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Design Considerations For PPTC Devices Used In Battery Packs

by Matthew Galla

 Materials and Process Development Manager 

Tyco Electronics Power Components/Raychem Circuit Protection Group

Circuit protection for rechargeable lithium-based battery cells and packs is a critical design

consideration. Overcurrent and overtemperature conditions that can result from accidental

shorting or abusive/runaway charging can raise temperatures high enough to damage

components and cause substantial equipment damage.

Early circuit-protection designs relied on fragile one-shot fuses to provide total current

interruption for overcurrent protection. However, because the majority of fault conditions that

a battery pack encounters are relatively infrequent or intermittent events, a resettable

 protection device is generally preferable.

Today, Li-ion packs typically include an active overvoltage and overcurrent detecting safetycircuit (IC and MOSFETs) as the primary pack protection, and a polymeric positive

temperature coefficient (PPTC) device in series as a second level of protection. Although the

semiconductor circuitry is considered reliable, there are conditions under which failure of the

 primary protection may occur, such as excessive electrostatic discharge, high temperature, or 

oscillation during a short circuit condition. In these cases, the PPTC device provides both

short-circuit and overcharge protection.

For emerging Li-ion cell technologies, such as Li-P and Li-Mn, the increased level of safety

inherent in the cell design has led some OEM pack designers to eliminate the IC safety circuit

and rely on a PPTC device as the primary short-circuit and overcharge protection (Fig. 1.) An

optional secondary passive element, in the form of a current fuse or thermal fuse, is

sometimes included if protection is needed beyond the rated voltage of the PPTC device.

Fig. 1: Some Li-ion Manufacturers Use PPTCs For Primary Protection,

Sometimes With A Secondary Passive Element

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Evaluation Of Primary PPTC Overcharge Protection In Li-Mn And Li-P Battery Packs

Raychem Circuit Protection recently purchased commercially-available OEM battery packs

that rely on PPTC devices for primary pack protection and subjected them to overcharge

testing. The Li-Mn packs were rated at 600 mA-hr, and the Li-P packs at 650 mA-hr. The

cells were removed from the packs with circuitry intact and each type was then subjected to2.5 C-12 V overcharge tests with and without PPTC protection. Test data were collected with

thermocouples connected to each cell. The same tests were then repeated using a thermal-

imaging camera.

The 2.5 C-12 V overcharge test conducted on a Li-Mn cell without PPTC overcharge

 protection resulted in a damaged, inoperative cell. The data (Fig. 2) show the rapid increase

in temperature that resulted from overcharge, and the thermal image captures the cell at

maximum temperature. In this test, the cell reached a maximum surface temperature of 

117°C, the can bulged from its original thickness of 6mm to 10mm, and the cell was rendered 

inoperative when the separator melted. Note that the drop in charge current occurred only

when the separator melted.

Fig. 2. 2.5-C 12-V Test Shows Temperatures In Li-Mn Cell Without Protection

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When the same test was performed on a Li-Mn cell with PPTC protection (here a PolySwitch

VTP210 device) the results were more favorable. The data (Fig. 3) show how the cell

temperature increased as the overcharge progressed, but when the cell reached 80°C, the

PPTC device interrupted the charge current, held off the excess charge voltage, and the cell

 began to cool. The PPTC device caused this drop in charge current by going into a high

resistance state. Note that the PPTC device latched in its high-resistance state at its triptemperature (~100°C) but did not heat the cell.

Fig. 3. 2.5-C 12-V Test Shows Temperatures In Li-Mn Cell With PPTC Protection

As with the Li-Mn tests, the 2.5-C 12-V overcharge test conducted on a Li-P cell without

PPTC overcharge protection resulted in a damaged, inoperative cell. Testing of the Li-P cell

with PPTC protection had positive results analogous to those in the Li-Mn case. For both Li-

P cells tested with PPTC protection, the cells reached a maximum temperature of 

approximately 50ºC, and were operational after the test. The only notable change was

loosening of the normally tight foil-laminate package.

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Fine Tuning Thermal Cutoff To Improve Overcharge Protection

When a PPTC device is used to help protect against damage from overcharge, the thermal

cutoff performance of the device must be matched to the protection needs of the cell. The two

most important criteria for tailoring the thermal cutoff temperature of a device are size and 

trip temperature.

The 2.5-C 12-V overcharge tests conducted on the Li-Mn and Li-P cells demonstrate a PPTC

device interrupting 1.5 A at 80°C and another PPTC device interrupting 1.65 A at 50°C.

Although the source current was slightly different in the two cases, the leading factor in the

difference observed in thermal cutoff temperature was simply the size of the PPTC device. A

PPTC device with smaller area is higher in resistance than a larger device made of the same

material at the same thickness, and will generate more heat when sourced with the same

current -- thus it will trip at a lower ambient temperature compared to a larger device.

Fig. 4 shows the thermal cutoff protection afforded by two different-sized PolySwitch VTP

devices at a 1 A simulated charge current. The smaller, higher-resistance device has a lower thermal cutoff temperature than the larger device. The cutoff temperature for either device

will be lower at higher currents.

Fig. 4: Smaller PPTC Device From Same Material Shuts Off At Lower Temperature

Another option for achieving lower thermal cutoff temperature is to use a device made with a

lower temperature PPTC material. Such devices can be lower in resistance and still maintain

a low thermal cutoff because of their reduced trip temperature. Combinations of device size

and material allow for fine-tuning of the thermal cutoff temperature, giving pack designers

the ability to specify appropriate protection and improve pack performance.

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Summary

Commercially-available OEM packs with Li-P and Li-Mn cells are now employing PPTC

devices as primary protection. Laboratory overcharge testing at 2.5-C 12-V demonstrated that

PPTC devices reduced maximum cell temperature by 40 - 50°C, while also helping to prevent

 bulging, rupture, separator melting, and cell shutdown. The latest generation of PPTCmaterials provides pack designers with another level of design flexibility in the form of low-

resistance devices capable of providing lower thermal cutoff.

Ultimately, pack designers must decide what level of protection is required for their 

applications and only a system test can determine whether or not a specific protection device

is appropriate. Recommendations from device manufacturers are useful in narrowing

 protection options, and benchmarking other pack protection schemes may provide a good 

lead for further investigation. However, specific testing of each protection option is the best

way to evaluate its effectiveness.

About the Author:

Matthew Galla is manager of the materials and process development group and is responsible

for battery product development for Tyco Electronics Power Components. He earned both BS

and MS degrees in Materials Science and Engineering from the Massachusetts Institute of 

Technology.