In the world of electronics, nothing is created equal. This is also applicable to batteries. A 10AH (ampere-hour) rated battery is not exactly 10AH in actual. This variation on capacity is a big problem when connecting battery in series because the battery with least capacity will be the first to full charge and full discharge. This is where you need a battery balancing circuit.
Battery Balancing Techniques
There are many methods but we will only focus on the 2 most common used for battery balancing.
PASSIVE BALANCING / Resistor loads
The first method is using resistor load also known as passive balancing. On this method we use resistor to discharge the battery with highest charge voltage. Some circuit designs only monitors per cell, the resistor loads will activate when the cell reaches its maximum voltage.
The main problem with this kind of method is that it is wasting power
Pros:
* Simple
* Cheap
Cons:
* Wasting power – not efficient
ACTIVE BALANCING / CAPACITOR based balancing
The second most common method is the active balancing using capacitors. In this method, we are using a capacitor to charge on the cell with higher voltage and discharge it to the cell with lower voltage. It is like you are charging the cell with lower voltage using the cell with the higher voltage. This method is vey efficient.
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Reference
https://www.analog.com/en/technical-articles/active-battery-cell-balancing.html