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Several versions of
the SmoothTalk [TM] battery management system (BMS) are in development to address the need
for taking care of individual Lithium based cells used in a traction pack of an electric
vehicle. Systems vary in complexity and functionality. There are two main types of the BMS
- pure hardware based and regular, controller-driven types. The hardware based system uses modular charging approach - all cells are charged as a group by a bulk charger while each one is monitored not to exceed max. voltage (which for lithium polymer cells corresponds to certain SOC). Then each cell receives finished equalizing charge from endividual isolated mini-chargers set up for max allowed finish voltage corresponding to 100% SOC. This way each cell in a traction pack takes what it needs regardless of other cells in the pack. There are common alarm and end of charge circuits as well as supervisors allowing to make sure there are no failed chargers in the chain. The system will take care of proper charging and - by definition - balancing, but will not prevent overdischarge, limit battery current or collect any battery related parameters. Most if this functionality is included in traction controller or inverter. Controller based BMS takes traditional approach - there are simple cell nodes sensing voltage and temperature and containing resistive shunt remotely controlled on/off. All the nodes, main controller and charger are linked via CAN bus. Main controller can address each cell's node, measure and store battery parameters. After processing collected information based on current state of charger (SOC) and other parameters controller can individually treat each cell by taking out or optionally adding small amounts of charge at the time until cells are balanced according to predefined criteria. For cell chemistries where voltage is fairly good and consistent indication of SOC (such as LiP), simple voltage balance method is sufficient, whereas for the types which maintain about constant voltage output over wide range of SOC )such as (LiFePo), coulomb Ah capacity balancing is deployed. The charge is taken out by shunting cells or by patented pseudo-shunting method. Pseudo-shunting allows to take charge out of cells out without applying resistive shunt which dissipates energy as heat, e.g. effectively electrically remove a cell out of the string being charged without physically disconnecting it. Adding the charge is done by individual boosters (see modular charging above) . Controller based BMS allows collecting historical data and other functions computerized and software based system might have. Physically the cell electronics PCBs have to match the cells they are installed on. Currently 3 types of cells are being manufactured: cylindrical (common 18650, A123 Systems, SAFT, GAIA), flat pouch (Kokam) or prismatic (Valence, Thunder-Sky, SAFT, K2) No two cells are created equal, so, connected in series and being cycled as one group, the cells will gradually drift out of balance. Lower capacity cells charge and discharge quicker so their terminal voltage may be higher or lower than the average; the temperature gradient across the battery pack results in further imbalance. This, however, is expected and does not constitute a problem with the pack, so may not require any special balancing actions. Featured BMS smart algorithm anticipates cell behavior learned from previous charge/discharge cycles to avoid pointless activity of trying to keep individual voltages the same at all times. A combination of terminal voltage and amount of amp-hours stored in a cell (adjusted to actual initial capacity) is used to determine the cell SOC and required action. Not only manufacturing differences or defects lead to non-uniform cells. If more than one location is used to place all the cells and no active temperature control is deployed, it is likely that groups of cells in different locations in a vehicle will have different temperatures. A BMS should take this into account and intelligently compensate for it. During discharge dynamic and thermal behavior of the battery is very complex and in general unpredictable as depends on the driving pattern, individual cell internal resistance, temp, age, amount of cycles accumulated, etc. The terminal voltage swings during driving or regenerative braking usually does not allow to make meaningful measurements. Therefore, during driving the system only tracks energy usage and uses this date to predict amount of charge needed to refill the battery. Few samples of the software based node construction as well as hardware based liquid cooled LiP battery module prototype assembly can be seen on the photos below. |
