Serial and parallel battery configurations
Cadex Electronics Inc.
www.buchmann.ca - www.BatteryUniversity.com
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.
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.
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.
2: Serial connection with one faulty cell.
Faulty cell 3 lowers the overall voltage to 4.2V, causing
the equipment to cut off prematurely.
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 series. The voltage of the pack remains at 1.2V
but the current handling and runtime are increased four fold.
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
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.
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
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
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
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:
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.
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
not recharge non-rechargeable batteries. Charging primary
cells will generate hydrogen that can lead to an explosion.
the right polarity. A reversed cell will deduct rather than
add the cell voltage to the string.
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
fully discharged batteries from the equipment. Old cells
tend to leak and cause corrosion. Alkaline is less critical
the batteries when the equipment is not used for a while
to prevent corrosion.
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.
keep batteries away from children.
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.
About the Author
Isidor Buchmann, founder and CEO of Cadex Electronics Inc.,
has studied the behavior of rechargeable batteries in practical,
everyday applications for two decades. As an award-winning
author of many articles and books on the subject, Mr. Buchmann
has delivered battery-related technical papers around the
world. Cadex is a Canadian company specializing in the design
and manufacturing of advanced battery testing instruments.
For product information please visit www.cadex.com