Protecting Your Cell’s Cells

April 9, 2009
There’s escalating consumer concern over the robustness and safety of mobile-phone batteries. In response, the Cellular Telecommunications and Internet Association (CTIA)

1. Here’s a typical single-cell lithium-ion battery-pack circuit-protection scheme.

2. The PolySwitch MXP strap device is designed for use under the PCB.

c3. Implementing the PolySwitch MXP device helps deliver lower resistance in a smaller form factor.

Ty Bowman

There’s escalating consumer concern over the robustness and safety of mobile-phone batteries. In response, the Cellular Telecommunications and Internet Association (CTIA), in partnership with leading cellular network operators, created a program that helps battery pack and handset manufacturers improve safety and product reliability.

Developed over the past two years, the CTIA’s certification program validates manufacturer compliance with the IEEE 1725- 2006 standard via a combination of audits and testing. The standard establishes criteria for design analysis to ensure a reliable user experience, as well as safe operation of rechargeable lithium-ion and lithium-polymer batteries for cellular telephone applications.

It encompasses system integration, the battery-cell design process, manufacturing considerations, assembly precautions, leakage protection, component and thermal considerations, overcharge, overcurrent and mechanical considerations, connectors and terminals, security and validation, and quality control. It also addresses external influences such as the host and auxiliary devices, including ac and dc adapters. Since no industry-wide standard currently exists, IEEE 1725-2006 is intended to standardise the evaluation of lithium-ion batteries for cell-phone applications. PCS Type Certification Review Board (PTCRB) compliance is granted on a system-wide basis, so all system components, including the cell, battery pack, charger, and the phone itself, must comply with this standard before the integrated system can be certified.

Rechargeable lithium chemistrybased cells and battery packs are particularly sensitive to overcurrent and/or overtemperature conditions caused by both accidental shorting and abusive or runaway charging. These conditions can raise the battery temperature, and may result in cell damage, equipment failure, or even venting, smoke, or flame.

Accidental short circuits can happen when a metal object bridges the exposed terminals of the battery pack. Such a scenario might occur when a spare battery pack is carried in a briefcase or purse, where the terminals may come into contact with a set of keys or some other metal object.

Battery-cell overcharge can result from either an overcurrent or overvoltage condition, or a combination of both. In either case, if current or voltage is allowed to exceed prescribed values, a significant rise in cell temperature may result in venting, smoke, or flame.

A runaway charging condition can cause overcharge. In this case, the charger fails to stop supplying current after the pack is fully charged. This is typically caused by a charger fault. Abusive charging can result from the pack being charged under the wrong conditions. The most likely cause of this condition, however, is when an aftermarket or incompatible charger is used.

IEEE 1725-2006 covers the design analysis, manufacturing, and testing of rechargeable Li-ion and Li-ion polymer battery packs to ensure reliable performance over the expected lifetime of cellular phones. Section 6 of IEEE 1725- 2006 specifically addresses certain key safety considerations, including external short circuits and limiting output current, thermal protection design, and overcharge and overcurrent protection.

The standard calls for a minimum of one overcurrent protection function and two overcharge protection functions, where one of the overcharge protection functions must reside in the battery pack. These requirements can be met by applying several design combinations between the battery cell, battery pack, host device (e.g., cellular phone) or charger, and a wide range of protection solutions, including “an active circuit and/ or devices such as a thermal fuse, PTC, or thermostat.”

A suggested solution is to integrate the primary protection function (active circuit) and redundant protection function by placing a polymeric positive temperaturecoefficient (PPTC) device within the battery pack. With this design approach, the PPTC is placed in close proximity to the cell for optimum temperature sensing. The device also helps provide resettable protection against damage from external short circuits when the pack is removed from the host device. Redundant protection, when designed into the pack rather than into the charger, also helps protect against hazards that may occur from using the wrong charger.

In addition to an NTC thermistor, lithium-ion packs typically include protection schemes (Fig. 1). In these cases, MOSFETs and a control IC provide overvoltage, undervoltage, and overcurrent protection, and a PPTC device provides overtemperature protection on charge and discharge, as well as redundant overcurrent protection.

PPTC devices are employed as series elements in a circuit. They help protect the circuit by going from a low-resistance to a high-resistance state in response to an overcurrent or overtemperature event.

The PPTC device’s low resistance helps overcome the additional series resistance that’s introduced by the MOSFETs. On top of that, the device’s low trip temperature helps provide protection against thermal runaway in the case of an abusive overcharge.

During a short-circuit fault, the PPTC device rapidly produces heat due to the excess current. As it nears trip temperature, the device increases in resistance by several orders of magnitude and limits the fault current to a low level. When the fault condition is removed and the power is cycled, the device cools and returns to a low resistance state. If the fault isn’t cleared and the power isn’t cycled, the device will remain latched in the high resistance state.

During a typical overcharge fault, cell temperature rises when excessive voltage across the fully charged cell causes chemical degradation of cell components. When a PPTC device is included in the circuit, as the cell temperature rises, the ambient temperature of the PPTC device increases accordingly. Thus, less current is required to trip the device.

PPTC devices are often used to replace bimetal or thermal fuse protectors. Bimetals are often bulky, higher-cost protectors, which frequently don’t latch in the protected position during a fault condition. This may result in a cycling batterypack fault and battery-cell damage.

One-shot secondary overcurrent protectors, such as thermal fuses, are difficult to set at the low temperatures required for charge protection. They may trip in high ambient temperatures, disabling an otherwise functional pack.

Low-temperature PPTC devices are uniquely suited to limiting the charge current close to the functional pack’s operating temperature. The device’s resettable functionality ensures that nuisance tripping, which can be caused by exposure to high storage temperatures (e.g., leaving the phone inside a vehicle on a hot day), doesn’t permanently disable the pack. Also, because the majority of fault conditions encountered by a battery pack are relatively infrequent or intermittent events, resettable protection is generally the preferred method.

PPTC devices come in a variety of form factors and current ratings and are designed for specific battery chemistries or usage profiles. They’ve evolved in the direction of lower-resistance, smaller form factors, and better thermal protection. Clearly, the trend toward more space-efficient packs requires smaller protection devices. Furthermore, locating protection circuitry in close proximity to the cell helps eliminate the need for long metal interconnects and assists in the enhancement of thermal sensing.

Tyco Electronics’ latest generation of PolySwitch devices for cellphone applications was specifically designed for use under the printedcircuit board. The PolySwitch MXP strap device incorporates conductive metal particles to achieve lower resistance than traditional carbon black filled PPTC devices (Fig. 2). In comparison with the prior generation VTP strap device (Fig. 3), having approximately the same hold current at 60ºC, the MXP device is 88% smaller in size and 68% lower in resistance.

Using PPTC devices for overcurrent and overtemperature fault protection is well established. The latest generation of PPTC devices provides pack designers with another level of design flexibility in the form of a very low-resistance device in a much smaller form factor.

Ultimately, battery-pack designers must decide what level of protection is required for their application. 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.

Ty BOWMAN is global battery market manager, Tyco Electronics, Raychem Circuit Protection Products.

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