Short Circuit Conditions:
An unprotected battery cell or pack can deliver a very high current when it is “hard shorted” by a low-resistance element. In this case, power dissipated in the battery cell’s internal impedance can lead to a rise in cell temperature. The severity will depend on the pack’s thermal characteristics and the battery cell chemistry. These short circuits can increase the cell temperature to levels high enough to damage the cell, other components or surrounding materials.
At a minimum, pack performance can deteriorate and with some packs, thermal runaway may occur and can result in damaged devices or even fire. Additionally, accidental short circuits can occur when a metal object, such as a keychain, bridges the exposed terminals of the battery cell/pack. If an unprotected pack is “soft shorted” by an element with even a small amount of resistance, (e.g., a few hundred milliohms), the potential problem changes from being power dissipated in the cell to power being dissipated in the shorting element. Tests have shown that the resistive shorting element can reach temperatures in excess of 600°C during this type of event, which may result in ignition of adjacent combustible materials.
Overcharge Conditions:
Individual battery chemistries require specific charging profiles to optimize performance and minimize safety issues. If this profile is not met, an overcharge condition may occur. A battery pack overcharge condition is most often caused by:
• A runaway charging condition in which the charger fails to stop supplying current to the pack once it is fully charged. This is typically caused by a charger fault.
• Abusive charging that occurs when the pack is charged under the wrong conditions by an incorrect or faulty charger. The most likely cause of this condition occurs when a consumer uses an aftermarket or non-compatible charger. Product reliability or safety issues may arise when using some aftermarket products due to the proprietary nature of cell chemistries and charger designs.
Battery cell overcharge can result from an overcurrent or overvoltage condition or a combination of both. If current or voltage is allowed to exceed prescribed values, a significant rise in cell temperature may result. During a typical overcharge fault, the cell temperature rises when excessive voltage across the fully charged cell causes chemical degradation of the cell components.
PolySwitch PPTC Resettable Devices vs. Traditional Solutions:
During a short circuit fault, Littelfuse’s PolySwitch PPTC resettable device rapidly heats up 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 lowresistance state. If the fault is not cleared and the power is not cycled, the device will remain latched in the high-resistance state.
When a PolySwitch PPTC device is included in a circuit, as the cell temperature rises, the temperature of the PolySwitch device increases accordingly and less current is required to trip the device. PolySwitch PPTC devices are often used to replace bimetal or thermal fuse protectors since traditional bimetals often result in bulky, high-cost protection solutions. Bimetals normally do not latch in the protected position during a fault condition, which may result in battery pack fault and battery cell damage.
Unlike resettable PolySwitch PPTC devices, one-shot secondary overcurrent protectors, such as fuses, are difficult to set at the low temperatures required for charge protection and may trip at high ambient temperatures. Since they do not reset, they can cause an otherwise functional pack to be disabled, which can result in unnecessary field returns.
PolySwitch PPTC devices can also help provide overtemperature protection in addition to overcurrent protection. The device’s resettable functionality provides that nuisance tripping caused by exposure to high storage temperatures, such as leaving a cell phone inside a vehicle on a hot day, does not permanently disable the pack.
MHP-TA devices from Littelfuse are resettable circuit breakers that are sensitive to over current and over temperature conditions arising from Li battery packs. In the fault condition, the bimetal spring within the MHP-TA device will open the contacts to stop the current flowing in the battery pack. Unlike the traditional bimetal protectors, MHP-TA will remain in the latched (open) condition until fault is removed or power cycled.
In some applications, a redundant FET is used as a 2nd 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.
