The Positive Temperature Coefficient (PTC) device is so named as it increases in resistance as the temperature increases. Hence as a device within a battery cell it protects the cell against over-current or short circuits.
In a cylindrical cell the PTC sits electrically between the positive output of the jelly roll and the top positive outer connection point.
Typically it is a washer of the PTC material that is clamped between the Current Interrupt Device (CID) and the Top Positive Cover.
One significant issue is that the resistance isn’t zero even at low temperature and resistance is very important in battery pack design.
Key Safety Features of Lithium-Ion Cells
Together, PTC, CID, safety vent, notched can bottom, and center pin form a multi-layered safety system. These features ensure that lithium-ion cells can operate reliably across diverse applications—whether optimized for high energy density or high power output.
NASA in their technical report on the limitations of these devices [1] state: The PTC protects the cells under external short conditions and the Current Interrupt Device (CID) protects the cells under overcharge conditions. While proven to be effective at the single cell and small-size battery levels, these devices do not always offer protection when used in high-voltage and high-capacity battery designs.
PTCs work fine in multi cell battery packs but there is a pack total voltage limit. With the polymer PTCs I have worked with, low temperature resistance was generally not a problem. But there are several types of PTC.
Robert Stodieck Consulting
Having a PTC trip in a high voltage battery pack puts the entire pack voltage across a single PTC device. As with CIDs, this can result in an arcing failure and a battery pack fire.
With the polymer type PTCs I am familiar with this safe pack voltage limit was around 40 VDC. This type of PTC was included in the very first cylindrical lithium metal batteries put on the market by Panasonic/National/Matsushita in 1985. These are often marketed now as a twin cell 6 volt pack called a CR2-P2. And also as single cells as the CR2.
References
- Judith Jeevarajan, Limitations of Internal Protective Devices in High-Voltage/High-Capacity Batteries Using Lithium-Ion Cylindrical Commercial Cells, NTRS – NASA Technical Reports Server
- Generic Safety, Handling and Qualification Guidelines for Lithium-Ion (Li-Ion) Batteries, NASA Aerospace Flight Battery Program
See also: Fire Suppression, DC Arc Flash Hazard