It’s a common question: do I need balancing if my cells are already balanced, and how much balancing current do I need?
To get the most out of your battery pack, every battery needs balancing from time to time. Even when the cells are matched.
Multiple battery properties have influence on the cell disbalance, for example self-discharge current and Coulomb efficiency. Let’s discuss one of these properties: the self-discharge.
Each cell has its own self-discharge. The exact value always varies from cell to cell. Especially if a cell in a battery is replaced by a new one, the self discharge of this new cell can be very different from the others. The result is that one cell will discharge faster than the other. This results in an unbalance. One cell is fuller than the other. When the cells are fully charged, the charging will stop as soon as the first cell is full, so that this cell is not damaged. The emptier cell(s) will not be completely full. As a result, the usable capacity is reduced. This effect becomes more and more noticeable: the usable capacity decreases more and more.
Balancing ensures that each cell can be fully charged, so that the usable capacity is also maximized.
This happens in the last stage of charging: called top balancing. When the cell is nearly full, the voltage of the cell will increase. This creates a voltage difference with the other cells and so the BMS will start balancing.
Balancing methods #
There are two balancing methods:
Passive balancing #
In this process, energy from the fuller cells is burned. This allows the charger to continue charging. The cells that are balancing will thus charge at a slower rate, while emptier cells charge at a greater rate. After a certain time all cells are fully charged and the charger can stop.
This method is used in most BMS since only a small amount of energy is lost during charging.
Active balancing #
Instead of dissipating energy, energy from a full cell will go to a leaner one. Depending on the balancing hardware, this can be done in different ways:
- One at a time: from fullest cell in the battery to emptiest.
- From fuller to emptier neighbor cell. Each cell receives or delivers energy to a neighbor.
- Cell to battery. Energy from fullest cell goes to all cells at once.
- Cell to cell. Energy from the fullest cells goes to the emptiest cells.
The advantage of this method is that it is more efficient and heats up a less than passive balancing. The disadvantage is that the hardware is quite expensive.
Balancing current #
Despite the fact that many BMS can balance, many systems have only a limited balancing current often on the order of 40-100mA. This means that it takes a long time for the system to be rebalanced. Especially with larger cell capacities, this small balance current is not enough.
The 123\SmartBMS balances at 1A per cell, so balancing is faster and also large cell capacities can be balanced.
Balancing time #
The time that balancing takes depends on the balancing method, balancing current and imbalance between cells.
In the following calculation example, we assume passive balancing and 1A balancing current, which the 123SmartBMS has. The emptiest cell has a charge of 38Ah and the fullest has a charge of 41Ah.
The imbalance is 41-39.5Ah = 1.5Ah. With a balancing current of 1A, balancing takes approximately 1.5Ah/1A = 1.5 hours.
Note: a BMS with a balancing current of only 100mA (0.1A) would need 15 hours!