Pre-Balancing Cells

Lithium ion cells are often shipped from the manufacturer at different states of charge from one another. This is sometimes due to the manufacturer pulling from different lots, cells with vastly different discharge rates, or a result of the manufacturer doing spot testing on some of the cells. This is an issue when assembling a battery pack since the cells must be brought to roughly the same state of charge before the pack can be used.

The Orion BMS and Orion Jr. BMS have balancing capabilities to bring cells into balance with each other. However, in order to lessen the amount of energy wasted and to reduce costs, the BMS is designed to maintain an already roughly balanced battery pack and is not designed to provide the initial gross balancing. Gross balancing only occurs once, and designing the BMS to handle this one event in the life of the pack would add dramatically to the cost, size and weight of the BMS.

The first step is to determine if you need to pre-balance cells or not. If your cells are already in balance, pre-balancing is obviously not necessary. Likewise, if the amount of time it will take the BMS to balance your cells is acceptable, no pre-balancing is necessary.

The Orion BMS balances at a maximum rate of about 200mA. This is enough current to maintain balance on most configurations with cells up to 1000Ah (balancing is only necessary to correct the difference in self discharge between cells which typically is measured in micro amps (uA.) A general rule of thumb is that an Orion BMS can correct about 25 amp hours of imbalance per week or about 3 amp hours per day. If a pack is being cycled and used while the initial balance is occurring, balancing may take considerably longer as the balancing only occurs when the pack is near full state of charge. For small battery packs such as 40 – 100 amp hours, it may be acceptable to simply allow the BMS to balance over the course of a few days regardless of how out of balance the pack is. For larger packs closer to 1000Ah, the BMS may take an extremely long time to balance, and pre-balance is strongly recommended. In general, it is a good rule of thumb to try and have the battery pack within 40 amp hours or less of being in balance before turning the pack over to the BMS to balance.

Keep in mind that part of the battery pack is usable while the initial balance is occurring, and for some applications, it is acceptable to use the battery while the initial balance is occurring (e.g. a standby power application). The unusable portion is the difference between the highest and lowest cells in the pack. For example, if a battery pack consists of 4 cells at 60%, 80%, 70% and 75% SOC, the usable portion of the battery is 80% since the difference in balance from the lowest to the highest cell is 20%.

With some chemistries, it is easier to determine the difference in balance from voltages than others. With iron phosphate in particular, it is difficult to determine a difference in state of charge from voltage alone. The difference between 3.2 volts (open cell voltage / no load) and 3.35 volts (open cell voltage) is far more significant than the difference between 3.35 and 3.65v. 3.2 volts (open cell voltage) may equate to around 30-40% while 3.35 volts may equate closer to 80-85% state of charge. This sometimes causes confusion as to whether or not a battery pack is significantly out of balance or not.


If pre-balancing is necessary, there are a couple common methods for pre-balancing cells:

Method 1: Charge all cells individually. Before assembling the cells into a pack, each individual cell may be charged independently by using a battery charger designed for a single lithium ion cell. It is essential that if this method is used, the charger MUST be configured so that it does not overcharge the cell. Never leave a cell charging without an automatic method for shutting of the charger.


Method 2: Put all cells in parallel and fully charge together. Before assembling the pack, cells can be connected in parallel and charged together. This method may not work with some types of cells or if the cells are significantly out of balance, as significant currents may flow from one cell to another. Current flow can be calculated by taking the difference in voltage from the lowest to highest cell and dividing by the internal resistance. If you are unsure if current will flow or if you are using a chemistry other than iron phosphate, use one of the other methods. The charger must be setup with a maximum voltage no higher than the maximum cell voltage specified by the cell manufacturer. In order for this method to work, cells must have a charge applied while in parallel (simply connecting in parallel will not allow enough current to flow to balance the pack.)


Method 3: Measure voltages of each cell and manually adjust using a charger or load. This is the most difficult method, but it may be significantly faster than the others if only a small number of cells are significantly out of balance while the rest of the pack is well balanced already. This is an especially useful technique after a cell in a pack has been replaced. Use an isolated load or isolated charger to manually adjust the state of charge of the particular cell requiring adjustment to bring it into balance with the rest of the pack. Careful attention must be given to ensure that a cell is not over charged or over-discharged with this method. This method can be used to balance a cell after it has been assembled into a series pack, but it is absolutely essential to ensure that the load or charger is isolated so that a short circuit is not created within the pack.


Method 4: Have the manufacturer “bin” the cells. While this is not usually an option for small scale systems, in production environments, cell manufacturers can “bin” the cells and match them based on current state of charge, internal resistance characteristics, and cell capacity. Building a pack using cells which are all approximately the same not only eliminates the need to pre-balance, but also keeps the pack in the best health long term since all cells are likely to age similarly.

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