Balancing on the Orion BMS only occurs when the BMS is powered in CHARGE mode (powered by pin 3 on the Main I/O connector). When any one cell in the battery pack exceeds the Start Balancing voltage, the BMS will begin the balancing algorithm for all cells. The BMS will look for the lowest cell, then place a load on all cells which are more than the maximum difference in voltage above the lowest cell. For example, if a battery pack consists of 4 cells at 3.5, 3.51, 3.65 and 3.49 volts and the maximum difference in voltage is configured for 10mV (0.01 volts), the BMS would only apply a load to the cell which is 3.65v, to bring it down to within 10mV with the rest of the cells. This algorithm continues until all cells are balanced to within the pre-defined maximum difference in voltage and continues even after the BMS has switched off the charger. Once all cells are within this voltage, balancing will stop until power is removed and re-applied to pin 3 on the BMS (i.e. the next charge cycle).
The BMS has a safety feature to prevent over-discharging any cell during balancing in the event of a defective or dead cell. A minimum balancing voltage threshold allows the programmer to specify a voltage threshold at which the BMS is not allowed to remove energy from a cell. While the rest of the cells will continue to balance, the BMS will not place a load on any cell which is below this threshold, even if a cell below this threshold is needing to be balanced. The purpose of this feature is to protect the cells from over-discharge and to prevent a possible race condition where the BMS removes charge from alternating cells.
The start balancing voltage setting should typically be configured to a voltage that indicates a cell is within approximately 5-10% of the maximum state of charge. For iron phosphate this is typically about 3.5v and varies with other chemistries. The maximum delta voltage (difference in voltage from the highest to lowest cell) recommended is 10mV for most lithium ion chemistries such as iron phoshpate, but may be adjusted slightly lower for certain chemistries with a linear discharge curve (such as many manganese or polymer type cells.) A value too low will cause a race condition reducing or eliminating the effectiveness of the balancing algorithm and 10mV is recommended unless research has been done on a lower setting. When balancing a grossly out of balance pack, choosing a higher number such as 20mV may increase the speed of bulk balancing, but should then be reduced back to 10mV for finer balancing.
The minimum balancing voltage setting is simply to prevent cells from becoming over-discharged. This value can be set to a fairly low voltage, often a voltage corresponding to around 25% state of charge. For iron phosphate a voltage of 3.0 to 3.2v is appropriate. The minimum balancing voltage setting must be low enough to allow the BMS to effectively perform balancing and must be below the settling voltage.
While the BMS is balancing, the balancing will pause every so often to allow cell voltages to settle and to re-evaluate the balance of the cells in the pack. This is a normal part of the balancing algorithm and happens at set intervals. If the BMS unit itself is at an elevated temperature, the BMS will pause for a longer period of time to prevent overheating. To prevent a burn hazard, the BMS will not balance at all when the heatsink temperature is above 50C.
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