PTC Coolant Heater for Battery and Cabin: One Loop or Two?

PTC coolant heaters can serve as a heat source for both the battery and the cabin; however, this does not mean that these two loads must share the same coolant circuit. Before selecting a heater, it is necessary to determine: Can these loads share the same coolant? Do they require independent temperature control? How should the system respond when both require heating simultaneously?

There are three options to consider: a shared coolant circuit, independent circuits connected via a heat exchanger, or completely independent heating circuits. The specific solution depends on the vehicle’s operating modes and the requirements of each component. It is important to distinguish between the two distinct design decisions: “shared heat source” and “shared coolant circuit.”

Before linking the two loads, they must first be clearly defined.

First, refer to the thermal operating conditions permitted by the battery supplier, as well as the heating requirements of the cabin system. Analyze how these requirements vary during cold starts, driving, charging, and when the vehicle is stationary. Never simply reduce the requirements of each load to a single maximum power value.

Confirm whether battery heating and cabin heating need to occur simultaneously, and identify operating conditions where their requirements conflict. For example, the cabin may still require heating after battery heating has ended; or, under certain operating conditions, the battery may need to be cooled while occupants still require cabin heating. The designed circuit must have clear response mechanisms tailored to these combined scenarios.

Document the coolant formulations, materials, pressure limits, and temperature limits for all associated components. Even if antifreeze protection requirements are similar, this does not imply compatibility between systems. Before combining coolant circuits or treating the certification of a single component as certification for the entire circuit, be sure to obtain written confirmation of the selected configuration.

Option 1: Interconnected Cooling Loop

In a hydraulically interconnected configuration, the coolant circulates through a shared piping network. The drawings must clearly specify how the coolant is delivered to each load and how the system is controlled or isolated when the demands of the various branches differ.

This option may be considered if the fluid requirements of each load are compatible and the desired operating mode can be achieved within the operating limits of the interconnected components. However, branch flow rates, pump operating conditions, valve positions, and temperature control must still be evaluated. A smaller number of heat sources does not necessarily mean simpler control logic or an easier validation process.

It is necessary to analyze the situation when a branch is shut off or demand changes, and to confirm the resulting changes in flow rates through the heater and other branches. Never assume that each component receives its required flow rate based solely on the total pump flow rate. Installation requirements and control limits must be provided by the relevant suppliers.

Option 2: Fluid Isolation via Heat Exchangers

Heat exchangers allow for heat transfer while keeping the two fluid circuits independent of each other. This option should be considered when fluid demands or operating boundary conditions render direct interconnection unsuitable.

Fluid isolation does not imply that the loads are thermodynamically independent. The heat transfer process still depends on the temperatures and flow rates on both sides, as well as the characteristics of the heat exchanger itself. The operating conditions on the heater side cannot be directly equated with the temperatures achieved on the battery side.

Target heat transfer rates under key operating conditions should be clearly defined; simultaneously, the corresponding pressure losses and control strategies during periods of low demand, startup, and mode transitions must be verified. When evaluating electrical and structural layouts, both the pump and the heat exchanger should be taken into account. Do not assume that this solution will necessarily be lighter, more economical, or more efficient than using a standalone heater.

MathWorks’ official vehicle thermal management documentation explains how to model multiple thermal loops and various operating modes. The model serves only as a design tool and cannot directly verify the performance or safety of a specific EVLINK installation.

Option 3: Independent Heating Circuits

Using independent circuits allows for an evaluation of the heating paths and control requirements specific to each of the two loads. This option should be considered when operating temperatures, availability requirements, or integration constraints make it difficult for the system to share resources.

The trade-offs include additional hardware, connectors, packaging space, and power requirements. Since independent circuits may still share the vehicle’s power limits or monitoring controller, they may not be completely independent at the vehicle level.

Power requirements during simultaneous operation should be evaluated, rather than simply adding the rated power of each heater and assuming the vehicle can meet the total demand. If available power is limited, a load allocation plan must be determined in collaboration with the vehicle and battery teams. This document does not define safety priorities among cabin comfort, battery operating conditions, and other vehicle functions.

Compare the various solutions under identical requirements.

All candidate solutions should be evaluated using a standardized operating condition matrix. Otherwise, evaluating the “shared solution” only under low-load conditions may make it appear superior to the “independent solution” evaluated under extreme load conditions.

This table lists the issues that need to be addressed. This does not imply that any specific system configuration will achieve the expected level of reliability, nor does it imply compatibility with all PTC coolant heaters.

Test dynamic requirements, not just steady-state heating.

At a minimum, the following operating conditions should be considered: simultaneous cold start requirements, the termination of a load heating request on a particular branch, changes in branch flow rate, and transitions to charging or driving modes. If the vehicle’s expected operating conditions include situations where “battery cooling” and “cabin heating” occur simultaneously, these should also be included in the test scope.

For each applicable mode, record heating requests, response acknowledgments, actual electrical input, relevant inlet and outlet temperatures, flow rates at specific boundaries, and controller limitations. Ensure that all records are synchronized so that temperature changes can be correlated with the states of the valves and pumps that caused those changes.

Before testing, a consensus on “acceptable switching behavior” should be reached. Stable performance in a single mode does not imply that switching between modes also meets requirements. Recovery following a failure of “shared equipment” functions should be evaluated through established validation procedures; never intentionally induce high-voltage faults or bypass protective mechanisms for demonstration purposes.

When consulting on heater selection, please provide the circuit diagram along with your request

When discussing the EVLINK high-voltage coolant heater product series, please provide circuit diagrams, heating requirements for the battery and passenger compartment, applicable operating modes, coolant-related technical data, installation constraints, and available power supply parameters. Please clearly indicate which input parameters have been confirmed and which are still at the estimation stage.

When discussing load requirements, please refer to the heater selection guide; when compiling supplier feedback, you may use the Request for Quote (RFQ) checklist. If the system involves a heat pump, please refer to the auxiliary heating load guidelines and specify its contribution separately.

Please contact EVLINK to evaluate your proposed heating circuit solution and confirm the applicability of the selected configuration. Product images, approximate power ranges, or similar comparative data alone do not constitute evidence that a “shared battery and cabin circuit” solution has been approved.