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Battery Management Systems

Battery Management Systems

Battery Management System (BMS) isthe biggest problems of the EV industry. Here is what I think itneeds to accomplish


Battery Management Systems (BMS)
]Virtually all types of batteries can be damaged by excessively high or low voltages, and in some cases the results can be catastrophic. Battery Management Systems can provide battery charge protections, discharge protections, state-of-charge monitoring.
In general, using flooded lead-acid batteries in series in EV applications can work without a BMS. A good charger can provide a float or equalization charge cycle that can effectively balance the batteries. Occasional checking and manual equalization is still required, but not on a daily basis.
Alternatives battery technologies such as Lithium and NiMh are more sensitive to over-charging and over dis-charging on a cell-by-cell basis. When the cells are used in a series, these battery technologies are generally implemented with a BMS.
Discharge Cycle Management:
Low-Voltage-Cutoff (LVC): As the cells discharge, the cell voltage drops the cell can be damaged by taking the cell voltage to zero. Low Voltage Cutoff must be provided on each cell to shut down the power draw when the LVC is reached.
However, a noted problem is when using LVC during high current loads in EV applications, the voltage sag across the cell may trip the LVC, so the LVC may need BMS to be calculated with loading of the cell.
A typical LVC for a NiMh 1.2 V cell is about .9V.
For Li-Ion, the LVC circuits for each cell will make sure the voltage for any one cell doesn’t drop below 2.1V.

(BMS)]Cell Temperature Monitoring During Discharge:TBD
(BMS)Current Limit During Discharge: TBD

Charge Cycle Management:
Each type of battery has a different charge profile that needs to be followed.
]Flooded Lead-Acid battery charging typically have a IUI charge profile of the following:
Bulk charge (I):[/B] Initially the battery is charged at a constant (I) rate until the cell voltage reaches a preset value - normally a voltage near to that at which gassing occurs. Constant Voltage (U): When the preset voltage has been reached, the charger switches into the constant voltage (U) phase and the current drawn by the battery will gradually drop until it reaches another preset level. [B]Equalize (I):[/B] Finally the charger switches again into the constant current mode (I) and the voltage continues to rise up to a new higher preset limit when the charger is switched off. This last phase is used to equalize the charge on the individual cells in a gassing process. [/INDENT]Flooded Lead Acid batteries are unique in that the 'gassing' allows each cell to equalize without individual cell monitoring. Typically flooded lead-acid batteries are grouped into 3 or more cells, making it impossible to 'balance' individual cells.
[B]NiMh battery Charging[/B] typically follows the figure above, NiMh batteries are charged until the cell voltage stops rising (dV=0) or begins to decrese (negative dV), or the temperature reaches a threshold (dT/dt). Delta V detection as a cutoff method is not suitable for charging currents less than 0.5 C since delta V becomes difficult to detect.
[B]Lithium battery charging[/B] has a charge characteristic as shown in the figure below.
Lithium batteries which are vulnerable to damage if the upper voltage limit is exceeded. Special precautions are needed to ensure the battery is fully charged while at the same time avoiding overcharging. For this reason it is recommended that the charging method switches to constant voltage before the cell voltage reaches its upper limit.
The lithium charge voltage rises rapidly to the cell upper voltage limit (typically 3.65V), and is subsequently maintained at that level. As the charge approaches completion the current decreases to a trickle charge. Cut off occurs when a predetermined minimum current point, which indicates a full charge, has been reached.
Cell Temperature Monitoring During Charge: TBD
Li-Ion Charge Management:A Li-Ion battery has stringent charge management requirements because the battery may ignite if overcharged. This problem is especially serious for the big packs in EVs and HEVs where it could cause a fatal accident. To ensure safety, the voltage of each Li-Ion cell must be measured very accurately since this is the best indicator of the SOC. Balancing the cell voltages (equalization) also is more difficult because the simple method of overcharge with a small current (trickle charge) cannot be used. Ideally, the Li-Ion cells should be charged individually during charge. (See "Balancing" section below as an alternative to individual cell charging)
Abnormal conditions for the Li-Ion cell voltage, battery current and temperature must activate an alarm and be handled promptly.
]Since the safety of the battery pack is dependent on the management system, the reliability of the management system becomes very critical.
Li-ion Group Balancing vs. Individual Cell Charging.  There appears to be two mechanism used for lithium pack charging. Individual cell charging is the best mechanism to obtain complete charge, but is technically difficult to do in a long series of small lithium batteries.
Balancing refers to the process of keeping all the cells in a series "pack" at roughly the same State of Charge. Balancing may involve the transfer of power from one cell to another or discharge of some cells.
Balanced charging. If a series of Li-Ion cells are in 'balance' then the entire string can be charged using a smart charger with a constant current (CC) until the upper cell voltage (3.85 * number of cells) is achieved then a Constant Voltage (CV) until a lower threshold of current is obtained.
One concept for an easy balancing method is a 'switched capacitor balancer', where a capaciter is switched between each of the cells as they charge. Balanced pack charging is not as precice as individual cell charging but works unless some of the cells are damaged or out of balance. Heavy discharge rates may unbalance cells. Minor differences in batteries may cause the pack to unbalanc.e Even with a well balanced pack, not all cells will achieve the upper cell voltage at the same moment during series charge to switch to CV, so the charger upper cell voltage for the pack is lowered by a small amount (safety factor) to avoid individual cells from overcharging. The net is that not all cells will achieve a full charge in this charge method, but should be able to achieve 90-95% capacity. Cell voltage monitoring and charger feedback can assist with the charge process.

NiMh Charge Management:
NiMH batteries are not as prone to the overcharge combustion problem as Li-Ion, their voltage measurement requirements are not as stringent. In addition, since voltage is not a useful indicator of the State of Charge (SOC) of this battery, the voltage measurements do not require the same accuracy as Li-Ion, and individual cell voltage measurements are desired but are not mandatory.
Cell charge voltage measurements for NiMh can be reduced to a few segments of series connected cells, e.g., 6 segments of 8 cells each in a 48 cell pack.
Parking Time Measurement as part of State of Charge: Leakage current when the battery is not used is much higher in NiMh than for Li-Ion, and the SOC can decrease considerably while the vehicle is parked for a few days. This high leakage along with the lack of correlation between voltage and SOC makes it much more difficult to determine the SOC for NiMh. Because of this leakage, the parking time should be measured to predict the energy loss.

Determine State of Charge:
Some battery chemistries allow the use of pack voltage to determine the state of charge. For lead-acid packs, voltage is a fair measure, but must be done under the right circumstances: a few minutes after loading the batteries. Under load, the lead-acid batteries voltage sags considerably, well below their cut-off voltage. Immediately after heavy load, the battery voltage will remain low and over a few minutes will rise up to a measureable voltage. (Not sure what the exact time is here).
Due to the fairly flat discharge curves, the SOC measurement for Li-Ion and NiMh is more difficult. {More to come here}

Communications and Reporting  Providing feedback to theoperator (driver) for the state of the battery pack is a desiredfeature, and a would be very useful to let the operator know ifthey are about to be stranded. The most useful information to theoperator is the battery pack State of Charge or SOC.
In general, determining SOC for an entire pack and reporting to operator is complex due to the difficulty of determining SOC and the wide voltage variation across an EV pack.
 
Summary Goals of a  BMS: 1. Measure the voltage of each cell and provide a signal to cut the power when the voltage gets too low (LVC). 2. Measure the voltage of each cell and throttle the charging current when the voltage gets too high. 3. Engage a shunt resistor across any cell that reaches the max. voltage threshold. This would not be needed if you are going with single cell chargers, but a single big charger is much handier. 4. Being software based, the upper and lower cell voltages can be programmable to accommodate different chemistries. 5. Not drain the batteries when they are in standby. This might require some kind of 'sleep mode'. When charging or discharging, the circuit needs to be activated, otherwise it needs a low current standby mode. 6. Easily saleable to any number of cells. 7. Needs to accomplish the above tasks at the least overall cost. 8. Optional features like 'fuel gauge' function and individual cell health displays. If a cell starts to go bad, it would be nice if the circuit would identify which one it is.