Serial and parallel battery configurations

Battery packs get their desired operating voltage by connecting several cells in series. If higher capacity and current handling is required, the cells are connected in parallel. Some packs have a combination of serial and parallel connections. A laptop battery may have four 3.6 volts lithium-ion cells connected in series to achieve 14.4V and two cells in parallel to increase the capacity from 2000mAh to 4000mAh. Such a configuration is called 4S2P, meaning 4 cell are in series and 2 in parallel.


Single cell applications

Single cell batteries are used in watches, memory back up and cell phones. The nickel-based cell provides a nominal cell voltage of 1.2V; alkaline is 1.5V; silver-oxide 1.6V, lead-acid 2V; primary lithium 3V and lithium-ion 3.6V. Spinel, lithium-ion polymer and other lithium-based systems sometimes use 3.7V as the designated cell voltage. This explains the unfamiliar voltages such as 11.1V if three cells are connected in series. Modern microelectronics makes it possible to operate cell phones and other low power portable communications devices from a single 3.6V lithium-ion cell. Mercury, a popular cell for light meters in the 1960s has been discontinued because of environmental concerns.

Nickel-based cells are either marked 1.2V or 1.25V. There is no difference in the cells but only preference in marking. Most commercial batteries are identified with 1.2V/cell; industrial, aviation and military batteries are still marked with 1.25V/cell.

Serial connection

Portable equipment with high-energy needs is powered with battery packs in which two or more cell are connected in series. Figure 1 shows a battery pack with four 1.2-volt cells in series. The nominal voltage of the battery string is 4.8V.

Figure 1: Serial connection
of four cells.
Adding cells in a string increases the voltage but the current remains the same.

High voltage batteries have the advantage of keeping the conductor and switch sizes small. Medium-priced industrial power tools run on 12V to 19.2V batteries; high-end power tools go to 24V and 36V to get more power. The car industry will eventually increase the starter-light-ignition (SLI) battery from 12V (14V) to 36V, better known as 42V. These batteries have 18 lead-acid cells in series. The early hybrid cars are running on 148V batteries. Newer models feature batteries with 450-500V; mostly on nickel-based chemistry. A 480-volt nickel-metal-hydride battery has 400 cells in series. Some hybrid cars are also experimenting with lead acid.

42V car batteries are expensive and produce more arcing on the switches than the 12V. Another problem with higher voltage batteries is the possibility of one cell failing. Similar to a chain, the more links that are connected in series, the greater the odds of one failing. A faulty cell would produce a low voltage. In an extreme case, an open cell could break the current flow. Replacement of a faulty cell is difficult because of matching. The new cell will typically have a higher capacity than the aged cells.

Figure 2 illustrates a battery pack in which cell 3 produces only 0.6V instead of the full 1.2V. With the depressed operating voltage, the end-of-discharge point will be reached sooner than with a normal pack and the runtime is severely shortened. Once the equipment cuts off due to low voltage, the remaining three cells are unable to deliver the stored energy. Cell 3 could also exhibit a high internal resistance, causing the string to collapse under load. A weak cell in a battery string is like a blockage in a garden hose that restricts water flow. Cell 3 could also be shorted, which would lower the terminal voltage to 3.6V, or be open and cut off the current. A battery is only as good as the weakest cell in the pack.

Figure 2: Serial connection
with one faulty cell.
Faulty cell 3 lowers the overall voltage to 4.2V, causing the equipment to cut off prematurely.

Parallel connection

To obtain higher ampere-hour (Ah) ratings, two or more cells are connected in parallel. The alternative to parallel connection is using a larger cell. This option is not always available because of limited cell selection. In addition, bulky cell sizes do not lend themselves to build specialty battery shapes. Most chemistries allows parallel connection and lithium-ion is one of the best suited. Figure 3 illustrates four cells connected in parallel. The voltage of the pack remains at 1.2V but the current handling and runtime are increased four fold
Figure 3: Parallel connection
of four cells.
With parallel cells, the voltage stays the same but the current handling and runtime increases.

A high resistance or open cell is less critical in a parallel circuit than the serial configuration but the parallel pack will have reduced load capability and a shorter runtime. It's like an engine running only on three cylinders. An electrical short would be more devastating because the faulty cell would drain the energy from the other cells, causing a fire hazard. Figure 4 illustrates a parallel configuration with one faulty cell.

Figure 4: Parallel connection
with one faulty cell.
A weak cell will not affect the voltage but provide a low runtime. A shorted cell could cause excessive heat and create a fire hazard.

Serial/parallel connection

Figure 5 illustrates a parallel/serial connection. This allows good design flexibility and attains the wanted voltage and current ratings by using a standard cell size. It should be noted that the total power does not change with different configurations. The power is the product of voltage times current.

Figure 5: Serial/ parallel connection of four cells.
The configurations will not affect the overall power but provide the most suitable voltage and current source for the application.

Serial/parallel connections are common with lithium-ion. One of the most popular cells is the 18650 (18mm diameter; 650mm long). Because of the protection circuit, which must monitor each cell connected in series, the maximum practical voltage is 14.4V. The protection must also monitor strings placed in parallel.

Charging and discharging lithium-ion batteries

Lithium-ion batteries are safe when used as directed. The same safety cannot be assured if individual cells of unknown nature are connected in series and parallel to build a battery pack. Not all lithium-ion cells are suited for muli-cell packs. Only cells that meet tight voltage and capacity tolerances can be used for serial and parallel connection. Mismatched packs are subject to overcharge, resulting in venting with flame and fire. Check with the cell manufacturer if the cells are suitable for multi-cell packs.

In the past, single lithium-ion cells were only made available to authorized battery assembles. Today, imports are becoming readily available and often fall into the hands of the inexperienced. While most brand name cells are equipped with an internal cell disconnect that permanently opens the current path on high pressure, some brands do not provide this safeguard. Neither do many brands use a separator that is designed to shut down on high temperature. The internal safety features are omitted for cost reasons.

Please follow the following guidelines when charging and discharging lithium-ion cell(s) and packs. Failing to follow these rules could result in venting with flame, explosion, fire and personal injury.

WARNING when charging lithium-ion cells and packs

- Never connect cells in parallel and/or series that are not designed for that purpose. A cell mismatch may cause overcharge and venting with flame.

- Never charge or discharge the battery without connecting a working protection circuit. Each cell must be monitored individually and the current disconnected if an anomaly occurs.

- Always attach a temperature sensor when charging and discharging the battery. The temperature sensor must disconnect the current on excess temperature.

- Only connect cells that are matched and have the identical state-of-charge.

- Pay special attention when using an unknown brand. Not all brands contain intrinsic safety features that protect the cell when stressed.

- During experiments, place the test battery into a well-ventilated fireproof container.

- Never leave the battery unattended while under charge or discharge.

- Do not charge a battery hat has physical damage.

Household batteries

The serial and parallel connections of cells described above addresses rechargeable battery packs in which the cells are permanently welded together. The same rules apply to household batteries except that we are dealing here with single cells that are put into a battery holder and form a serial configuration. When using single cells, some basic guidelines must be observed:

- Keep the battery contacts clean. A four-cell configuration has eight contacts (cell to holder and holder to next cell). Each contact exhibits some resistance which, when added, can affect the overall battery performance.

- Never mix batteries. Replace all cells when weak. (Remember the 'weak link of a chain' and 'a battery is only as good as the weakest cell'.) Use the same cell type for the whole string.

- Do not recharge non-rechargeable batteries. Charging primary cells will generate hydrogen that can lead to an explosion.

- Observe the right polarity. A reversed cell will deduct rather than add the cell voltage to the string.

- Charging a secondary battery with a reversed polarity will cause the affected cell to develop an electrical short. If left unattended, the damaged cell will heat up and create a fire hazard.

- Remove fully discharged batteries from the equipment. Old cells tend to leak and cause corrosion. Alkaline is less critical than carbon-zinc.

- Remove the batteries when the equipment is not used for a while to prevent corrosion.

- Do not store a box of cells in a way that can create an electrical short. A short cell will
heat up and create a fire hazard. Place lose cells in small plastic bags for electrical insulation.

- Always keep batteries away from children.

- Primary batteries such as Alkaline can be disposed in regular trash. It is recommended, however, to bring the spent batteries to a depot for recycle or disposal.

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Created: September 2004, Last edited: May, 2005


About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC. Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.