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Battery Management (BMS)

RE Solutions > Energy Storage

LiFePO4 cells have a nominal or working voltage of 3.2 Volts.

The discharge curve is very flat between 3.3V and 3.0V. The Voltage will drop exponentially from 3.8V, or 3.6V to 3.3V in the first couple of minutes, pending the discharge rate, and 95% of the time will operate between 3.3V and 3.0V. After 3.0V, it will start to drop exponentially again to 2.8V, or 2.5V which is the minimum. Discharging for all practical purposes can stop at 3.0V or 2.9V.

When we are building a Battery with LiFePO4 Cells, 3.2V cells will be placed in series to form anything from 12V to 800V battery.

Let`s take a 24V battery for an example. There will be 8 cells in series. 25.6V nominal with a Maximum of 30.4V and a minimum of 20.8V.

BMS has several functions. Two of the most important functions are protection and balancing.

Protection

In a charge cycle, although the total pack did not reach the Maximum voltage of 30.4V, there might be cells that will already be at 3.8V. Should charging continue, the cell will be damaged.

Similarly on the discharging side, if the Voltage did not reach 20.8V, some cells might be at 2.5V which can cause damage to the cells. LiFePO4 batteries are not that forgiving when it comes to over charging and under discharging, unlike their Lead acid counter parts.

In order to manage the individual cells, we need a Battery Management System (BMS) that can protect the Cells. The BMS will protect the cells from overcharge, under discharge, over current, too high charge or discharge rate, and many more protection variables.

The BMS can manage up to 256 Cells in series. Very large batteries packs of up to 1 MW can be managed with a good BMS.

ZettaJoule is using state of the art BMS systems from Lithium balance in Denmark. We received training on this system and can assist in the setup and management of such an installation.

Balancing

When there are 8, 16 or even 256 Cells in series, charging a battery pack is a bit more complex than charging just one cell. During a charge cycle, the voltage of the cells will be different. The difference between the cells is usually 30mV to 60mV which is very small, but when then cells reach the nonlinear region, the difference can be a bit more.

With a new set of Cells, the State of Charge (SOC) of the individual cells can differ quite a lot, and can be anything between 400mV up to 800mV. In a case like this, the Charger will stop charging if any individual cell in the pack reaches the maximum charging voltage of 3.6V or 3.8V.

This will mean that the rest of the pack is still not fully charged, leaving the pack in a state where the SOC is not optimized to its maximum.

Balancing is a feature in the BMS that will ensure that the SOC of the battery pack is fully optimized after charging.

When a Cell reaches the maximum voltage, and all the other cells are not in the fully charged zone (inside the 50mV window around maximum level), then the BMS will start to `Bleed` current, technically discharging the cell. This will allow the Cell to move away from the maximum level, and the charger will start to charge the rest of the pack. This will continue up until ALL the cells are in the fully charged zone.

This process of protecting cells from overcharging, discharging, and charging them again to fully charge all the other cells in a battery pack, is called balancing the cells in a Battery pack.

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