How to Validate a BTMS for an Electric Truck or Bus

The performance of a Battery Thermal Management System (BTMS) during component bench testing may differ from its performance when integrated into a battery pack, the vehicle’s control system, and the actual cooling circuit. For electric bus or truck projects, validation efforts should adhere to the operating conditions and acceptance limits agreed upon by the vehicle development team, rather than relying solely on the vague statement that “testing has been passed.” This checklist is designed to help engineers and technical procurement personnel identify the key metrics that should be measured before approving a configuration.

Establishing Acceptance Criteria Before Testing

These should be based on temperature limits approved by the battery supplier and the vehicle’s operating conditions (duty cycles). Identify which operating conditions place the highest demands on cooling capacity, which place the highest demands on heating capacity, and how the charging process alters system requirements.

For each operating condition, clearly specify the permissible temperature range, required response time, test duration, and permissible power consumption. These limits should be set for the specific project; never directly adopt temperature targets used for other vehicle models.

In addition, test boundaries must be clearly defined. Do the reported cooling performance results include the power consumption of the water pump and fan? Is the coolant flow rate forced by the test bench, or is it generated by the system under test itself? Without these definitions, even a passing test may not resolve issues at the system integration level.

Distinguishing Between Three Levels of Validation Criteria

1. Component or Unit Bench Testing

Measure the system’s actual thermal management performance under specified environmental conditions, coolant inlet conditions, and electrical conditions. Record coolant inlet and outlet temperatures, flow rate, pressure drop, power input, control commands, and steady-state operating performance.

Test conditions should cover key scenarios jointly confirmed by the supplier and the original equipment manufacturer (OEM), rather than merely testing the most ideal rated operating point.

2. Integrated Cooling Circuit Testing

Connect the unit under test to the hoses, battery-side flow resistance components, water pumps, valves, and sensors used in actual applications. Verify whether the system can maintain the required flow rate and check whether changes in control commands lead to temperature instability or unexpected pressure losses.

At the same time, based on the predetermined installation plan, verify filling, venting, airtightness (leak prevention), and serviceability.

3. Complete Vehicle or Representative System Testing

Conduct tests under operating conditions such as key route driving, charging, and cold starts, in coordination with the actual Battery Management System (BMS) and vehicle control interfaces. Confirm control command priorities, CAN bus messages, fault reporting mechanisms, and fault recovery logic.

Performance metrics derived from bench testing cannot replace the evidence required to verify that the entire system can ensure the battery pack remains within its permissible temperature limits under the aforementioned actual operating conditions.

What data should be included in the test report?

• Configuration and software versions of the BTMS (Battery Thermal Management System), battery interface, and vehicle controller.
• Environmental conditions, coolant composition, initial temperature, and test duration.
• Coolant inlet/outlet temperature, flow rate, and pressure readings, as well as sensor locations.
• Battery temperatures at agreed-upon measurement points, and the method used to summarize temperature distribution (temperature differences).
• Voltage, current, or power data from equipment within the test boundaries.
• Commands, operating states, warnings, faults, and a log of state transitions with timestamps.
• Acceptance limits, pass/fail determinations, deviations, and the person who approved the results.

Both raw data traces and summarized information must be recorded. Relying solely on final temperature values may obscure slow responses, transient fluctuations, or unstable control cycles.

Where applicable, include cold-weather and charging conditions

For vehicles sold in cold regions, testing should be conducted for typical cold starts and transitions to the charging state within the limits authorized by the battery supplier. Specify the battery pack temperature and coolant temperature at startup, the available electrical power, and the conditions under which charging may begin. Never estimate preheating times based solely on the heater’s rated power.

For high-power charging conditions, include charging curves and thermal response characteristics; the U.S. Department of Energy notes that battery thermal dissipation capacity is one of the limiting factors in the design of fast-charging systems.

Convert Operating Conditions into a Test Matrix

First, create a table that maps each operating condition to the corresponding test conditions and acceptance criteria. Test content should cover normal driving, scheduled high-load conditions, charging, cold starts, shutdowns, and restarts following a failure.

Each row of the table should clearly specify: the initial battery pack temperature and coolant temperature, environmental conditions, power supply status, expected operating mode, test duration, and the response performance permitted by the battery supplier. If the test team cannot clearly identify which real-world operating condition a particular test corresponds to, that test is of no value.

Distinguish between continuous requirements and time-limited requirements. If the vehicle permits temperatures to exceed normal ranges for short periods, the limits and duration must be clearly documented; if not permitted, do not arbitrarily set tolerances.

Any accelerated testing or simulated conditions must be linked to specific vehicle requirements and clearly labeled as “simulated”; they must not be confused with completed field validation.

Select measurement metrics that aid in analyzing the root cause of failures

Battery temperature alone may only indicate whether the battery is approaching its limits, but it cannot reveal the underlying cause. Sensors should be placed at agreed-upon locations, and data—including coolant inlet and outlet temperatures, flow rate, pressure before and after key throttling points, equipment power, ambient temperature, and controller status—should be collected under a unified time reference.

The report should document the sampling intervals and the calibration status of the instruments. If the thermal response is too slow, these data curves can be used to determine whether the problem stems from insufficient component output, low flow rate, the presence of air pockets, improper sensor placement, or control delays.

During cold-start testing, the time required from issuing a command to a change in coolant temperature should be recorded, as well as the time required for the battery to reach the project-specified operating or charging conditions.

In cooling tests, attention should be paid not only to the final steady state but also to the response process following load changes. In both types of tests, it is necessary to compare the dispersion (distribution) of sensor readings within the battery pack as well as the average temperature. Average values may mask localized areas of overheating or overcooling.

Targeted Testing of Control Logic and Fault Responses

First, define the normal command sequence: activation, mode selection, setting or request issuance, status confirmation, and shutdown. Subsequently, test predetermined fault scenarios, such as: lost CAN messages, abnormal (unreasonable) temperature signals, reduced coolant flow, or interruption of critical power supplies.

Original Equipment Manufacturers (OEMs) and battery system integrators should clearly define the expected safety response measures. This document does not define a uniform “safe state,” as the specific measures to be taken depend on the vehicle’s system architecture.

For each fault, record the detection time, reported fault codes, protective measures taken, and recovery procedures. Additionally, confirm whether the system automatically restarts or requires manual intervention.

If changes are made to the software or control strategies (control mapping tables), revalidation is mandatory; the fact that a previous version passed thermal management testing does not guarantee that the final configuration will also be compliant.

Define the delivery process from prototype development to mass production

The validation report should detail the tested hardware and software versions, outstanding deviations, corrective actions, and test conditions not covered. Each acceptance criterion should be linked to specific requirement items and the responsible party.

If test results are non-compliant, determine whether the issue lies with the unit itself or with the control loop or interface level before replacing the hardware. Both parties should reach a consensus on “what retest results would indicate that the issue has been resolved” and assess whether the changes made will affect other operating conditions.

To ensure test repeatability, test layout diagrams, sensor distribution maps, raw data, acceptance checklists, and approval records should be properly archived. Compared to a simple statement such as “BTMS (Battery Thermal Management System) test passed,” these detailed records provide greater reference value for subsequent mass production or after-sales service teams.

Furthermore, this enables more precise communication between the vehicle manufacturer and the supplier—both parties can discuss the same operating conditions, the same measurement data, and the same software version.

Brief Answers to Common Questions About Validation

Does passing a bench test mean the vehicle is ready for use?

Not necessarily. Bench testing only verifies performance within specific test boundaries. Actual flow rates, heat transfer characteristics, and responses to control commands may differ when the system is integrated into the vehicle’s control environment.

Should all projects use the same temperature limits?

No. Use the limits approved for the specific battery pack and its application scenario, and document the source and version information of these limits in the test plan.

When should testing be repeated?

When changes occur to hardware, cooling line layout, control software, sensor placement, or acceptance criteria, retest the affected test items. Document the reason for the change and the version to ensure that the final validation results match the actual delivered configuration.

Before final validation, consider the following questions:

• Have tests covered all critical operating conditions, or were only rated operating points tested?
• Are the records of sensor locations and test boundaries detailed enough to ensure the test results are reproducible?
• Do the integrated circuits meet the expected flow and pressure limits?
• Has the system’s behavior been verified in the event of communication interruptions, sensor failures, and entry into a safe state?
• For any remaining risks, are in-vehicle tests, design adjustments, or revisions to acceptance criteria required?

No single document can directly prove the compliance of a specific configuration. The value of a validation plan lies in its ability to transform a supplier’s claims into reproducible, project-specific decision-making criteria.

For information on related hardware, please refer to EVLINK’s Battery Thermal Management System (BTMS) product line; for project evaluation, please provide your duty cycle, circuit schematics, and proposed acceptance criteria.