long leads. Currently 3 types of cells are being manufactured: cylindrical (for instance common 18650/28650 and larger form factor by A123 Systems, SAFT, GAIA), flat pouch (Kokam, K2, EIG and others) or prismatic (Valence, Thunder-Sky/Headway, SAFT and many others).
No two cells are created equal. So, connected in series and being cycled as one group, the cells’ SOC guaranteed to gradually drift out of balance. Lower capacity cells charge and discharge quicker so their terminal voltage swing will be higher or lower than the average (at the end of charge and discharge respectively). This means they end up working harder which over time further decrease their capacity compounding contribution to the imbalance. The temperature gradient across the battery pack results in further SOC imbalance. Identical initial capacity cells might have different self-discharge rate, and so on. MMC BMS systems’ smart algorithm anticipates cell behavior learned from previous charge/discharge cycles to avoid pointless activity of trying to keep individual voltages appear the same at all times. A combination of terminal voltage near 0% SOC and 100% SOC, measured capacity of each cell, tracking amount of amp-hours in and out the pack and measuring actual capacity of each cell is used to determine running SOC of each cell. Terminal voltage swings during driving or regenerative braking usually do not allow making meaningful measurements. Therefore, during driving the system only tracks energy usage (amount of Ah in and out) and this determines amount of charge needed to refill partially discharged battery to exactly 100% (or other preferred amount) of SOC. MMC implements custom top performance designs, so when enough data is available, we model a battery behavior as well as vehicle subsystems in matlab environment – that allows optimize the system for intended application and the vehicle. If no data is available from the manufacturer, we can learn the battery by bench testing a sample using battery cycler and data acquisition system (data logger) and obtaining empirical data for the simulator as well as for real EEPROM later. The differences between MMC BMS and most of other designs on the market: - The BMS does not use individual cells to power node electronics. As long as main 12V, 24V or 48V power is applied, the BMS will work even if all the cells in the series string are flat 0V. Few prototypes of the software based node construction as well as purpose built hardware based liquid cooled LiP battery module prototype assembly can be seen on the photos below. These are samples of custom BMS systems designed per customer’s specifications. |
EXAMPLES OF CUSTOM BMS DESIGNS
 Example of custom
main BMS controller |
 16 cell slave controller
designed for remote sensing |
 Cell module installed
on pouch type cell dummy |
 Another example of
main BMS controller |
 Separate cell nodes under
test with small 18650 cells |
 32 cell 4.7kWh module assembly |