The most common cause of cell phone battery failure is battery aging, which results in a dead battery after a large number of charge/discharge cycles. However, under certain circumstances the battery can short internally. Since batteries are energy storage devices with a large amount of available energy, they can cause a serious thermal runaway event. This type of failure can result in catastrophic events such as fire, and may occur if the battery is defective, utilizes inferior knock-off peripheral circuitry or hasn't been properly sized for the end application.
There are a number of passive components that, when designed into cell phones and their battery packs, can greatly reduce the possibility of a malfunction. Specifying the right components is an integral aspect to the durability of the entire design.
One of the first considerations for a cell phone designer is determining what level of electrostatic discharge (ESD) can be handled by the charge control IC. Most ICs can't handle extreme ESD described in IEC specifications at 8 kV. It is, therefore, important to find an "ESD hardened" IC. A conservative design would also include an external transient voltage suppressor like a multilayer varistor (MLV) to protect the hardened IC.
Since ICs with "on-chip" protection are not always adequate to meet the IEC specification and real world ESD requirements, external transient voltage suppressors will become essential. The device of choice is an 0402 MLV. An 0402 is electrically large enough to withstand a surge, but small enough not to negatively impact the total size or weight of the circuit. An additional advantage of MLVs is their off-state capacitance. This feature allows off-state MLVs to act as EMC filter capacitors. This helps reduce the emitted radiation and/or the susceptibility to E fields in the charge IC. With this approach battery failure due to damaged IC charge controllers is virtually eliminated.
Another vulnerable aspect of the charge circuit is temperature fluctuation that can easily be monitored by adding a negative temperature coefficient thermistor (NTC) to the battery pack. The most desirable NTC is an 0402 size with values in the range of 10 k to 100 k. When choosing NTC thermistors, design engineers need to be sure they are properly sized, not only in terms of power dissipation, but also in response time. The NTC needs to be fast enough to accurately measure temperature transitions but not so fast that you give up other important parameters such as heat capacity or dissipation factor on the NTC.
One additional potential preventative measure is the inclusion of a fusing device in the battery pack. Without a fuse, if the battery were discharging faster than normal in a best- case scenario the battery life would just be greatly diminished and continue to erode well beyond the normal rate of wear out. In a worst-case scenario, the faulty battery could cause a fire. However, the use of SMT fast blow fuses can prevent this scenario.
Choosing the right circuit protection devices including MLVs, NTCs and surface-mount fuses in cell phone designs can significantly reduce safety issues caused by faulty battery packs.
Ron Demcko, application engineering manager, AVX Corp.