PTC vs PCM: Why Use Both in Lithium Battery Protection?

Introduction

Battery protection is often discussed after the cell capacity, voltage, and dimensions have already been determined. In practice, however, the protection system can be just as important as the cell itself—especially when a custom lithium battery will be integrated into a commercial product and may need safety certification.

One question we recently received from a customer was:

 “A&S Power suggested that the PTC and PCM combination to better protect the cell for when it comes time to get the 2054 certification.”

Our answer was:

 “Yes, incorporating the PTC would be helpful for battery safety and facilitate the UL2054 certification process.”

This question reflects a common situation in custom battery development. A customer may already understand the need for a PCM but wonder why an additional PTC is necessary.

The reason is that PTC and PCM work in different ways. When properly selected and integrated, they can provide complementary layers of protection.

What Is a PCM in a Lithium Battery?

PCM stands for Protection Circuit Module. It is an electronic protection circuit commonly used in rechargeable lithium batteries, including lithium-ion and lithium polymer battery packs.

The PCM monitors electrical conditions and can disconnect or limit the battery when abnormal conditions are detected. The exact functions depend on the PCM architecture, protection IC, MOSFET configuration, and battery requirements.

Depending on the design, a lithium battery PCM may provide protection against:

• Overcharge
• Over-discharge
• Over-current
• Short circuit
• Other abnormal operating conditions defined by the protection design

For example, if a battery experiences an excessive discharge current, the PCM can detect the condition and turn off the protection MOSFETs according to the configured protection threshold and delay time.

This makes the PCM an important part of the battery's electronic protection architecture. However, the PCM is still only one part of the complete battery safety design.

What Is a PTC Battery Protection Device?

PTC stands for Positive Temperature Coefficient. A PTC is a passive protection component whose electrical resistance increases as its temperature rises.

Under normal operating conditions, the PTC can have relatively low resistance, allowing the required current to flow. When excessive current causes the component to heat up, its resistance increases significantly. This helps limit the current flowing through the battery circuit.

Because the PTC works through a physical resistance-temperature response, it does not perform the same function as an electronic PCM.

This difference is important. A PTC is not a replacement for a PCM. Instead, the two can be designed to provide complementary protection.

PTC vs PCM: What Is the Difference?

Although both components are used for battery protection, their operating principles are different.

Why Combine PTC and PCM?

The main reason for combining PTC and PCM protection is that they use different protection mechanisms.

The PCM provides active electronic protection. The PTC provides a passive current-limiting response when excessive current causes the component to heat up.

By combining them, the battery can have more than one layer of protection. If an abnormal electrical condition occurs, the PTC can help limit current while the PCM provides electronic monitoring and cutoff according to its protection settings.

This layered approach can be useful in applications where battery safety is a significant design requirement.

It is important, however, not to treat the PTC and PCM as completely independent safety systems without considering the complete circuit. Their electrical characteristics, current ratings, resistance, protection thresholds, response times, and thermal behavior must be evaluated together.

How Can PTC + PCM Support UL 2054 Certification?

For many custom battery projects, UL 2054 is an important safety standard to consider during battery development. This is especially true when a battery is intended for commercial products that require a recognized safety evaluation.

A common misunderstanding is that a battery becomes UL 2054 compliant simply by adding a PTC and PCM. That is not correct.

UL 2054 certification evaluates the battery as a complete system, not simply one individual protection component.

Nevertheless, incorporating a PTC into an appropriate battery protection design can be helpful because it may provide an additional layer of current-limiting protection and contribute to the overall safety architecture.

This is why A&S Power may recommend a PTC when a customer plans to pursue UL 2054 certification. The PTC can be considered as part of the battery safety design rather than being treated as a standalone certification solution.

Whether a PTC is appropriate depends on factors such as:

• Battery chemistry
• Cell construction
• Nominal voltage and capacity
• Continuous operating current
• Peak or pulse current
• Short-circuit behavior
• PCM protection thresholds
• Battery wiring and connector design
• Mechanical construction
• End-product application
• Applicable certification requirements

Therefore, the correct approach is to evaluate the entire protection architecture early in the project rather than adding a PTC at the final certification stage without reviewing the rest of the design.

A Real Customer Question: Why Add a PTC Before UL 2054?

At A&S Power, many battery protection decisions come directly from practical customer requirements. One customer asked us about the benefit of combining PTC and PCM protection when preparing a battery for UL 2054.

This question illustrates an important point in custom battery development: protection design should be considered together with the customer's certification target.

A battery designed only around electrical performance may require changes later if certification requirements introduce additional safety or construction considerations. By discussing the certification target at the beginning, the battery manufacturer can evaluate whether additional protection components such as a PTC are appropriate.

Does Every Lithium Battery Need a PTC?

No. Not every lithium battery requires a PTC.

The appropriate protection architecture depends on the application and battery design. Some battery packs may use a PCM alone, while other designs may benefit from additional passive protection.

For example, a battery designed for a low-current application may have different protection requirements from a battery used in a device with high startup current or repeated pulse loads.

Similarly, a battery designed for a medical, industrial, consumer, wearable, or IoT product may have different safety and certification requirements.

Therefore, adding a PTC simply because another battery uses one is not necessarily the correct engineering decision. The component must be selected based on the actual electrical and mechanical requirements of the battery.

How A&S Power Selects PTC and PCM Protection

For a custom battery, we normally start with the customer's actual application rather than selecting a protection component from a standard list.

Several parameters are important when selecting the protection design.

1. Normal Operating Current

The normal current consumption of the device determines the basic operating requirements for the protection circuit. The protection system must allow the battery to operate normally without unnecessary interruptions.

2. Peak and Pulse Current

Some products have short-duration current peaks. These may occur during motor startup, wireless transmission, high-power audio operation, communication events, or other transient conditions.

If the protection threshold is selected without considering these peaks, the battery may experience unwanted protection activation during normal product operation.

3. Charging Conditions

Charging voltage, charging current, charger behavior, and the customer's charging system all need to be considered when designing the battery protection architecture.

4. Cell Chemistry and Construction

Lithium-ion cylindrical cells, lithium polymer pouch cells, and LiFePO4 batteries can have different electrical and thermal characteristics. Protection selection therefore needs to match the actual cell and pack configuration.

5. Physical Space

Custom batteries are often designed for compact products. Adding a PTC or changing the PCM layout can affect the overall battery dimensions, wiring, insulation, and assembly method.

6. Certification Requirements

If UL 2054 or another certification is planned, the certification target should be communicated during the battery design stage.

This allows the battery manufacturer to evaluate protection components, cell selection, construction, wiring, and other design details together instead of treating certification as a final-stage inspection.

PTC and PCM Should Be Considered Early in Battery Development

Protection components are not simply accessories that can always be added at the end of a battery project.

A PTC may affect the electrical resistance of the battery circuit. The PCM may require a specific PCB layout, MOSFET configuration, sensing arrangement, and wiring structure. These factors can influence both electrical performance and mechanical packaging.

For a custom battery, the protection architecture should therefore be discussed during the initial specification stage.

A good development process typically considers:

1. Customer application requirements
2. Cell selection
3. Voltage and capacity
4. Continuous and peak current
5. Charging requirements
6. PCM protection parameters
7. PTC requirements, if applicable
8. Mechanical dimensions
9. Connector and wiring requirements
10. Certification requirements

Evaluating these requirements together can reduce the risk of redesign later in the development process.

Can a PTC Replace a PCM?

In most lithium battery protection designs, a PTC should not be considered a replacement for a PCM.

The fundamental reason is that they provide different types of protection. A PTC is a passive current-limiting component. It does not provide the complete electronic monitoring and cutoff functions normally associated with a PCM.

For example, a PTC cannot independently perform the same type of controlled overcharge and over-discharge protection provided by an appropriate PCM.

The PTC is therefore better understood as a complementary protection component when the battery design calls for it.

Why Protection Design Matters for Custom OEM Batteries

In standard off-the-shelf batteries, protection parameters may already be fixed. Custom OEM batteries are different because the protection architecture must be matched to the customer's product.

A custom battery may require a specific voltage, capacity, size, connector, current capability, PCM configuration, and certification target. The protection design therefore becomes part of the battery engineering process.

For example, two lithium polymer batteries with the same nominal voltage and capacity may still require different protection configurations if their end products have different peak current requirements or certification targets.

This is why A&S Power evaluates the complete application before recommending a PTC, PCM, or other protection configuration.

Our engineering team works with customers to evaluate the practical requirements of the battery, including operating current, pulse current, charging conditions, physical dimensions, protection requirements, and certification objectives.

Conclusion

PTC and PCM are not competing battery protection technologies. They are different components that can complement each other when the application requires a layered protection strategy.

The PCM provides electronic monitoring and protection functions, while the PTC provides passive current-limiting behavior through its positive temperature coefficient.

Combining the two can provide an additional layer of battery protection. For projects where UL 2054 certification is planned, incorporating a PTC may also be helpful for the overall battery safety design and can facilitate the certification process, depending on the complete battery design and applicable requirements.

However, a PTC + PCM combination should never be treated as an automatic guarantee of UL 2054 compliance. Certification depends on the complete battery construction, components, protection design, and applicable testing requirements.

For custom lithium battery projects, the most effective approach is to define the protection and certification requirements early. This allows the battery manufacturer to select the appropriate cells, PCM, PTC, wiring, mechanical structure, and other components as one integrated design.