Lithium Batteries packs are different to AGM in the way they are manufactured and designed.

Let’s get a few of the terms on the table before we begin.

Then we can discuss one of features of the BMS, which is Balancing

Cells

Cells are the individual building blocks for Lithium Batteries and are broadly either cylindrical or prismatic cells.

Each cell has a nominal voltage of 3.2 volts.

Each cell has a capacity rating in Ah and small cylindrical are sometimes 5Ah and larger Prismatic are 50/120/200Ah.

You either use a single cell or group a number together in parallel to build a battery pack of the intended capacity.

You would use a single 110Ah Prismatic Cell if you were building a 100Ah Battery Pack.

Packs

To create a 12V Battery Pack you place the cells or groups of cells and place them in series to create the battery pack.

Pack refers to the complete battery and you refer to the pack voltage as being the nominal 12V

Parallel Vs Series

Parallel is when you connect cells together with a positive-to-positive link and a negative-to-negative link.

Series is when your daisy chain the connections and go from the positive of the first cell into the negative of the 2nd cell.

Then positive of the 2nd cell to the negative of the third cell and so forth along the chain.

The Pack voltage is the most negative of the first cell and pack positive is the positive of the 4th cell once they are put into series.

……

The images below show the cells in a 12150 Battery.

The pack is a 12V Battery and the Capacity is 150Ah.

To get the capacity of 150Ah there are 2 x 75Ah prismatic cells placed in parallel (POS to POS and NEG to NEG) first.

Then to get the 12V there are 4 of these placed in series (POS to NEG along the 4 cell string)

This arrangement is called a 2P4S Pack.

BLITH packs use a CLB (Cell Link Board) which brings all of these connections together.

BMS 

The BMS (Battery Management System) is the controlling brain of the battery pack. 

It looks at each individual cell in the pack and also looks at the total pack. 

Some decisions are based on how the whole pack is operating and some operations are based on looking at individual cells. 

One of the functions is cell balancing which is looking at each cell in the pack and keeping cells in balance: 

What is cell balancing? 

This is when the BMS looks at individual cells and tries to keep them at a similar voltage to the others in the pack. 

Most BMS will disconnect when any individual cell is below 2.5V on LVD (Low Voltage Disconnect) 

Also when charging it will disconnect the pack when any individual cell is charged above 3.65V HVD (High Voltage Diisconnect) 

Towards the bottom of a cycle and also at the top of cycle it is common for the cell voltages to diverge. 

The BMS will use one of the following techniques to keep the cells in balance. 

Active Balancing 

Active balancing is moving charge from the highest voltage cell in the pack to the lowest voltage cell in the pack. 

This technique is constant and requires circuitry to control and monitor this energy movement all the time the battery is operating. 

Complex hot circuitry is required because the current path resistors generate heat to control the flow of energy. 

Smart Passive Balancing 

Smart Passive Balancing is bleeding energy from the highest voltage cell to bring into line with voltage of the lowest cell. 

It is smart because instead of over bleeding it is only bleeding to the voltage point of the lowest cell. 

Passive Balancing 

Passive Balancing is bleeding energy when cells are at the top of the cycle and bleeding to a set threshold of usually 3.4V. 

It is a simple technique but not as efficient as Smart Passive. 

It is important to keep the cells in balance. 

When cells are out of balance you loose capacity at both ends of the cycle. 

LVD (Low Voltage Disconnect and HVD (High Voltage Disconnect) are triggered by either the lowest of highest cell voltage. 

Similar to the old adage of the weakest link in the chain. 

Luckily unlike chains, the modern Lithium Battery is constantly strengthening the weakest link. 

Baintech is here to help. Contact our friendly team if you have questions about our products and best practices for lithium batteries.

Leave us a comment

Your email address will not be published. Required fields are marked *