Serial and parallel battery configurations (BU10A) 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.
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.
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
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.
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.
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 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. Click here for a printer-friendly version © Copyright 2003 - 2005 Isidor Buchmann
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