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IEC-EN60950-1

Use of Low Resistivity Surface Mount PPTC in Li-ion Polymer Battery Packs

Littelfuse, Inc 

Application Note:

Use of Low Resistivity Surface Mount PPTC

in Li-ion Polymer Battery Packs

1. Short-Circuit tests and Forced Discharge tests:

These tests are conducted by discharging the

battery with a low resistance load and then allowing

the battery to protect itself or fail by re or

explosion; the latter being a test failure. A test pass

is when battery returns to a safe temperature. Tests

are done at room temperature and elevated

temperatures.

2. Abnormal Charging test, Overcharging test, High

Charging Rate test: These tests are conducted by

subjecting the battery pack to several times more

than the normal charging current or charging at an

abnormally fast rate. When there is a non-resettable

over-current device present, the test is repeated at

a current below which the device activates.

3. Heating and Temperature Cycling tests. These tests

are conducted by raising and cycling the battery

pack to high temperature and then checking to see

if the pack responds safely. Fire, explosion, and

venting would be considered failures.

The purpose of the safety standards is to ensure the

battery pack and cells have protection mechanisms

designed into the overall system to prevent rapid

thermal runaway, re, explosion, rupture, venting, or

even gas bloating of the battery packs. All of these

events can create a hazard to the user or any equip-

ment used with the battery pack.

Typical Li-ion and lithium-polymer battery packs have

several levels of protection in order to meet the

required safety standards and to protect the user and

equipment from battery failure hazards. In addition to

internal cell level protection, external protection

solutions are added to provide further safety mea-

sures. Some battery packs will use what is called a

Battery Management Unit (BMU), which is a small

print circuit board with several protection components

(see Figure 2). The BMU will have a central processing

device, which is usually an IC that controls the battery

charge and monitors the pack for unsafe conditions.

The battery controller IC controls two FETs, which act

as the charge and discharge switches. The battery IC

will turn these FETs off as the primary way to shut

down the battery pack. The IC will use thermistors and

temperature cut-outs (TCO) to sense temperature,

current sense resistors to monitor current, gas gauges

to monitor gas buildup, and fuel gauges to monitor

charge. Upon any unsafe condition, the IC will turn the

FETs off to shut down the pack and stop the fault

event. Because the Li-ion chemistry is so dangerous in

certain conditions, there must be a secondary method

for protection. This secondary protector can be a PPTC

(polymeric positive temperature coef cient) resettable

fuse, thermal fuse, or a controllable battery protector

(see Figure 3).

Battery

cell

PCM

Figure 2. A typical Battery Management Unit (BMU) design

Battery Pack

SMD PTC

Switch Switch

+

Discharge Charge

Control IC

Battery

Cell

–

Figure 3. A secondary method of battery protection

©2012 Littelfuse, Inc

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