BU Summarized: Learn How to Arrange Batteries to Increase Voltage or Gain Higher Capacity

December 05, 2017

One of the most popular recent articles on the Cadex sponsored educational website BatteryUniversity.com presents a detailed look at series and parallel battery configurations. Here, on the Cadex blog, we've condensed the article and presented the following key points. After reading this, if you want to learn more, please click here.

Batteries achieve the desired operating voltage by connecting several cells in series; each cell adds its voltage potential to derive at the total terminal voltage. Some packs may consist of a combination of series and parallel connections. Laptop batteries commonly have four 3.6V Li-ion cells in series to achieve a nominal voltage 14.4V and two in parallel to boost the capacity from 2,400mAh to 4,800mAh. Such a configuration is called 4s2p, meaning four cells in series and two in parallel. 

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, as a battery is only as strong as the weakest link in the chain.

Single Cell Applications

The single-cell configuration is the simplest battery pack; the cell does not need matching and the protection circuit on a small Li-ion cell can be kept simple. Typical examples are mobile phones and tablets with one 3.60V Li-ion cell. Other uses of a single cell are wall clocks, which typically use a 1.5V alkaline cell, wristwatches and memory backup, most of which are very low power applications.

Series Connection

Portable equipment needing higher voltages use battery packs with two or more cells connected in series. 

Serial connection of four NiCd or NiMH cells

Figure 2: Series connection of four cells (4s).
Adding cells in a string increases the voltage; the capacity remains the same.

High voltage batteries keep the conductor size small. Cordless power tools run on 12V and 18V batteries; high-end models use 24V and 36V. Most e-bikes come with 36V Li-ion, some are 48V. The car industry wanted to increase the starter battery from 12V (14V) to 36V, better known as 42V, by placing 18 lead acid cells in series. 
Some mild hybrid cars run on 48V Li-ion and use DC-DC conversion to 12V for the electrical system. 

Parallel Connection

If higher currents are needed and larger cells are not available or do not fit the design constraint, one or more cells can be connected in parallel. Most battery chemistries allow parallel configurations with little side effect. 

Parallel connection of four cells

Figure 4: Parallel connection of four cells (4p).
With parallel cells, capacity in Ah and runtime increases while the voltage stays the same.

Series/parallel Connection

The series/parallel configuration shown in Figure 6 enables design flexibility and achieves the desired voltage and current ratings with a standard cell size. The total power is the product of voltage-times-current; four 3.6V (nominal) cells multiplied by 3,400mAh produce 12.24Wh. Four 18650 Energy Cells of 3,400mAh each can be connected in series and parallel as shown to get 7.2V nominal and 12.24Wh. The slim cell allows flexible pack design but a protection circuit is needed.


Figure 6: Series/ parallel connection of four cells (2s2p).
This configuration provides maximum design flexibility. Paralleling the cells helps in voltage management.


Safety devices in Series and Parallel Connection

Positive Temperature Coefficient Switches (PTC) and Charge Interrupt Devices (CID) protect the battery from overcurrent and excessive pressure. While recommended for safety in a smaller 2- or 3-cell pack with serial and parallel configuration, these protection devices are often being omitted in larger multi-cell batteries, such as those for power tool. 

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