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Bluetooth BMS Cell Balancing

Balancing the cells of a LiFePO4 battery allows the most energy to be used from the battery. Since LiFePO4 cells require a Battery Management System (BMS) to protect the cells from damage, the BMS will prevent charging or discharging if any cell gets too high or low respectively. If any cell is out of balance, then the BMS will cut off charging or discharging prematurely due to the imbalanced cell.

As a bit of an extreme example, consider the case when a 100Ah’s cell 1 is at 100% state of charge and cell 2, 3, and 4 are at 60% state of charge. In this case, the battery is “full” since cell 1 is preventing any more charge from going into the battery. After 60Ah (60%) discharge, cell 2, 3, and 4 will now be at 0% and the BMS will cut off discharging. Cell 1 is now at 40%. The effective capacity for this battery is only 60Ah.

The above example is extreme to help illustrate the issue. In reality the imbalance will be at most only a few percent.

How the SOK BMS Balances the Cells

There are two ways BMS’s designed for LiFePO4 cells balance:

  1. Draining energy from the cells with higher voltage (passive balancing)
  2. Transferring energy from cells with higher voltage to cells with lower voltages (active balancing)

The SOK Battery BMS uses passive balancing to balance the cells. It drains/discharges higher voltage cells through a resistor until the voltage gets within a specified range (see below for more details). The process of balancing is very slow but it doesn’t need to be fast. Once the cells are balanced, the slow balancing is sufficient to keep the cells in balance.

When first receiving a battery

the cells will almost certainly be a bit out of balance. Many new SOK owners may notice the cells are not balanced and may think the BMS isn’t balancing the cells. Cell imbalance on first delivery is normal and as noted above, the BMS’s balancing is a very slow process, so the cells will eventually balance over time. The BMS balancing timing and resistors changed through various versions. Below is a summary of each version balance function as well as approximately how long it will take to overcome a 1% imbalance if the balance function is active 100% of the time (see below for balancing criteria):

Balance Time Example Scenario: 100Ah overcoming 1% imbalance

Version 07272189109230

  • Approximate balance time required: 13 hours
  • Balance resistor: 47Ω
  • Balance duty cycle: 100%?

Version 07272289029201 to 07272289039202

  • Approximate balance time required: 22 hours
  • Balance resistor: 47Ω
  • Balance duty cycle: 60%

Version 07272289049202 to 07272289069230

  • Approximate balance time required: 12 hours
  • Balance resistor: 33Ω
  • Balance duty cycle: 80%

Cell Balancing Criteria Based on Version

Note: the balance function will be active when all of the criteria listed for a specific version are met.

Version 07272189109230

  • any cell >3.3V
  • charge current of at least 1A
  • cell voltage difference of at least 50mV

Version 07272289029201 to 07272289039202

  • any cell >3.4V
  • charge current of at least 1A
  • cell voltage difference of at least 25mV

Version 07272289049202 to 07272289069230

  • any cell >3.4V
  • charge current of at least 1A or a charger is detected if CMOS is OFF (charger outputting a voltage less than 0.25V less than the battery voltage is considered disconnected)
  • cell voltage difference of at least 25mV
  • if any cell is >3.6V then cell voltage difference requirement drops to 10mV

Getting the BMS to Balance the Cells

Using the above criteria as a guide, we can get the BMS to having the balancing function on indefinitely or for as long as possible to get the cells balanced. Below are some general points to help ensure the balance function is either on permanently or for as long as possible:

  • Set charge to a constant 14.6V if possible
  • Set charge current to as low as possible (ensuring each battery shows at least 1A charge current)
  • For BMS versions that don’t have charger detection (see above), once the battery is full, discharge for a while (until the cell voltages are something like 3.35V) and start charging again

The Cells Get Out of Balance when Full but are Balanced when the Battery Starts to Discharge

LiFePO4 cell voltage won’t change very much through most of the charge curve. However, when the cell is full, the voltage begins to rise exponentially (likewise, when empty it drops fairly quickly but not as drastically as when full). Because of this, any imbalances between cells will only be apparent and amplified at high or low state of charge. Once the battery starts to discharge, the cell voltages will converge again so it will appear like the battery is getting balanced, but in reality it’s simply a characteristic of LiFePO4 cell voltage. Another way to put it, if the battery is 0.1% out of balance, then at Vmax 3.65V this could mean a cell Vdiff of 0.1V, while the same 0.1% imbalance at Vmax 3.3V would mean a Vdiff of somewhere around 0.001V. The imbalance was the same but the Vdiff will be very different depending on where Vmax is.

Monitoring Cell Balancing Progress

Sometimes it can seem that the battery is getting more out of balance over time when monitoring the cell voltages. Since the cell voltage is so incredibly sensitive at above 3.6V, if the point of observation of the cell voltage differences is not the same each time, the balance can seem like it’s getting worse when in reality it’s actually balancing. To effectively monitor the cell balance:

  • Pick a voltage somewhere above 3.6V (3.670V for example)
  • When watching the app, wait for the BMS to turn off CMOS due to overvoltage and charging is stopped
  • Wait until the Vmax voltage drops to the picked voltage (3.670V in this example and note Vdiff
  • Each time the Vdiff is observed, the same Vmax observation point must be used to get an accurate assessment of the balancing progress