High-voltage PTC heaters: the true meaning of ‘self-regulating’

Even when selecting a high-voltage PTC heater, you must still be clear about how the vehicle controls the heating process. ‘Self-regulating’ merely describes a characteristic of the heating element itself; it neither sets a target temperature for the battery, nor specifies coolant flow requirements, nor defines how the system should respond should the heating command be lost.

When consulting EVLINK, we ask that you clearly set out these specific requirements. Suppliers should not make assumptions about your control strategy based solely on the term ‘PTC’; similarly, you should not infer which protective functions a heater possesses based solely on brief marketing descriptions.

What does a high-voltage PTC heater regulate?

PTC stands for ‘Positive Temperature Coefficient’. Within a specific operating range, the resistance of the heating element increases as the temperature rises. With a constant input voltage, this increase in resistance leads to a reduction in electrical power. Consequently, the local temperature of the heating element directly influences its own heat output.

This is a completely different concept from monitoring the temperature of each individual battery cell. The heating element, coolant outlet, battery pack inlet and individual cells are distinct thermal locations. They respond to temperature changes at different rates, and their respective temperatures depend on the circuit environment and operating conditions in which they are situated.

A ‘self-regulating’ element cannot, by itself, tell you which of the above locations the vehicle should control the temperature of. The battery team and the vehicle engineering team must collaborate to define the target temperature, sensor placement and permissible operating range.

‘Self-regulation’ does not refer to the coolant temperature setpoint

Take a particular vehicle as an example: it may need to preheat the battery before setting off and then heat the passenger compartment once underway. Although the material of the heating element remains the same, the heat required and the available electrical power may vary significantly.

The vehicle must still be equipped with appropriate mechanisms to issue heating requests, limit heating power, and cease heating once the task is complete. Depending on the components selected, these functions may involve integrated electronic components or the vehicle’s central control unit. It is essential to request the actual interface specifications; never assume that all PTC coolant heaters employ the same architecture.

In its technical overview of coolant PTC heaters, Mitsubishi Heavy Industries discusses how ECUs (Electronic Control Units) and sensors are integrated. This clearly illustrates that ‘material characteristics’ and ‘system control’ are two distinct design domains – but this does not imply that EVLINK’s configuration employs the same electronic control scheme.

Not all high-voltage coolant heaters are PTC heaters

‘High-voltage coolant heater’ is merely a description of the product’s function. PTC refers to a type of heating technology. These terms should not be conflated as if they always describe the same structure.

Manufacturers also supply thick-film coolant heaters. For example, in its discussion of 800-volt technology, Eberspächer explicitly distinguishes between its thick-film coolant heaters and PTC air heaters. This distinction applies solely to these specific products; it should not be extrapolated as a basis for a general ranking of the merits and demerits of all PTC and thick-film designs.

When comparing quotations, be sure to enquire about the heating technology used in the selected model, its operating data, and the documentation relating to the control and protection of the complete assembly. Even if the outer casing appears similar or the rated power is the same, this does not imply that their performance is equivalent.

Please provide output data for specific operating conditions.

A rated power value alone does not clearly define the specific operating conditions. For example: What is the supply voltage? What are the inlet temperature and flow rate of the coolant? Does this value refer to the power during a cold start, a limited-time output power, or a sustainable operating power point?

Please request a ‘power map’ or validated performance data points specific to your operating conditions. As the resistance of PTC elements varies with temperature, it cannot be assumed that their output power remains constant when operating conditions change. Furthermore, electronic control systems may also limit output power; therefore, material characteristic curves alone cannot fully describe the performance of the entire heater assembly.

Please refer to our ‘Heater Power Verification Guide’ to distinguish between ‘required power’, ‘electrical input power’ and ‘thermal power transferred to the coolant’. These concepts must not be confused.

Clarify protection mechanisms and vehicle control responsibilities

Never misinterpret the ‘self-regulating’ characteristic as permitting dry burning, insufficient circulation or the bypassing of safety interlocks. Whether a heater assembly can detect specific faults, how it responds and how it restarts must be determined in accordance with its officially published technical documentation.

When conducting an integration assessment, the following points should be clarified:

• Which controller issues the heating request, and which controller is responsible for executing the vehicle’s power allocation (power budget).
• Which temperature parameters are monitored, and via which signal the control targets are set.
• Which operational authorisation conditions the heater must satisfy before being energised (activated).
• How communication interruptions, invalid measurement data or fault reports are handled.
• What countermeasures the vehicle should take when heating requirements cannot be met.

The above are consensus requirements at the engineering design level and do not imply that every heater possesses all of the aforementioned functions. Our ‘High-Voltage Coolant Heater (HVCH) Control Interface Checklist’ can be used to assist with assessments at the communication level.

Three levels to be distinguished in quotations

At the ‘component’ level, it is necessary to understand how resistance and available power vary with temperature and voltage. At the ‘assembly’ level, it is necessary to clarify which monitoring, control and feedback functions the heater possesses. At the ‘vehicle’ level, it is necessary to determine the priority of each heating task and to clarify the actual requirements of the battery system or the passenger compartment. Verification evidence at one level does not automatically cover the other two levels.

For example, certain material properties may cause the power output of a heating element to decrease as the temperature rises. However, this does not prove that changes in the coolant outlet temperature follow the expected temperature rise curve, nor does it prove that the remote battery pack has reached the target temperature before the vehicle sets off. These fall within the scope of system response, for which measurement points and acceptance criteria vary.

Why the same heater may produce different operating results

Consider two integration assessments carried out on the same heater. In the first scenario, the battery cooling circuit is at a low temperature, and the heater receives a heating command within the available electrical power budget. In the second scenario, the coolant itself is already at a higher temperature, and the vehicle controller limits the heater’s input power because other loads have higher priority. The difference in measurement results does not, in itself, imply that the PTC heating element has changed or is faulty.

This is merely an illustrative comparison and not an actual customer test report. To interpret such data accurately, it is necessary to simultaneously record the heating command, the actual electrical power input, the coolant status and any power limitation conditions. Focusing solely on the power percentage specified in the command overlooks the most critical explanatory factors.

Similarly, a low temperature rise at the coolant outlet does not necessarily indicate insufficient heat transfer. At the same heat generation rate, if the mass flow rate of the coolant is higher, the temperature rise before and after the heater will actually be smaller. Furthermore, the battery pack requires some time to respond, as the coolant, pipework and the hardware structure of the battery pack itself must first be brought up to temperature. Never judge the status of the entire heating system based on a single temperature reading.

A simple energy calculation should be carried out before committing to a preheating time

Assuming a heater has an average power of 10 kW over 15 minutes, the electrical energy input is 10 × 15/60 = 2.5 kWh. However, this does not mean that all of this 2.5 kWh of energy is transferred to the battery cells. Heat may be used to heat the coolant and surrounding hardware, or it may be dissipated through other means; furthermore, the water pump and other auxiliary loads will also consume energy.

The same calculation logic cannot directly determine the battery’s preheating time. To reach an accurate conclusion, one must also consider the initial thermal state, the thermal characteristics of the battery and circuitry, heat loss, coolant distribution, and the permissible electrical power input during that time period. The above example is intended solely for energy budget calculations and does not constitute a heater specification, efficiency claim or performance guarantee. If two quotations promise the same preheating time, it is essential to verify whether they are based on the same initial temperature, battery pack configuration, target measurement points and available input power. If the underlying conditions differ, these time figures are not comparable.

How should one respond when a supplier claims ‘no external control required’?

It is essential to clarify the specific scope of this statement. This may imply that power electronics or a local temperature controller are integrated within the component; however, it does not mean that the vehicle system can dispense with enable signals, operational authorisation, power allocation or fault handling.

Request that the supplier specify: which operating modes the system supports; which parameters can be controlled by external commands; which feedback signals are provided; and which restrictions remain the responsibility of the vehicle integrator. Furthermore, enquire how the system should respond when cycling functions are unavailable or necessary signals are invalid. Unless explicitly stated in officially published technical specifications, never assume that the system is capable of detecting specific faults.

Only approved validation procedures may be used. The content described in this document does not constitute operating procedures for high-voltage measurement or fault injection. Relevant tests must be carried out by trained personnel in strict compliance with vehicle isolation and testing specifications; bypassing protective mechanisms to demonstrate the system’s ‘self-regulating’ capabilities is by no means an acceptable shortcut.

Three separate procurement decisions

• Is the heating technology suitable? Please refer to the output power and operating limits documented in the selected design solution; do not base your selection solely on the ‘PTC (Positive Temperature Coefficient)’ label.
• Is the component configuration suitable? For the specific configuration, verify the interfaces, sensing, control, established protection mechanisms and installation requirements.
• Is the vehicle integration solution feasible? Clarify where control lies and determine the evidence required to validate the necessary heating functions within the actual circuit.

Prepare a request for quotation for a specific configuration

When evaluating different heating components, comparisons should be based on standardised requirements, including: voltage range, heating requirements over time, coolant conditions, sensing limits, control interfaces and established protection characteristics. Before comparing prices, ensure that installation and validation requirements are clearly defined.

Should discussions regarding EVLINK be required, please provide the proposed cooling circuit design, the vehicle’s voltage range, the target temperature requirements and a description of control authority. Specify the exact configurations being compared and list any parameters that remain undetermined. It is recommended that heater configurations be evaluated to ensure that decisions are based on technical specifications agreed upon by both parties, rather than relying solely on the vague concept of ‘self-regulating’.