Balancing LiPo Cells

Things change fast in the electronics world, and that’s also true for recharge- able batteries. The rate of development of new types of rechargeable batteries has been accelerated by the steadily increasing miniaturisation of electronic equipment. LiPo cells have conquered the market in a relatively short time. Their price and availability have now reached a level that makes them attractive for use in DIY circuits.

Circuit diagram

Balancing LiPo Cells Circuit diagram

Unlike its competitors Elektor Electronics has already published several articles about the advantages and disadvantages of LiPo batteries. One of the somewhat less well-known properties of this type of rechargeable battery is that the cells must be regularly ‘balanced’ if they are connected in series. This is because no two cells are exactly the same, and they may not all have the same temperature. For instance, consider a battery consisting of a block of three cells. In this case the outer cells will cool faster than the cell in the middle. Over the long term, the net result is that the cells will have different charge states. It is thus certainly possible for an individual cell to be excessively discharged even when the total voltage gives the impression that the battery is not fully discharged. That requires action – if only to prolong the useful life of the battery, since LiPo batteries are still not all that inexpensive. 

One way to ensure that all of the cells have approximately the same charge state is limit the voltage of each cell to 4.1 V during charging. Most chargers switch over to a constant voltage when the voltage across the batter terminals is 4.2 V per cell. If we instead ensure that the maximum voltage of each cell is 4.1 V, the charger can always operate in constant-current mode. 

When the voltage of a particular cell reaches 4.1 V, that cell can be discharged until its voltage is a bit less than 4.1 V. After a short while, all of the cells will have a voltage of 4.1 V, with each cell thus having approximately the same amount of charge. That means that the battery pack has been rebalanced. 

The circuit (Figure 1) uses an IC that is actually designed for monitoring the supply voltage of a microcontroller circuit. The IC (IC1) normally ensures that the microcontroller receives an active-high reset signal whenever the supply voltage drops below 4.1 V. By contrast, the out-put goes low when the voltage is 4.1 V or higher. In this circuit the output is used to discharge a LiPo cell as soon as the voltage rises above 4.1 V. 

When that happens, the push-pull output of IC1 goes low, which in turn causes transistor T1 to con-duct. A current of approximately 1 A then flows via resistor R1. LED D2 will also shine as a sign that the cell has reached a voltage of 4.1 V. The function of IC2 requires a bit of explanation. The circuit built around the four NAND gates extends the ‘low’ interval of the signal generated by IC1. That acts as a sort of hysteresis, in order to prevent IC1 from immediately switching off again when the voltage drops due the internal resistance of the cell and the resistance of the wiring between the cell and the circuit. The circuitry around IC2 extends the duration of the discharge pulse to at least 1 s.

Figure 2 shows how several circuits of this type can be connected to a LiPo battery. Such batteries usually have a connector for a balancing device. If a suit-able connector is not available, you will have to open the battery pack and make your own connections for it. The figure also clearly shows that a separate circuit is necessary for each cell.