PROTECTING
RECHARGEABLE LI-ION AND
LI-POLYMER BATTERIES
in Portable Electronics
Littelfuse offers designers many different protection devices to choose
from in an array of form factors and device characteristics that meet the
needs of their particular design.
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PROTECTING
RECHARGEABLE LI-ION AND
LI-POLYMER BATTERIES
in Portable Electronics
As sleeker designs and thinner portable consumer electronics, such as smart phones, tablets, power
banks, other advanced handheld electronics and emerging drone, e-bike and e-cigar market become
increasingly popular, Lithium-ion and Lithium- polymer batteries, known collectively as Li-batteries,
have become the “go-to” power sources of choice in these applications. As battery technology and
form factors for consumer devices expand beyond traditional cylindrical cells, Li-batteries are in
increasing demand due to their higher energy density, small form factors and design flexibility. These
batteries, in turn, require ever-smaller circuit protection devices to help provide robust protection in
thinner, lower-profile and more compact portable products.
Need for Battery Protection
Li-batteries are particularly sensitive to faults caused by Batteries in Cellular Telephones”; and IEC/EN 60950 and
external shorts, runaway charging conditions and abusive IEC 62133 specifications.). Moreover, certain end product
overcharging that can result in potentially damaging applications require that the power output of a battery be
overcurrent and overtemperature conditions. The limited to reduce the risk of device failures. The Limited
overcharge, deep-discharge, or short circuit conditions that Power Source (LPS) Test described in UL2054 is used
create heat can cause a Li-battery cell to bloat, rupture, or to determine whether a cell or battery is suitable in such
experience other issues, even fire . applications where safety issues may otherwise exist.
Although internal cell failures are less common, an adverse This application note discusses the need for protecting Li-
event may affect any of the complex electronics on the batteries against short circuit and overcharge conditions
battery pack’s PCM (Protection Circuit Module), such as the and shows how devices from Littelfuse’s can help designers
fuel gauge or charge controller. Because these components achieve robust and safe battery solutions. As a pioneer
are vulnerable to these events, Li-cells using PCMs require of polymeric positive temperature coefficient (PPTC)
many levels of protection against overcharge shutdown, resettable devices, Littelfuse has developed PolySwitch
over-discharge shutdown, overtemperature shutdown, and PPTC resettable devices of different form factors including
overvoltage and under-voltage lockout of a cell that may strap, disc, surface-mount, weldable and reflowable
lead to thermal runaway and possibly failure. products to meet different assembly requirements. These
devices are supported by several material platforms for
Organizations such as UL, IEC and IEEE have enforced safety protecting battery applications. Each materials platform
regulations and established test requirements for Li-ion and offers different performance characteristics and a range of
Li-Polymer packs to demonstrate their resilience to both thermal cut-off, or activation temperatures. In the recent
short circuit and overcharge events. (For additional details years, Littelfuse also developed MHP-TA resettable compact
refer to UL2054, “Standard for Household and Commercial thermal cut-off device by combining the company’s bimetal
Batteries”; IEEE 1725-2011 “Standard for Rechargeable and PPTC technologies for the high capacity Li-Polymer and
prismatic cell applications.
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Protecting Rechargeable Li-Ion and Li-Polymer Batteries
PolySwitch PPTC Resettable Devices vs.
Short Circuit Conditions Traditional Solutions
An unprotected battery cell or pack can deliver a very During a short circuit fault, Littelfuse’s PolySwitch PPTC
high current when it is “hard shorted” by a low-resistance resettable device rapidly heats up due to the excess current.
element. In this case, power dissipated in the battery cell’s As it nears trip temperature, the device increases in resistance
internal impedance can lead to a rise in cell temperature. The by several orders of magnitude and limits the fault current
severity will depend on the pack’s thermal characteristics to a low level. When the fault condition is removed and
and the battery cell chemistry. These short circuits can the power is cycled, the device cools and returns to a low-
increase the cell temperature to levels high enough to resistance state. If the fault is not cleared and the power is not
damage the cell, other components or surrounding materials. cycled, the device will remain latched in the high-resistance
At a minimum, pack performance can deteriorate and with state.
some packs, thermal runaway may occur and can result in
damaged devices or even fire. Additionally, accidental short When a PolySwitch PPTC device is included in a circuit, as
circuits can occur when a metal object, such as a keychain, the cell temperature rises, the temperature of the PolySwitch
bridges the exposed terminals of the battery cell/pack. If an device increases accordingly and less current is required to
unprotected pack is “soft shorted” by an element with even trip the device. PolySwitch PPTC devices are often used to
a small amount of resistance, (e.g., a few hundred milliohms), replace bimetal or thermal fuse protectors since traditional
the potential problem changes from being power dissipated bimetals often result in bulky, high-cost protection solutions.
in the cell to power being dissipated in the shorting element. Bimetals normally do not latch in the protected position
Tests have shown that the resistive shorting element can during a fault condition, which may result in battery pack
reach temperatures in excess of 600°C during this type of fault and battery cell damage.
event, which may result in ignition of adjacent combustible
materials. Unlike resettable PolySwitch PPTC devices, one-shot
secondary overcurrent protectors, such as fuses, are difficult
Overcharge Conditions to set at the low temperatures required for charge protection
and may trip at high ambient temperatures. Since they do
Individual battery chemistries require specific charging not reset, they can cause an otherwise functional pack to be
profiles to optimize performance and minimize safety issues. disabled, which can result in unnecessary field returns.
If this profile is not met, an overcharge condition may occur.
A battery pack overcharge condition is most often caused by: PolySwitch PPTC devices can also help provide
• A runaway charging condition in which the charger overtemperature protection in addition to overcurrent
fails to stop supplying current to the pack once it is fully protection. The device’s resettable functionality provides
charged. This is typically caused by a charger fault. that nuisance tripping caused by exposure to high storage
• Abusive charging that occurs when the pack is charged temperatures, such as leaving a cell phone inside a vehicle on
under the wrong conditions by an incorrect or faulty a hot day, does not permanently disable the pack.
charger. The most likely cause of this condition occurs
when a consumer uses an aftermarket or non-compatible MHP-TA devices from Littelfuse are resettable circuit breakers
charger. Product reliability or safety issues may arise when that are sensitive to over current and over temperature
using some aftermarket products due to the proprietary conditions arising from Li battery packs. In the fault
nature of cell chemistries and charger designs. condition, the bimetal spring within the MHP-TA device will
Battery cell overcharge can result from an overcurrent or open the contacts to stop the current flowing in the battery
overvoltage condition or a combination of both. If current or pack. Unlike the traditional bimetal protectors, MHP-TA will
voltage is allowed to exceed prescribed values, a significant remain in the latched (open) condition until fault is removed
rise in cell temperature may result. During a typical overcharge or power cycled.
fault, the cell temperature rises when excessive voltage across
the fully charged cell causes chemical degradation of the cell In some applications, a redundant FET is used as a 2nd
components. protection instead of a PPTC or MHP-TA thermal cut-off
device. Although FET offers very precise current protection
in this application, it is not sensitive to over-temperature in
the battery packs.
3
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Protecting Rechargeable Li-Ion and Li-Polymer Batteries
PolySwitch Devices for Li-Battery Protection PolySwitch Strap Products:
Installation Method – Spot Weld
Li-packs typically include ICs capable of detecting and
implementing an overvoltage lockout, undervoltage lockout, PolySwitch strap products offered by Littelfuse include the
overtemperature protection and overcurrent protection. ICs SRP, LR4, VTP, VLR, VLP, MGP and MXP families. Strap
and MOSFETs are often used as the primary pack protection devices, which come in a flat, tab-like form factor, can be
in conjunction with a fuel gauge device to track the battery incorporated into cylindrical based packs, prismatic cells or
cell capacity, state-of-charge (%), run-time to empty even pouch packs and can be applied to specific battery
(minutes), battery voltage (mV) and temperature. chemistries or usage profiles. Their installation method is to
be spot welded to cells or straps in the battery pack (Figure
A PolySwitch PPTC device placed in series with the battery 2).
helps provide a second level of protection in the event of
a control circuit malfunctioning (Figure 1). Although the
semiconductor circuitry is considered reliable, there are
conditions under which failure of the primary protection
may occur, including excessive electrostatic discharge, high
temperature or oscillation during a short circuit condition. In
these cases, the PolySwitch PPTC device helps provide cell
overtemperature protection on charge and discharge, as well
as redundant overcurrent protection. When a PolySwitch
PPTC device is included in the circuit, the temperature of the
device increases accordingly as the cell temperature rises
and less current is required to trip the device. Figure 2. PolySwitch strap devices offer space-saving solution for protecting
battery cells.
The evolution of Littelfuse’s strap devices has progressed to
lower resistance, smaller form factors and increased thermal
protection, as shown in Figures 3 and 4.
Figure 1. PolySwitch PPTC device in a typical protection circuit for a Li-ion or
Li-Polymer cell. SRP
Figure 3. VTP
A wide range LR4strap devices
of PolySwitch VLR are available
VLP for MXP
specific
pack requirements.
A wide selection of PolySwitch PPTC products is offered for
Li-battery protection. The PolySwitch family includes devices
offering a range of thermal cutoff (activation temperatures)
from 85°C to 125°C. The PolySwitch PPTC device’s low
resistance helps meet the battery pack’s resistance budget
requirements, and its low trip temperature helps provide
protection against thermal runaway in case of an abusive
overcharge. PolySwitch PPTC resettable devices are also
available in a variety of form factors and current ratings.
Figure 4. PolySwitch strap device enables current to be interrupted at different
4 ambient temperatures.
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Protecting Rechargeable Li-Ion and Li-Polymer Batteries
The PolySwitch low-resistance (low rho) MXP strap device, PolySwitch L-Tab Devices: Installation
shown in Figure 5, incorporates conductive metal particles
Methods – Reflow Solder and Spot Weld
to achieve lower resistance than traditional carbon black-
filled PPTC devices. The MXP device is 88% smaller than
The PolySwitch L-Tab device helps provide a weldable and
the prior-generation VTP strap device (Figure 6), while also
reflowable devices. The L-Tab device can be reflow soldered
providing approximately the same hold current at 60°C.
onto the battery PCM at one end and the device’s L-shaped
Regardless of the pack chemistry, device hold current is
tab/terminal can be directly welded to the battery cell tab at
selected on the basis of the maximum average charge or
the other end, therefore providing cost savings. Additionally,
discharge current and takes into account the maximum
its “L” shape assists in reducing manufacturing steps when
operating temperature. The form factor will depend on the
the PCM is folded into the pack.
available space within the pack. PolySwitch PPTC strap
With operating current up to of 4A at room temperature,
devices with activation temperatures (thermal cutoff) from
they are suitable for use for battery protection in high-
85°C to 125°C are offered in a wide range of custom and
performance tablets. They also offer ultra-low resistance
standard configurations.
to help maintain the system impedance budget. Locating
protection circuitry in close proximity to the cell helps
eliminate the need for long metal interconnects and helps
improve thermal sensing (Figure 7).
Figure 5. The low rho MXP strap device and surface-mount device are
placed under the PCB. (The MXP Strap length and configuration can also be
customized per customer requirements.) Figure 7. PPTC L-Tab devices occupy the same space as the existing battery
terminal block.
PolySwitch Surface-Mount Products:
Installation Method – Reflow Soldering
Littelfuse surface-mount products are well suited for battery
PCMs since their smaller size helps save board space and
eases design complexity. Our standard 1206 (nano) and
1210 (micro) form factor offer low profiles and small form
factors that can be reflow soldered. The lower resistance of
these devices help maintain the system impedance budget.
Figure 6. Compared to the prior-generation VTP device, the PolySwitch MXP
device has lower resistance in a smaller form factor.
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Protecting Rechargeable Li-Ion and Li-Polymer Batteries
PolySwitch Disc Products:
Installation Method – Press Fit
A PolySwitch disc device is a bare disc made of PPTC
material that is placed inside a Lithium-ion 18650 cylindrical
cell header to help protect cells during shipping and handling
prior to assembly in packs (Figure 8). The disc device also
helps provide protection for cells that are sold individually
as AA and AAA-sized, non-rechargeable lithium batteries
for consumer use. Each disc device is custom designed for
the cell it will be used with.
Figure 8. A PolySwitch disc device’s placement on an 18650 battery cell.
MHP-TA Devices:
Installation method – Spot Weld
The MHP-TA devices offer a space-saving solution for
protecting higher energy Lithium-polymer and prismatic
battery pack applications such as ultra-thin notebooks
and tablets. These resettable thermal cut-off (TCO)
devices consist of a PolySwitch PPTC device in parallel
with a bimetallic protector. They activate thermally at
temperatures from 72°C to 90°C, while also offering a
high withstand voltage and high hold currents from 6A to
20A at room temperature. Available in an ultra-low-profile
package (Figure 9), Their benefits include their extremely
low resistance, ability to open by thermal activation and
their resettability. The MHP-TA device eliminates the non-
latching properties of traditional bimetals because the built-
in PPTC keeps the bimetal contacts latched open during a
fault condition.
Figure 9. MHP-TA devices target thinner Li-battery cell applications such as
ultra-thin notebooks.
Selecting Circuit Protection Devices
Table 1 shows a selection of Littelfuse PolySwitch devices The protection requirement is cell chemistry-dependent
that are suitable for Li-battery protection: PolySwitch PPTC and precise protection requirements should be obtained
devices (strap, surface-mount, disc, L-Tab), as well as the from the cell manufacturer. Recommendations from device
MHP-TA devices. manufacturers are useful in narrowing protection options
and benchmarking other pack protection schemes may
When adding protection devices, battery pack designers help provide a good lead for further investigation. However,
must decide what level of protection is required for each specific testing of each protection option is the best way to
application. A system test should be used to determine evaluate its effectiveness.
whether or not a specific protection device is appropriate.
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Protecting Rechargeable Li-Ion and Li-Polymer Batteries
Battery
Protection Strap L-Tab Strap Surface-Mount MHP-TA® Disc
Device
Installation Reflow and
Spot Weld Reflow Spot Weld Crimp
Methods: Spot Weld
SRP MHP-TAC6
LR-SMD Custom Made
LR4 MHP-TAC15 (Contact
Product VTP Nano
L-Tab MHP-TAM6 Littelfuse
Family VLR Micro
MHP-TAM15 for more
MXP
information)
MGP MHP-TAT18
Li-Ion
Li-Ion
Battery NiMH Li-Polymer Li-Polymer Li-Polymer
(Secondary and
Technology NiCd Li-Ion Li-Ion Li-Ion
Primary)
Li-Polymer
Table 1. Selection of Littelfuse PolySwitch products for battery applications.
Summary
Battery applications designers must respond to the trend toward more space-efficient battery packs that require ever-
smaller protection devices. Littelfuse PolySwitch offers them many different protection devices to choose from in an array of
form factors and device characteristics that meet the needs of their particular design.
Notice:
Information furnished is believed to be accurate and reliable. However, users should independently evaluate the suitability of and test each product selected for their own
applications. Littelfuse products are not designed for, and shall not be used for, any purpose (including, without limitation, military, aerospace, medical, life-saving, life-
sustaining or nuclear facility applications, devices intended for surgical implant into the body, or any other application in which the failure or lack of desired operation of the
product may result in personal injury, death, or property damage) other than those expressly set forth in applicable Littelfuse product documentation. Warranties granted
by Littelfuse shall be deemed void for products used for any purpose not expressly set forth in applicable Littelfuse documentation. Littelfuse shall not be liable for any
claims or damages arising out of products used in applications not expressly intended by Littelfuse as set forth in applicable Littelfuse documentation. The sale and use of
Littelfuse products is subject to Littelfuse Terms and Conditions of Sale, unless otherwise agreed by Littelfuse.
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