2.2 Cell Balancing

Cells in series drift apart over time: one reaches full charge before the others and limits the whole battery. Balancing brings the cells back to the same voltage. This page helps you switch balancing on and set it for your battery.

Applies to

Boards: BMS1820, BMS3610, BCU2002, BMS1601, BMS1810, and the slave modules BMS1802 and BMS1202.

Balancing is off by default

The factory setting of Balancing Method is Balance Restricted: the BMS does not balance at all. Select a method when you commission the battery.

2.2.1 How It Works

LiBat™ boards balance passively: a resistor on the board takes charge from a cell that is higher than the others and turns it into heat. Each board balances its own cells.

Fig.2.2.1: Which Cells Are Balanced

3940 3950 3960 3970 3980 3990 4000 Cell voltage (mV) Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell Balance Threshold lowest cell + Balance Start Delta lowest cell + Balance Stop Delta lowest cell 3958 mV balancing now (highest cells first) waiting: Simultaneous Balancing Cells = 2 no balancing needed Example voltages with the factory thresholds. A cell balances until it is down to the stop line.

  • Start: a cell needs balancing when it is at least Balance Start Delta above the lowest cell of the whole battery.
  • Stop: it keeps balancing until it is down to Balance Stop Delta above the lowest cell.
  • Threshold: only cells at or above Cell Balance Threshold are balanced.
  • Highest first: the highest cells are balanced first. On one board at most Simultaneous Balancing Cells cells balance at the same time; the others wait.
  • Method: Balancing Method decides in which pack state the BMS balances (Table.2.2.2). The pack state follows the current: below 0.5 A the pack is idle (2.4 Current Measurement).
  • Duty cycle: the balancing resistor of a cell is switched on for Balance Duty Cycle percent of the time. The average balancing current is the board's balancing current times the duty cycle.
  • Board temperature: while the board is at or above Cell Balance O.T. Warning (2.3 Temperature Limits), balancing stops. It starts again when the board has cooled down.

2.2.2 Settings

Table.2.2.1: Balancing Settings (LiMon™ Cell tab)

Setting What it does Unit Default
Cell Balance Threshold Lowest cell voltage at which a cell can be balanced. mV 3950
Balance Start Delta Difference to the lowest cell at which a cell starts balancing. mV 25
Balance Stop Delta Difference to the lowest cell at which a cell stops balancing. mV 5
Simultaneous Balancing Cells Highest number of cells that balance at the same time on one board. — 2
Balancing Method Pack state in which the BMS balances (Table.2.2.2). — Balance Restricted
Balance Duty Cycle Share of the time the balancing resistor is on, 10…100 %. % 30

The LiMon™ reference describes every field of the screen: 7.2 Cell Settings.

Table.2.2.2: Balancing Methods

Balancing Method The BMS balances
Balance Restricted never
While Charging only while the battery is charging
While Discharging only while the battery is discharging
When Not Idle while the battery is charging or discharging
Always in every pack state, also at rest

Table.2.2.3: Balancing Current per Board (at 4.2 V cell voltage, 100 % duty cycle)

Board Balancing current
BMS1820, BMS1202 420 mA
BMS3610 up to 600 mA
BMS1802 1000 mA

The current falls with the cell voltage, because the resistor is fixed. See the board pages under BMS Hardware for the other boards, for example BMS1820.

2.2.3 Choosing the Values

  • Balancing Method: While Charging suits most batteries. Near the end of the charge the voltage differences show the real differences in charge, and the charger replaces the energy that the resistors turn into heat. Use Always only where the battery rests near full charge for long times, for example in standby systems.
  • Cell Balance Threshold: in the upper part of the charge curve of your cells, where a difference in voltage means a difference in charge. Lower in the curve the voltage depends more on the current and the temperature, and the BMS would balance the wrong cells. Take it from the cell datasheet; for cells with a flat curve, set it near the end of the charge.
  • Balance Start Delta: large enough that measurement noise and the voltage drop under current do not start balancing. A few tens of mV suits most systems.
  • Balance Stop Delta: clearly below Balance Start Delta, so that a cell does not switch on and off at the limit.
  • Simultaneous Balancing Cells and Balance Duty Cycle: together they set the heat on the board. A higher duty cycle balances faster but warms the board; if balancing stops on Cell Balance O.T. Warning, lower the duty cycle or the number of cells. Keep enough air around the boards.
  • Large differences: balancing corrects small drift. A cell that keeps drifting far after balancing points to a weak cell or a bad connection; a difference that reaches Imbalanced Max. Cell Delta stops the battery (2.1 Cell Voltage Limits).

2.2.4 Example

Calculation example: time to correct a difference of 1 Ah

Chosen values: a BMS1820 at about 4.2 V per cell, Balance Duty Cycle 30 %, Balancing Method While Charging.

  • Average balancing current: 420 mA × 30 % ≈ 126 mA.
  • Time to take 1 Ah from the highest cell: 1 Ah ÷ 0.126 A ≈ 8 hours of balancing.
  • The BMS balances only while charging and only above Cell Balance Threshold, so these 8 hours are spread over several charges. With a duty cycle of 60 % the time halves, and the board gets warmer.

2.2.5 Checking the Result

  • At the end of a charge: the difference between the highest and the lowest cell in Live View becomes smaller over a few charges.
  • While balancing: the cell chart in Live View marks the cells that are balancing.
  • No Cell Balance O.T. Warning in Events › Event Status. If it appears often, lower the duty cycle or the number of cells.

2.2.6 Common Mistakes

  • Balancing left on Balance Restricted. The factory setting never balances, so the cells drift apart over months and the battery loses capacity. Select a method at commissioning.
  • Cell Balance Threshold too low. The BMS balances in the middle of the charge curve, where the voltage does not show the charge well, and takes charge from the wrong cells.
  • Balance Start Delta too small. Small voltage differences under current start balancing again and again; the board warms and charge is wasted.
  • Expecting balancing to fix a large difference fast. With a small average current a difference of several Ah takes days of charging. Bring the cells to the same voltage before you assemble the battery.
  • Balancing stops on board temperature. A high duty cycle with many cells at once in a closed box reaches Cell Balance O.T. Warning. Lower the duty cycle or improve the airflow.
  • While Discharging chosen for a battery that is rarely full. Balancing then runs at lower voltages, where the cell differences are less clear. Prefer While Charging.