Construction of a Simple Multi Cell Battery Pack

Q1a. How weakest cell limits the usable capacity of the battery pack?

The weakest cell limits the because it sets a barrier for the entire battery pack to work with limitations. The weak cell may not fail immediately but will get exhausted more quickly than the strong ones when on a load. On charge, the low cell fills up before the strong ones because there is less to fill and it remains in over-charge longer than the others. On discharge, the weak cell empties first and gets hammered by the stronger brothers. Cells in multi-packs must be matched, especially when used under heavy loads.

Most battery chemistries lend themselves to series and parallel connections. It is important to use the same battery type with equal voltage and capacity (Ah) and never to mix different makes and sizes. A weaker cell would cause an imbalance. This is especially critical in a series configuration because a battery is only as strong as the weakest link in the chain. 

Example

Consider a lower voltage cell connected in a series configuration

A battery pack in which “cell 3” produces only 2.8V instead of the full nominal 3.6V. With depressed operating voltage, this battery reaches the end-of-discharge point sooner than a normal pack. The voltage collapses and the device turns off with a “Low Battery” message.

Consider a lower capacity cell connected in a parallel configuration

A cell that develops high resistance or opens is less critical in a parallel circuit than in a series configuration, but a failing cell will reduce the total load capacity. It’s like an engine only firing on three cylinders instead of on all four.

A weak cell will not affect the voltage but provide a low runtime due to reduced capacity. A shorted cell could cause excessive heat and become a fire hazard. On larger packs, a fuse prevents high current by isolating the cell.

Q1b.What is the solution?

To overcome the problem of reduced voltage and capacity we need to monitor the voltage and soc of each and every cell in a battery pack. This way technique of maintaining the overall balance in the battery pack is called cell balancing. For example, cells are connected in series the voltage should be equal, and cells connected in parallel the capacity or the soc must be equal to derive the battery pack with maximum efficiency. The method of maintaining all the cells is called cell balancing. 

Active balancing

Active cell balancing methods remove the charge from one or more high cells and deliver the charge to one or more low cells. Since it is impractical to provide independent charging for all the individual cells simultaneously, the balancing charge must be applied sequentially. Taking into account the charging times for each cell, the equalization process is also very time consuming with charging times measured in hours. Some active cell balancing schemes are designed to halt the charging of the fully charged cells and continue charging the weaker cells till they reach full charge thus maximizing the battery's charge capacity.

• Charge Shuttle (Flying Capacitor) Charge Distribution

With this method, a capacitor is switched sequentially across each cell in the series chain. The capacitor averages the charge level on the cells by picking upcharge from the cells with higher than average voltage and dumping the charge into cells with lower than average voltage. Alternatively, the process can be speeded up by programming the capacitor to repeatedly transfer charge from the highest voltage cell to the lowest voltage cell. Efficiency is reduced as the cell voltage differences are reduced. The method is fairly complex with expensive electronics.

• Inductive Shuttle Charge Distribution

This method uses a transformer with its primary winding connected across the battery and a secondary winding that can be switched across individual cells. It is used to take pulses of energy as required from the full battery, rather than small charge differences from a single cell, to top up the remaining cells. It averages the charge level as with the Flying Capacitor but avoids the problem of small voltage differences in cell voltage and is consequently much faster. This system obviously needs well-balanced secondary transformer windings otherwise it will contribute to the problem.

Passive balancing

Dissipative techniques find the cells with the highest charge in the pack, indicated by the higher cell voltage, and remove excess energy through a bypass resistor until the voltage or charge matches the voltage on the weaker cells. Some passive balancing schemes stop charging altogether when the first cell is fully charged, then discharge the fully charged cells into a load until they reach the same charge level as the weaker cells. Other schemes are designed to continue charging till all the cells are fully charged but to limit the voltage which can be applied to individual cells and to bypass the cells when this voltage has been reached.

This method levels downwards and because it uses low bypass currents, equalization times are very long. Pack performance determined by the weakest cell and is lossy due to wasted energy in the bypass resistors which could drain the battery if operated continuously. It is however the lowest cost option.

References

1. https://circuitdigest.com/article/cell-balancing-techniques-and-how-to-use-them

2. https://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations

3. https://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations