Download How to Construct a Lithium-Ion/Polymer Battery with Imbedded

Constructing a Lithium-Ion/Polymer Battery with
an Imbedded Protection Circuit Module (PCM)
Michael Kovalcik
Team 10
ECE_480_FS08
Executive Summary:
Rechargeable lithium- ion (Li-Ion) and lithium-polymer (Li-Po) cells have a
higher energy density and greater nominal voltage than conventional, onetime use
batteries. Li-Ion and Li-Po batteries are the ideal power source for many portable
electronic devices. In order to maximize their lifecycle and increase safety, both LiIon and Li-Po cells require the use of a Protection Circuit Model (PCM) when
assembled together to form a battery. The PCM will generally provide the battery
with overcharge/discharge protection, short circuit protection, current limitation, and
voltage and current balancing in each cell. All of these factors are important in
obtaining the maximum output and longest lifecycle from a rechargeable lithium
battery. These notes will discuss the need for and configuration of a PCM, the
advantages and disadvantages of these configurations, and step-by-step instructions
for the construction of a rechargeable lithium battery with an onboard PCM.
Keywords:
Battery, Cell, Lithium-Ion (Li-Ion), Lithium-Polymer (Li-Po), Protection Circuit Module
(PCM), Energy Density, Nickel-Metal-Hydride (Ni-MH), Nickel-Cadmium (NiCad),
Lead-Acid
Introduction:
When compared to other types of rechargeable batteries, the high energy
density associated with lithium-ion and lithium-polymer cells makes them the ideal
power source for portable electronic devices. Because of this, rechargeable lithium
batteries can be found in a wide variety of consumer electronic devices such as
cellular telephones, laptop computers, digital cameras and cordless power tools. In
order to maximize their lifecycle and reduce safety concerns, both lithium-ion (Li-Ion)
and lithium-polymer (Li-Po) cells require the use of a Protection Circuit Model (PCM)
when assembled together to form a battery (See Figure 4.). The PCM may be
imbedded in the charger or permanently attached to each individual battery pack (See
Figures 1a. & 1b.).
Figure 1a.
Figure 1b.
Objective:
The purpose to these application notes is to explain the advantages and
disadvantages of rechargeable lithium battery technology, explain the need for
protection circuitry, describe the two main methods for implementing a PCM, and
instruct the reader on how to construct a Li-Ion or Li-Po battery that includes an
onboard PCM.
Warning!
These application notes are intended for theoretical use in the Michigan State
University, ECE 480 course only. While the author does have experience in this field and for certain
types of electrical systems discussed here in, the reader is asked not to build any such battery or circuit
discussed in this paper without a thorough, professional education in electrical engineering and having
read all data available from the manufacturer of the cells, PCM, Charger, and any other equipment or
devices used on, in, or near the circuit and/or system. Furthermore, the author accepts no liability for
any bodily harm or destruction that may occur from an individual, other than the author, attempting to
construct an apparatus as described in these notes.
If you have not met the above requirements; DO NOT BUILD THIS DEVICE!
Background:
As Electrical Engineers, it is likely at some point in our careers we will have
some level of professional involvement with battery operated systems. Rechargeable
Nickel-Metal-Hydride (Ni-MH) and Nickel-Cadmium (NiCad) cells and Lead-Acid
batteries are no longer the standard rechargeable battery technologies. These three
technologies suffer from lower energy densities (50-112, 33-75, & 18-55 J/kg
respectively) than conventional, one time use Zink-Carbon or Alkaline cells. This has
made these onetime use batteries and cells more desirable for many consumer
applications.
The new generation rechargeable cells have made it possible to combine
higher energy density (95-335 J/kg), relatively high nominal voltage (between 3 & 4
volts), and the ability to hold a charge for years, into a compact rechargeable battery
that outperforms onetime use batteries. The ability of lithium ion (Li-Ion) and lithium
polymer (Li-Po) cells to charge and discharge more than 1000 times, without a
reduction in performance, makes them environmentally friendly as well. Increased
mass production of these cells has made this technology more economical. The
culmination of these factors is quickly making rechargeable lithium cells the preferred
power source for portable electronic devices
Rechargeable lithium cells are not without their drawbacks. The primary
ingredient, lithium, is highly reactive, making cells that contain it more dangerous
than those utilizing another rechargeable cell technology. Because of the increased
safety risk, both lithium ion and lithium polymer batteries require the inclusion of
four safety features to reduce the risk of fire or explosion:
-
Shutdown Separator (Disables a cell if its temperature is too high)
Tear-Away Tab (Disables a cell if its internal pressure is too high)
Vent (Releases built up internal pressure in a cell)
Thermal Interrupt (Disables a cell if it is being overcharged or if the current
level used to charge it is too high)
Aside from increasing the overall complexity of these cells, these safety features take
up space, add weight, and limit design options. Because the activation of a safety
feature can permanently and irreversibly disable a cell, rechargeable lithium batteries
usually include active circuit technology for protection, especially during the charging
cycle.
This protection circuitry is often contained in a PCM (Protection Circuit Module, See
Figure 4.). The PCM may be imbedded in the charger or permanently attached to an
individual battery pack. The remainder of these notes will discuss the advantages and
disadvantages to each method of implementation and will go over the details for the
construction of a Li-Ion or Li-Po battery that contains a permanently attached PCM.
Comparing Methods:
There are two methods for implementing a lithium ion or lithium polymer
battery utilizing a Protection Circuit Module or PCM. First, the PCM may be imbedded
in the charger pack (See Figure 1a.). While on the charger, the battery will have the
standard protection offered by a PCM including:
- Overcharge Protection
- Short Circuit Protection
- Charge Current Limiting
- Balancing Functions for Each Cell.
This system, however, will not be able to protect the battery once it is removed from
the charger. This means that the battery its self has few or no attached active circuit
elements which will monitor its operation. The advantages and disadvantages of this
system are shown below in Table 1.
Advantages
Disadvantages
Reduced Battery Weight
Uses Only One PCM
Reduced Cost For Multiple Batteries
No Active Battery Protection During Use
Increased Safety Risk
Reduced Battery Performance & Shorter Life
Table 1. Advantages and disadvantages of charger imbedded PCM.
Next, the PCM can be permanently attached to each individual battery pack (See
Figure 1b.). This system is generally used in rechargeable lithium batteries that have
multiple cells, are more expensive, and where the small amount extra weight from a
PCM will not effect performance. The advantages and disadvantages of this system
are shown below in Table 2.
Advantages
Fulltime Active Battery Protection
Increased System Safety
Optimal Battery Performance & Life
Ability to Recharge Battery In-Place
Use of Unspecialized Power Sources
for Recharging
Use of Onboard battery Fuel Gauge
Disadvantages
Increased Battery Weight & Complexity
Each Battery Requires Its Own PCM
Increased Cost For Multiple Batteries
Table 2. Advantages and disadvantages PCM onboard battery.
The advantages of the onboard PCM are usually worth the initial increased cost and
weight. The cost increase is often recouped by the extended battery life, in-place
charging, and increased battery performance.
How to:
Battery/Cell Size & Rating: This section includes instructions on how to assemble
the second rechargeable Li-Ion/Po battery system with an onboard PCM. In order to
construct a battery system that is the correct size for the intended application, and
assuming you have already determined the voltage needed, you must calculate the
amp hours that will be needed by using the formula below:
(Current Required for Operation) X (Duration of Operation) = AH
After determining the amp hours the item to be powered will use, it is usually a good
idea to add 10 to 20% to the calculated amp hour result.
Next you will need to select a lithium-ion or lithium-polymer cell that has an amp
hour rating nearest the level calculated. If you cannot find the exact rating you
require, you should select a cell that has a higher amp hour rating.
Next you will need to determine how many of these cells you will require to obtain the
voltage level your system requires. Both types of rechargeable lithium cells usually
have a nominal voltage of 3.7 volts. The choice of battery voltages is usually limited to
some multiple of this number.
# of Cells
Voltage
1
2
3
4
5
6
7
10
13
3.7v 7.4v 11.1v 14.8v 18.5v 22.2v 25.9v 37v 48.1v
n
n x 3.7
Select the appropriate number of cells in series required to reach the voltage level you
require.
Where to Order: Next you will need to select a PCM that is designed to utilize this
voltage and rated for the current that you require. A useful website for finding both
batteries and a PCM is:
http://www.batteryspace.com/
Assembly: Once you have received the cells and the PCM it is a simple matter of
soldering the cells together in series and then connecting it to the PCM as shown in
Figure 4.
Figure 4. Connection schematic for a three cell (11.1v) Li-Ion/Po PCM.
The PCM in Figure 4. utilizes three Li-Ion/PO cells in series to produce a
combined voltage of 11.1v. On the left you can see the positive and negative terminals
(P- & P+) and the connector for the fuel gauge. On the right you can see the points
where the battery and individual cells are to be connected. The individual cells are
also connected so that the PCM can perform balancing functions to ensure that each
cell maintains an equivalent voltage level, and does not exceed the individual cells
overcharge and overdischarge limits (Usually ranges from about 4.2 - 4.35v and 2.4 –
2.5v respectively).
While the nominal voltage level of the battery will usually be equal to the
number of cells times the nominal voltage of each cell (N x 3.7v), you should be aware
that this is an average. The maximum and minimum voltages of the battery will be the
number of cells times the overcharge and overdischarge limits respectively. This may
or may not be the same number indicated in the instructions for the PCM you have
selected. For the 11.1v system in Figure 4. the maximum battery voltage may be
calculated to be higher then the PCM specification. This is fine because the onboard
PCM system is also used when charging the battery, so as long as it is charged through
the P+ and P- terminals of the PCM, it will never reach the higher overcharge
and overdischarge limits of the cells.
Charging: In order to charge your PCM onboard battery simply connect the
appropriate PCM terminals to a DC power supply, ensuring that the current level used
is in accordance with the level specified by the manufacturer of the individual battery
cells used or the PCM, which ever is lower.
Warning!
These application notes are intended for theoretical use in the Michigan State
University, ECE 480 course only. While the author does have experience in this field and for certain
types of electrical systems discussed here in, the reader is asked not to build any such battery or circuit
discussed in this paper without a thorough, professional education in electrical engineering and having
read all data available from the manufacturer of the cells, PCM, Charger, and any other equipment or
devices used on, in, or near the circuit and/or system. Furthermore, the author accepts no liability for
any bodily harm or destruction that may occur from an individual, other than the author, attempting to
construct an apparatus as described in these notes.
If you have not met the above requirements; DO NOT BUILD THIS, OR ANY OTHER DEVICE mentioned
in this paper!
Sources:
http://www.mpoweruk.com/lithiumS.htm
http://en.wikipedia.org/wiki/Lithium_ion_polymer_battery
http://www.nexergy.com/battery-density.htm
http://en.wikipedia.org/wiki/Lithium_ion_batteries
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=4345