Home › Technical Training › HV Battery Cooling Systems
HV BATTERY COOLING SYSTEMS – DIFFERENCES BETWEEN HEV AND PHEV AND THERMAL FAULT DIAGNOSIS (2026)
Updated October 2025 — by Gianni Tritella. High-voltage (HV) battery cooling systems are one of the most critical elements of hybrid and electrified vehicles. In this article, we analyze the technical differences between HEVs and PHEVs , the types of cooling (air, liquid, shared refrigerant), and the most common thermal faults detectable via OBD2 diagnostics (codes P0A7F, P0A80, P0A82, P0A9E).
LIQUI MOLY – UNIVERSAL GARDEN EQUIPMENT OIL 10W-30
10W-30 multi-purpose oil for four-stroke engines in lawnmowers, generators, tractors, and garden tools . It ensures optimal lubrication, reduces wear, and maintains high performance in all seasons.
🛒 Buy now on Amazon✅ Introduction
With the evolution of hybrid and electric systems, thermal management of HV batteries has become a key factor in efficiency, longevity, and safety. High-voltage batteries operate within an ideal temperature range of 20°C to 40°C : exceeding these limits rapidly reduces capacity and compromises the safety of the battery pack.
A malfunctioning cooling system can cause localized overheating, power derating, or permanent fault codes. Understanding the control logic and types of cooling used is essential for workshops working on electrified vehicles today.
⚙️ Differences between HEV and PHEV
Hybrid vehicles are not all the same: the differences between HEVs (Hybrid Electric Vehicles) and PHEVs (Plug-in Hybrid Electric Vehicles) also extend to thermal management systems. Battery capacity, usage, and average thermal cycle duration require different technical solutions.
- HEV – Traditional Hybrids: Compact batteries (1–2 kWh) almost always air-cooled. The fan or ducts draw air from the passenger compartment or trunk. Airflow can be reduced by clogged filters or the presence of dust and hair. Typical problems: noisy fans, P0A82 error, or thermal imbalances between modules.
- PHEV – Plug-in hybrids: Larger batteries (8–18 kWh) with higher charging power and therefore greater heat generation . They use dedicated liquid cooling systems or a combined inverter/electric motor. HV electric pumps regulate flow based on the temperature measured by sensors inside the BMS.
- Control strategies: The BMS (Battery Management System) activates pumps, fans, and proportional valves based on temperature and load data. In PHEVs, the system can interface with the vehicle's air conditioning system to use refrigerant gas as a cooling element.
🧊 Types of HV cooling
Manufacturers adopt different thermal management solutions depending on the vehicle type and the battery pack location. The main technologies in use in 2026 are:
- Air cooling: Inexpensive and simple, typical of early hybrids (Toyota Prius, Honda Insight). The fan draws air from the cabin and passes it through the battery pack. Weaknesses: Reduced efficiency at high outside temperatures and frequent duct maintenance.
- Liquid cooling: Independent closed circuit with electric pump, HV radiator, and thermostatic valve. Widely used in PHEVs and BEVs. Advantage: Precise and quiet thermal control; disadvantage: Risk of leaks or air locks that reduce flow.
- Refrigerant cooling (A/C gas): the most advanced technology used by BMW, Hyundai, Kia, and Tesla. The battery pack includes a chiller that uses R134a or R1234yf gas to rapidly reduce the temperature under load. It ensures maximum efficiency but requires specialized maintenance and specific HV-safe refrigerants.
Each system must maintain a uniform temperature between cells: a difference of more than 10°C between modules can already generate errors or power limitations.
🚨 Thermal faults and OBD2 codes
Faults related to HV battery cooling manifest themselves with symptoms such as the hybrid light coming on, a noisy fan, reduced electrical power, or charging stopping. The most common OBD2 codes are:
| Code | Description | Probable cause | Effect on the vehicle |
|---|---|---|---|
| P0A7F | HV battery capacity degradation | Excessive temperatures or unbalanced modules | Power reduction, hybrid warning light comes on |
| P0A80 | Deteriorated HV battery | Faulty pump or fan, insufficient cooling | Safety mode, reduction of electricity supply |
| P0A82 | Faulty HV cooling fan | Fan motor, fuse or PWM control | Derating and hybrid system shutdown |
| P0A9E | Battery temperature sensor error | Faulty sensor or broken wiring | Thermal management disabled, risk of overheating |
Many of these codes may appear in conjunction with secondary BMS or inverter faults (e.g., P0A94, P1A80). It is essential to analyze the freeze frame data to identify the exact conditions under which the fault occurred.
🧰 Diagnosis and procedures in the workshop
Diagnosing HV thermal faults requires careful attention and appropriate instrumentation. The correct approach includes:
- Full DTC reading: advanced scanner with access to the HVBMS control unit and auxiliary modules (pumps, fans, inverters).
- Actuator Test: Manually activate fans and pumps to check actual power consumption and speed.
- Real-time parameter verification: check module temperatures, ΔT differences, liquid flow and HV pump voltage.
- Check wiring and connectors: oxidation or loose connections cause false thermal alarms. Use HV-safe dielectric sprays.
- Software update: some faults (e.g. false P0A82) can be resolved with BMS updates or thermal calibrations.
Recommended tools for HV diagnosis:
- TOPDON Phoenix EV – HV analysis and battery insulation test.
- Launch X431 EV Diag – real-time data reading and actuators on HV modules.
LAUNCH X431 EV – DIAGNOSTIC KIT + ACTIVATION CARD
Complete upgrade kit for LAUNCH X-431 PRO3 V+ 5.0, PRO3S+ V5.0, PRO3 ACEX and PRO3 APEX tools. Includes EV activation card for advanced diagnosis of electric and hybrid vehicles.
🛒 Buy now on Amazon🛡 Prevention and maintenance
Preventive maintenance of HV cooling systems is often overlooked but essential to preserving battery life. Here are some recommended best practices:
- Clean HV ducts and fans every 40,000 km (HEV) or sooner if the car is driven in dusty environments.
- Check HV coolant and electric pumps every 60,000 km (PHEV). Use only fluids with dielectric specifications.
- Do not block any air vents or rear grilles: poor ventilation can cause thermal errors to occur within minutes.
- Monitor module temperatures and fan/pump operation with periodic diagnostics after workshop interventions.
❓ Frequently Asked Questions
Can an HEV run with a faulty battery fan?
Only for short trips. The HV battery overheats quickly and the BMS reduces power for protection. Continuing to drive under these conditions will permanently damage the modules.
Can I use engine coolant for the HV circuit?
No. HV fluids have different dielectric and anti-corrosion properties. Using the wrong fluid can short-circuit sensors or damage the battery pack.
How much does an HV cooling pump cost?
€150 to €400, depending on the model. Replacement requires trained personnel and high-voltage safety devices.
