NESC tech bulletin

National Aeronautics and Space Administration

www.nasa.gov

Applicability

This information is applicable to those considering the use of 

Li-Ion batteries comprised of cells with PTC and CID internal 

protective devices.

Background

The internal protective devices (PTC and CID) used in the 

most common commercial-off-the-shelf (COTS) Li-Ion 

cells (cylindrical 18650’s) have been extremely reliable at a 

single-cell level and have resulted in total prevention of the 

cell reaching a hazardous condition. However, test programs 

have indicated that batteries built with cylindrical COTS 

cells in multi-cell configurations (series and/or parallel) have 

experienced thermal runaway under various test conditions. 

Test data analysis indicated that the two major causes for the 

thermal runaway are overvoltage (overcharge) and external 

short conditions. 

In these cases, the internal protective devices were either 

not protecting as expected or were a cause for the hazards 

encountered. PTC ignition above its withstanding (threshold) 

voltage has been shown to cause thermal runaway under

external short conditions in high-voltage battery modules. 

Thermal runaway has also been observed during overcharge 

conditions in high-voltage and high-capacity modules, 

indicating that the CIDs did not protect the cells from 

catastrophic events, as seen in single cells. The NESCsponsored study was conducted to understand the causes for 

the thermal runaway in high-voltage and high-capacity battery 

modules, and to determine the limitations of the cell internal 

protective devices.

Data and Analysis

PTC characteristics and limitations vary with cell manufacturer 

and are rarely provided. This information should be obtained 

by testing prior to considering a battery design for a specific 

application. The cell series voltage should not exceed the PTC 

withstanding voltage. For high-voltage batteries, diodes added 

to a series string of cells can improve their safety under external 

short conditions. The diodes must be carefully matched to 

battery characteristics.

In high-voltage and high-capacity batteries where the CID 

is used as a level of safety control, overcharge tests need to 

be performed to confirm its safe operation. The number of 

cells recommended for use in parallel depends on the charge 

current. The total charge current used to charge a bank (cells in 

parallel) should in no way cause an increase in PTC resistance 

of any single cell. In other words, in the event that all cell 

CIDs but one have opened, the current seen by the remaining 

cell should not cause an increase in PTC resistance. The CID 

voltage tolerance should also be characterized. The charger 

voltage limit should be set so that the difference between the 

voltage limit value and the end-of-charge battery voltage does 

not cause CID arcing. The main causes of failure that prevent 

the CID from proper safing are the charge current (causing 

inadvertent PTC activation), high temperatures (causing 

PTC activation or uncontrollable thermal runaway), and high 

voltages (causing PTC ignition).

References

NASA Aerospace Flight Battery Program Year 1 Report – Part 

1, Volumes 1 and 2, Generic Safety, Handling and Qualification 

Guidelines for Lithium-Ion (Li-Ion) Batteries, Li-Ion Batteries, 

Maintaining Technical Communications Related to Aerospace 

Batteries (NASA Aerospace Battery Workshop), NESC Document 

Number RP-08-75.

This work was led by Judith Jeevarajan, NASA Johnson Space 

Center, and Michelle Manzo, NASA Glenn Research Center.

For information contact the NESC at www.nesc.nasa.gov

Limitations of Internal Protective Devices in High-Voltage/HighCapacity Batteries Using Lithium-Ion Cylindrical Commercial Cells 

Cross-Section of a Typical 18650 Cylindrical Li-Ion cell showing the PTC (in 

green) and CID (in white)

Most commercial cylindrical 18650 Lithium-Ion (Li-Ion) cells have two internal protective devices: the Positive 

Temperature Coefficient (PTC) and the Current Interrupt Device (CID). The PTC protects the cells under external 

short conditions and the 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 highvoltage and high-capacity battery designs. 

NASA Engineering and Safety Center Technical Bulletin No. 09-02

NESC-TB-09-02

Scored Disk Vent + Top Cover

Crimped 

Can

CID Button

+ Tagmounting Disk

PTC

Gasket 

Seal

CID