Battery Thermal Management

Battery temperature affects charging, power output, regenerative braking, efficiency, and durability because cells cannot accept or deliver energy equally well at every temperature.

A Battery Thermal Management System (BTMS) therefore heats, cools, and equalises pack temperature according to chemistry, state of charge, required power, and the manufacturer’s control strategy.

Why Battery Temperature Matters

Battery cells are electrochemical devices. Their resistance, reaction speed, available energy, and ageing behaviour all change with temperature.

The thermal-management system must balance several priorities:

Making sufficient power available for driving
Allowing high charging and regenerative-braking power
Limiting electrical losses
Keeping cells within safe limits
Reducing long-term degradation
Maintaining similar temperatures across the pack
Using as little energy as practical for heating and cooling

A battery can be within its safe temperature range without being at the best temperature for every task.

There is no single ideal battery temperature. A temperature suitable for efficient cruising may still be too low for maximum fast-charging power, while a battery prepared for high-power charging may be warmer than necessary for parking or long-term storage. Audi, for example, describes a 30–35°C preferred range for the e-tron GT battery under demanding operating and charging conditions, but this is not a universal target for every EV. (Audi)

What Happens When the Battery Is Cold

Low temperature slows ion movement through the electrolyte and electrode materials. Internal resistance increases, causing a larger voltage drop and more heat generation for a given current.

The driver may notice:

Reduced acceleration
Lower peak and continuous power
Restricted regenerative braking
Slower DC charging
Reduced immediately available energy
Higher consumption during short winter journeys

Cold temperature by itself does not normally damage a battery. The greater risk occurs when a cold cell is charged at high current.

In conventional lithium-ion cells with graphite-based negative electrodes, lithium ions may reach the electrode surface faster than they can be inserted into the graphite. Metallic lithium can then form on the surface through a process known as lithium plating.

Lithium plating can:

Consume lithium that would otherwise remain available for cycling
Increase capacity loss
Raise internal resistance
Form uneven deposits
Increase safety risks in severe cases

The BMS therefore restricts charging and regenerative braking until the cells are warm enough to accept more current. Fast charging without lithium plating remains one of the key engineering challenges for cold or high-rate charging. (US Department of Energy)

What Happens When the Battery Is Hot

A warmer battery initially has lower resistance and can often deliver or accept more power. This does not mean hotter is always better.

High temperature accelerates unwanted reactions between the electrodes and electrolyte. Prolonged exposure can contribute to:

Faster calendar ageing
Loss of active lithium
Growth of surface layers such as the SEI
Electrolyte decomposition
Gas generation
Increased internal resistance
Reduced cycle life

Temperature and state of charge interact. Parking a battery for a long period at both high temperature and high SOC is generally more stressful than briefly warming it before a charging session.

High temperatures during extreme fast charging are also difficult to manage. US Department of Energy research identifies battery heat generation and temperatures above roughly 45°C as important barriers to sustained extreme-fast-charging performance and battery life. The exact limit remains chemistry- and design-dependent. (US Department of Energy)

Where Battery Heat Comes From

A battery generates heat whenever current flows.

Sources include:

Electrical resistance inside the cells
Ion transport through the electrolyte and electrodes
Electrochemical reactions
Cell tabs and current collectors
Busbars and electrical connections
Contactors and pack junctions
Charging and discharging inefficiency
Heat transferred from nearby power electronics

Resistive heat increases approximately with the square of current:

Resistive heat ∝ Current² × Resistance

Doubling current can therefore generate roughly four times as much resistive heat if resistance remains unchanged.

This helps explain why demanding conditions create high thermal loads:

High-power DC charging
Repeated hard acceleration
Sustained high-speed driving
Long climbs
Towing
Track use
Rapid regenerative braking

Heat is not always generated uniformly. The centre of a large cell may be warmer than its outer surface, while cells close to coolant outlets or pack edges may operate at different temperatures from cells elsewhere in the pack. NREL research has linked temperature inhomogeneity with local differences in state of charge, lithium plating, SEI growth, and degradation. (US Department of Energy)

There Is No Single Ideal Temperature

The preferred battery temperature depends on what the vehicle is preparing to do.

Normal Driving

For moderate driving, the system may prioritise low auxiliary consumption rather than heating or cooling the battery to its fast-charging target.

High-Power Driving

Repeated acceleration, towing, climbing, or high-speed driving may require additional cooling to maintain available power.

DC Fast Charging

The battery often needs to be warmer than it would during ordinary winter driving. Cell resistance must be low enough to accept high current without excessive voltage rise or lithium plating.

Parking

The car generally does not keep the battery at its fast-charging temperature while parked. Doing so would waste energy. The pack is normally allowed to move within a wider protective temperature range.

Long-Term Storage

High temperature and high SOC are undesirable for long-term storage, even though a temporarily warm battery may charge or deliver power more effectively.

Thermal management is therefore a continuous compromise between immediate performance, energy use, and ageing.

Thermal-Management System Architectures

EV manufacturers use several methods to move heat into and out of the battery.

The main approaches are:

Passive thermal management
Air cooling
Indirect liquid cooling
Direct refrigerant cooling
Integrated coolant and heat-pump systems

Most modern EVs designed for high charging power use active liquid cooling, often connected to the vehicle’s refrigerant circuit through a heat exchanger or chiller.

Passive Thermal Management

A passively managed battery relies mainly on heat moving through:

Cell housings
Module structures
The battery enclosure
The vehicle underbody
Surrounding air

This approach has few components and consumes little energy.

Advantages

Low cost
Low weight
No pumps or coolant
Few service points
Low standby consumption

Limitations

Limited ability to remove heat
Strong dependence on ambient temperature
Slow heating and cooling
Difficulty maintaining uniform cell temperatures
Poor suitability for repeated high-power charging

Passive thermal management can be sufficient for a low-power urban EV but becomes increasingly restrictive as charging speed, battery size, and sustained power increase.

Air Cooling

Air-cooled systems move ambient or cabin air through or around the battery.

The airflow may be driven by fans or by movement of the vehicle.

Advantages

Simpler than liquid cooling
Lower mass
No battery-coolant circuit
Reduced risk of coolant leakage

Limitations

Air carries much less heat than liquid coolant
Cooling depends heavily on air temperature
Air ducts occupy space
Temperature uniformity can be difficult to maintain
Fan noise and power consumption increase under high load

An air-cooled battery may complete one fast-charging session successfully but struggle to remove enough heat for repeated charging. Nissan documented declining charging power from the second rapid-charging session onward in testing of the air-managed LEAF e+ battery. (Nissan)

Indirect Liquid Cooling

Indirect liquid cooling is the most common architecture in current high-power EVs.

A water-glycol coolant flows through:

Cooling plates beneath the cells
Channels beside cell rows
Extruded sections inside the pack floor
Plates contacting the broad faces of cells
Cooling structures integrated into modules

The coolant does not normally flow inside the cells. Heat must cross several layers:

Cell interior
Cell enclosure
Thermal-interface material
Cooling plate
Coolant

Thermal-interface material can be a pad, paste, adhesive, gel, or another conductive layer used to fill small gaps between the cell or module and cooling plate.

The Audi Q4 e-tron uses a dedicated low-temperature battery circuit with a water-glycol mixture flowing through flat channels beneath the cell compartments. Thermal paste connects the cells to the cooling structure, while a high-voltage heater warms the coolant in cold conditions. (Audi)

Advantages

High heat-transfer capability
Better temperature uniformity than passive or air cooling
Effective heating and cooling through the same plates
Suitable for high-power driving and charging
Can be connected to a radiator, chiller, or heat pump

Trade-Offs

Adds coolant, pumps, valves, hoses, and heat exchangers
Increases weight and cost
Creates possible leak and service points
Requires careful sealing and corrosion control
Cooling performance depends on contact between the cells and plates

Radiator Cooling

When ambient air is cool enough, the battery coolant can release heat through a radiator.

This is often the most energy-efficient cooling mode because the system may need only:

A coolant pump
Airflow through the radiator
A cooling fan when vehicle speed is insufficient

Radiator cooling cannot reduce battery temperature below ambient air temperature. It can also become insufficient during hot weather or high-power charging.

Refrigerant Chiller Cooling

When the radiator cannot remove enough heat, the coolant can pass through a chiller connected to the vehicle’s air-conditioning refrigerant circuit.

The chiller transfers heat from the battery coolant to the refrigerant. The electric compressor then moves that heat through the air-conditioning system and rejects it outside the vehicle.

A simplified heat path is:

Battery cells
    ↓
Cooling plate
    ↓
Water-glycol coolant
    ↓
Refrigerant chiller
    ↓
Air-conditioning refrigerant
    ↓
Condenser and outside air

This allows the vehicle to cool the battery below ambient temperature or maintain it under sustained thermal load.

The Audi e-tron GT uses four interconnected coolant circuits. Under high drive or charging load, valves connect the battery circuit with the air-conditioning refrigerant circuit for more intensive cooling. (Audi)

Direct Refrigerant Cooling

Some designs place refrigerant channels or evaporator surfaces directly against the battery cooling structure instead of first using a separate water-glycol loop.

This can reduce the number of heat-transfer stages and provide strong cooling performance.

Trade-offs include:

More difficult refrigerant distribution
Greater sensitivity to pressure and sealing
More complex service procedures
Risk of uneven cooling if refrigerant flow is not well controlled
Tighter integration between the battery and HVAC system

Indirect liquid cooling remains common because it separates the battery coolant circuit from the high-pressure refrigerant circuit.

Integrated Thermal Management

Modern EV thermal systems increasingly connect the battery, motors, power electronics, charging hardware, and cabin.

Depending on current conditions, heat can be:

Removed from the battery
Reused to warm the cabin
Moved from the drivetrain to the battery
Rejected through a radiator
Moved through the refrigerant circuit
Drawn from outside air by a heat pump

The Audi Q6 e-tron uses coolant and refrigerant heat exchangers, while its predictive control system coordinates battery heating and cooling. Nissan similarly describes an integrated system that reuses heat generated during driving and charging for the battery and cabin. (Audi)

Battery Heating Methods

Heating a cold battery requires energy. Manufacturers differ in how that energy is supplied.

High-Voltage Coolant Heater

A dedicated electric heater can warm the coolant flowing through the battery cooling plates.

Positive Temperature Coefficient, or PTC, heaters are commonly used because their electrical resistance increases as they become hotter, helping limit excessive heating.

Advantages

Works in almost all ambient conditions
Provides predictable heating power
Can warm the battery without driving
Relatively straightforward to control

Trade-Offs

Electrical energy is converted directly into heat
High heating power can reduce available driving range
Adds hardware, weight, and cost

Audi uses high-voltage heaters in several EV thermal systems, including the Q4 e-tron’s coolant circuit. (Audi)

Heat-Pump Heating

A heat pump transfers heat rather than producing all of it through electrical resistance.

Possible heat sources include:

Outside air
Motor and inverter coolant
Charging hardware
Cabin exhaust air
The refrigerant circuit

Under suitable conditions, a heat pump can move more thermal energy than the electrical energy consumed by its compressor.

Its performance falls as the temperature difference increases. Many vehicles therefore combine a heat pump with resistance heaters for rapid heating or very cold conditions.

Waste-Heat Recovery

Motors, inverters, onboard chargers, and DC-DC converters generate heat during operation.

An integrated thermal system can redirect some of this heat toward:

The battery
The cabin
Another coolant loop

Waste-heat recovery is useful during driving and charging but cannot always provide enough heat after a cold start. Nissan describes reusing powertrain and charging heat as part of its integrated thermal-conditioning strategy. (Nissan)

Heat Generated During Driving

Driving itself warms the battery because current flow generates heat.

A cold pack may gradually recover charging power and regenerative braking after:

Motorway driving
Climbing
Strong acceleration
Sustained power use

This warming is not free. The heat comes from energy lost through resistance and other inefficiencies.

Some vehicles may deliberately create additional electrical load or alter drivetrain operation before charging. Whether this is used, and how aggressively, depends on the vehicle’s thermal architecture.

Cooling-Plate Placement

The position of the cooling surface influences how effectively heat can move through the cell.

Common arrangements include:

Bottom cooling
Side cooling
Cooling between cell rows
Broad-face cooling
Tab or terminal cooling
Double-sided cooling

Bottom Cooling

A plate beneath the modules or cells is relatively easy to integrate into a flat pack. Heat must travel from the upper parts of a tall cell down to the cooling plate.

Side or Broad-Face Cooling

Cooling the large faces of prismatic or pouch cells can shorten the heat path but requires consistent mechanical contact and compression.

Cooling Between Cell Rows

Channels between cylindrical cells provide a large contact area but take up pack space and create many repeated coolant paths.

Double-Sided Cooling

Cooling a large cell from both sides reduces the distance heat must travel and can improve temperature uniformity.

The Porsche Cayenne Electric uses cooling plates above and below each of its six modules. Porsche says the plates can both heat and cool the battery, helping the large pouch cells reach their target temperature more effectively. (Porsche Newsroom)

Temperature Uniformity

Reducing average pack temperature is not enough. The thermal system must also minimise differences between cells and within each cell.

Uneven temperature can cause:

Different cell resistance
Uneven current distribution
Different states of charge
Local ageing
Local lithium plating
One cell group reaching a voltage limit early
Reduced usable capacity

Potential problem areas include:

The centre of large cells
Modules far from the coolant inlet
Cells close to contactors and busbars
Densely packed central modules
Cells with poor cooling-plate contact
Areas exposed to road or exhaust heat

The thermal-management system, BMS, cooling-plate design, cell format, and pack compression all contribute to temperature uniformity. DOE research into extreme-fast-charging cells identifies internal temperature gradients as a cause of uneven degradation and a limitation for large cells cooled from only one side. (US Department of Energy)

Battery Preconditioning

Battery preconditioning brings the pack closer to the temperature needed for a planned task.

The term most commonly refers to preparation for DC fast charging, but it can also include:

Heating before departure
Cooling before charging
Preparing for high drive power
Maintaining a suitable temperature while connected to AC charging

Preconditioning is not always heating. A hot battery may need active cooling before reaching a fast charger.

Fast-Charging Preconditioning

When a DC fast charger is selected in the onboard navigation system, the vehicle can estimate how much heating or cooling is needed before arrival.

Inputs may include:

Current battery temperature
Ambient temperature
Distance and driving time
Expected arrival SOC
Recent driving load
Expected charger capability
Route elevation and speed

The system then adjusts heating or cooling during the journey.

Audi’s e-tron GT thermal management can intensify battery cooling roughly half an hour before arriving at a planned high-power charger, or heat the pack in cold conditions.

Preconditioning does not guarantee peak charging power. Charging still depends on:

Arrival SOC
Charger voltage and current
Cell voltage
Cell imbalance
Pack temperature
Battery condition
The vehicle’s charging curve
Power sharing at the charging site

Manual Preconditioning

Some EVs let the driver start battery preparation manually.

This is useful when:

A phone or third-party route planner is being used
The charger is missing from the onboard database
The planned charging stop has changed
The driver is not using route guidance
The vehicle has selected the wrong charger type
Charging is expected sooner than the navigation system predicts

Manual preconditioning may still have restrictions:

A minimum SOC
Temperature-dependent availability
A fixed maximum operating time
Automatic shutdown
Reduced heating power at low SOC
Cancellation if the driver changes operating mode

Starting preconditioning also does not prove that the battery has reached its target temperature. The available preparation time and heating capacity still matter.

Preconditioning Before Departure

When the EV is connected to home charging, scheduled preconditioning can use grid energy to warm the cabin and, in some vehicles, the battery.

This can reduce the amount of stored battery energy used immediately after departure.

Three functions should not be confused:

Cabin preconditioning
Battery preconditioning
Charging scheduled to finish near departure

Some EVs perform all three. Others heat only the cabin.

Finishing charging shortly before departure can still help because charging generates heat inside the cells. A battery that finished charging recently may start the journey warmer than one that completed charging many hours earlier.

US Department of Energy analysis notes that preconditioning while connected allows energy to come directly from the grid and can reduce cold-start energy losses during short winter trips. (US Department of Energy)

Cooling Before Fast Charging

In warm weather or after demanding use, the battery may be too hot rather than too cold.

This can occur after:

Sustained high-speed driving
Towing
Mountain driving
Track use
A previous fast-charging session
Parking in strong sunlight
Repeated acceleration

The vehicle may begin active cooling well before the charging stop. A large pack has substantial thermal mass and cannot change temperature instantly.

Preconditioning therefore needs enough travel time to remove heat from the cell interiors, rather than simply cooling the measured surfaces.

Why Preconditioning May Stop at Low SOC

Battery heating and cooling consume energy.

At low SOC, the manufacturer must balance:

Reaching the charger
Preparing the battery
Preserving a reserve
Avoiding excessive cell discharge
Maintaining cabin heating

The system may reduce or stop battery preconditioning when SOC falls below a defined threshold.

The threshold differs between vehicles and can change with software. The Kia EV9 example above shows one implementation and should not be interpreted as a general 12% rule.

Energy Use and Driving Range

Battery preconditioning uses energy. When the vehicle is unplugged, that energy comes from the traction battery and reduces the amount available for propulsion.

The main benefits are:

Shorter charging stops
More available drive power
More regenerative braking
Reduced risk of cold-charging damage
More predictable performance

A warmer battery can have lower resistance, but this does not mean that preconditioning always increases total driving range. The energy saved through reduced losses may be smaller than the energy used for heating.

For a long journey with a fast-charging stop, spending battery energy on preconditioning can still reduce total travel time substantially.

When plugged in, grid-powered preconditioning normally provides the clearest range benefit because less stored energy is consumed before departure.

Repeated Fast Charging and Thermal Soak

Peak charging power describes only one part of road-trip performance.

After one charging session, heat remains in:

Cell interiors
Module structures
Cooling plates
Coolant
Busbars and terminals
The battery enclosure

This accumulated heat is often called thermal soak.

The next charging stop may begin with a warmer battery, even if the dashboard shows no warning. The thermal system must remove heat during the journey and maintain a sufficiently even temperature before charging again.

A strong long-distance system needs to:

Prepare the battery before the first stop
Remove heat during charging
Continue cooling after departure
Avoid excessive temperature rise at the next stop
Maintain repeatable charge power
Limit unnecessary compressor and fan consumption

The Audi e-tron GT uses interconnected coolant and refrigerant circuits to support repeated high output and fast charging. Its route-based thermal strategy can heat or cool the battery before the charging stop. (Audi)

Two EVs with the same quoted peak power can therefore perform differently over several charging sessions.

Sustained Performance

Thermal management also determines how long an EV can maintain high drive power.

A battery may support a very high output for several seconds but require lower power continuously because of:

Cell heat generation
Inverter or motor temperature
Coolant temperature
Radiator capacity
Refrigerant-system capacity
Ambient conditions

This matters during:

Autobahn driving
Towing
Long mountain climbs
Track driving
Repeated acceleration

A powerful motor does not guarantee repeatable performance if the battery and thermal system cannot support it.

Thermal Management While Parked

The battery can continue heating or cooling after:

Fast charging
A demanding drive
Parking in extreme heat
Connecting to AC charging
Completing a scheduled trip

Drivers may hear:

Coolant pumps
Fans
Valves
The air-conditioning compressor
Electrical heaters

This can be normal even after the car is locked.

The vehicle does not normally maintain the battery at its ideal fast-charging temperature indefinitely. It instead allows a wider parked-temperature window and intervenes when required for protection, charging, or a planned departure.

Strategies vary between vehicles. Some cars actively condition the battery only while plugged in, while others may use traction-battery energy when temperatures approach protective limits.

Chemistry-Specific Thermal Behaviour

The thermal-management requirement depends partly on cell chemistry, but the complete vehicle design remains decisive.

Nickel-Rich Lithium-Ion

NMC, NCA, and NCMA cells can provide high energy density and power.

Their performance depends on careful control of:

High temperature
High cell voltage
Charging current
Cooling uniformity
Time at high SOC

Nickel-rich cells are widely used in vehicles requiring long range and high power, but they place substantial demands on cooling and control.

Lithium Iron Phosphate

LFP offers strong thermal stability and long cycle life.

Its main thermal limitation is cold operation. Cold LFP cells can show:

Greater charging restriction
Reduced regenerative braking
Lower power
More difficult SOC estimation
Greater dependence on active heating

A well-heated LFP battery can still provide strong winter charging performance. Chemistry alone does not determine the result.

Sodium-Ion

Some sodium-ion designs are being developed specifically for broad-temperature operation.

CATL claims that its Naxtra passenger-vehicle sodium-ion cell retains 90% usable power at −40°C. This is a manufacturer specification for that particular product, not a universal property of all sodium-ion cells. (CATL)

Sodium-ion packs still require thermal management because:

Charging creates heat
Cell ageing remains temperature-dependent
Temperature uniformity matters
Power and charging limits still change with temperature
The passenger cabin and powertrain share thermal resources

Thermal Management and Battery Ageing

Thermal management affects both cycle ageing and calendar ageing.

A good system limits:

Long periods at excessive temperature
Fast charging when cells are too cold
Large differences between cell temperatures
Repeated high-power use without sufficient cooling
Local hotspots
Long-term exposure to high temperature and high SOC

The goal is not to hold the battery at one temperature at all times. That would consume too much energy.

Instead, the system uses different temperature targets for:

Driving
Fast charging
Parking
AC charging
High power
Battery protection

A vehicle with strong cooling may deliberately allow the cells to become warm during fast charging and then remove that heat afterward. Brief controlled warming is different from prolonged heat exposure.

Normal Cooling Is Not Thermal-Runaway Protection

Normal thermal management removes heat created during ordinary charging and driving.

Thermal runaway is a different condition. It involves self-accelerating internal reactions that can release heat faster than the standard cooling system can remove it.

If thermal runaway begins inside a cell, opening the contactors and running coolant pumps may not stop the reaction. The energy is already stored chemically inside the cell.

Protection also depends on:

Cell chemistry
Manufacturing quality
Cell spacing
Thermal barriers
Vent direction
Gas-management paths
Pack pressure relief
Fault detection
Crash protection
Prevention of propagation to neighbouring cells

Thermal management helps prevent cells from reaching abusive conditions during normal use, but it is not a substitute for dedicated thermal-propagation protection. NREL treats ordinary pack thermal control and thermal-abuse containment as related but separate engineering problems. (NREL)

Thermal-System Trade-Offs

A more capable thermal system is not free.

It adds:

Weight
Cost
Coolant
Pumps
Valves
Heat exchangers
Refrigerant components
Sensors
Software complexity
Electrical consumption
Service and leak points

A large chiller and compressor can support repeated high-power charging but occupy more space and consume more energy.

A simpler system may be entirely appropriate for a low-cost urban EV that rarely fast charges. The same system could be inadequate in a long-range vehicle intended for repeated charging, towing, or sustained high-speed use.

Thermal-management technologies therefore involve trade-offs between performance, cost, efficiency, packaging, and durability. DOE technology roadmaps likewise identify added weight, cost, and complexity as central thermal-management constraints. (US Department of Energy)

What EV Buyers Should Compare

Manufacturers rarely publish enough technical detail to compare cooling-system hardware directly. Real-world behaviour provides better evidence.

Useful questions include:

Does the car automatically precondition when navigating to a charger?
Can preconditioning also be started manually?
Does a scheduled departure warm the battery or only the cabin?
How long does the battery need to prepare in winter?
At what SOC does preconditioning stop?
Can the car maintain charging speed over repeated stops?
How does charging perform in hot weather?
Is regenerative braking heavily restricted when cold or full?
Does the car retain drive power during towing or sustained high speed?
Can battery temperature be shown to the driver?
Does the vehicle continue cooling after charging?
Is a heat pump standard, optional, or unavailable?

Peak charging power alone says little about thermal performance. The more useful measure is whether the EV can arrive at a charger with the battery prepared, maintain a strong charging curve, and repeat that performance throughout the journey.

For related battery behaviour, see Battery Charging and Charging Performance, Battery Degradation, and EV Battery Safety and Failure Management.