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Glossary · Energy industry & energy transition

Battery thermal management

Battery thermal management Battery storage safety cooling Degradation thermal runaway

Battery thermal management refers to the targeted measurement, limitation, distribution and control of heat in a battery system. It does not mean cooling alone. A complete thermal management system keeps the battery cells within a temperature range in which they operate with good performance, efficiency, low ageing and safety. It also keeps the temperature differences between cells small. This includes temperature sensors, air or liquid circuits, thermal interface materials, the housing design, software in the battery management system, limits for charging and discharging and, in many applications, active heating.

The term is central because batteries are electrochemical storage devices and their behaviour depends strongly on temperature. Losses occur during every charging and discharging process, and a large part of them becomes heat. The higher the current, the greater the internal resistance and the less favourable the state of charge or cell temperature, the more a cell heats up. Heat is therefore not a side issue but an operating variable. It influences performance, service life, safety and economic viability.

Temperature as an operating variable

Temperature is usually measured in degrees Celsius. Technically, however, what matters is not a single value but the distribution and the change of temperature. A cell has a surface temperature, a usually higher core temperature and a temperature difference to neighbouring cells. Lithium-ion cells conduct heat poorly, especially across their layers. A storage system can therefore look unremarkable on average and still have local hotspots. Such hotspots arise, for example, in poorly cooled groups of cells, at connections with high contact resistance, in densely packed modules or with uneven air distribution.

For many lithium-ion batteries, the most favourable operating range is around room temperature. Laboratory studies show that service life drops markedly in heat and in cold. The exact range depends on cell chemistry, cell design, ageing state, state of charge and application. Ultimately, the manufacturer's data sheets and warranty conditions are binding. High temperatures accelerate side reactions that attack the electrolyte, electrodes and interface layers. As a result, usable capacity falls and internal resistance rises. Low temperatures slow down the reactions and likewise increase resistance. Then the available power drops and charging becomes more difficult. If cold cells are charged too quickly, lithium plating can occur, that is, the deposition of metallic lithium on the anode. This damages the cell permanently and can increase the safety risk.

The heat comes mainly from ohmic losses, that is, from current flowing through resistances in the cell and in the electrical connections. Polarisation losses at the electrodes add to this. Ohmic heat rises with the square of the current: at double the current, about four times as much heat is produced. In addition there is a reversible, so-called entropic heat. It depends on the state of charge and, depending on the direction of the current, can heat or cool. In practice this means: thermal management must know the load profile. A storage system that rarely runs at high power places different demands than one with frequent fast charging and discharging.

Distinction from battery management, cooling and fire protection

Battery thermal management is often equated with the battery management system (BMS). That is imprecise. The BMS measures cell voltages, temperatures and currents, estimates state of charge and ageing state, i.e. the state of health, provides cell balancing and sets limit values. Thermal management is part of this overall system or closely coupled with it. However, it also includes mechanical, thermal and fluid-technology components that are not software.

Cooling, too, is only one part of it. Electric vehicles often have to preheat their battery in winter so that fast charging or strong regenerative braking is possible. Stationary storage systems need heating if containers are located in cold regions or must be brought back into a permissible charging range after a longer standstill. A system that can only cool, but has no strategy for cold starts, preheating or reduced charging power, does not have a complete temperature concept.

Thermal management must likewise be separated from fire protection. Thermal management is meant to prevent critical temperatures and limit differences between cells. Fire protection deals with detection, containment, extinguishing, venting, structural separation and response concepts in case a fault escalates. The two areas touch each other but do not replace each other. Strong cooling does not protect against all internal cell faults. And a good fire protection concept does not make weak thermal management acceptable if, in normal operation, it leads to ageing, failures or shutdowns.

Why batteries need to control heat actively

A battery installation is not a passive store like a tank. How much capacity is usable, how high the efficiency is and what power is available depends on the current state of the cells, and temperature is among the most important state variables. A cold storage system can contain energy and still be unable to absorb or deliver it at the desired power. A warm storage system appears powerful in the short term but ages faster if it is operated too warm over the long term.

This is particularly evident with battery storage systems in the power system. Large installations, often called BESS (Battery Energy Storage Systems), are used for balancing energy, power trading and arbitrage, load shifting, grid services or together with solar and wind farms. These applications stress the battery thermally in very different ways. Frequency regulation produces many short power changes. Arbitrage can mean charging and discharging at high power over several hours. Grid-supporting functions are rarely called upon but must then be reliably available. If thermal management does not match the duty profile, the available power drops precisely when the storage system is needed.

The difference between energy and power matters here. Capacity is stated in kilowatt-hours or megawatt-hours, power in kilowatts or megawatts. A storage system with a large amount of energy can be thermally limited if it is to absorb or deliver this energy quickly. A high C-rate, i.e. a high charging or discharging current relative to capacity, produces more heat and places higher demands on cell chemistry, cooling paths and control. An example: a 100 MWh storage system discharged in one hour (1C) has to remove considerably more heat than the same system with a four-hour discharge duration (0.25C). Decisive factors are therefore power, the duration of the power phases, the packing density of the cells and the course of the ambient temperature.

Cell chemistry, design and application

Not every battery needs the same thermal management. Lithium iron phosphate (LFP) cells are considered thermally more stable than cells with an NMC cathode (nickel manganese cobalt). Nevertheless, LFP batteries also cannot do without temperature control. They too age faster at unfavourable temperatures, lose charging ability in the cold and can become dangerous in the event of local defects. The higher stability changes the risk profile. The requirements for operation, monitoring and design remain.

The cell format also plays a role. Cylindrical cells, prismatic cells and pouch cells release heat differently. Large prismatic cells develop different temperature differences than small round cells. Pouch cells can be cooled well over their surface, but must be mechanically clamped and they expand. In modules and racks, further obstacles to heat removal arise: cell holders, busbars, insulation and fire protection layers and housing parts determine how the heat gets from the cell to the cooling medium.

Electric vehicles and stationary storage systems have different boundary conditions. In a vehicle, installation space, weight, fast-charging capability, crash safety and comfort count, and thermal management is usually coupled with the vehicle's heating and air conditioning. For stationary installations, land requirements, maintainability, fire protection distances, container climate control, noise, self-consumption and availability are the priorities. They usually have more space for cooling, but concentrate high heat loads in a confined space inside a container. They also stand in the same place for years, where dust, salt air, heat waves, frost or high humidity can stress the thermal concept.

Cooling concepts and their consequences

Air cooling is simple, robust and comparatively inexpensive. Fans and air ducts carry heat away from modules or racks. How well this works depends on the airflow guidance, the condition of the filters and the temperature difference to the surroundings. Per volume, air absorbs much less heat than liquids and conducts it less well. Air cooling therefore reaches its limits at high power density or when cell temperatures are to be very uniform.

Liquid cooling absorbs heat closer to the cells and removes it more evenly. It enables higher power densities and more precise temperature control. In return it needs additional components that can fail: pumps, lines, seals, heat exchangers and coolant monitoring, as well as more maintenance. A leak near electrical components is a different risk from a failed fan. The choice between air and liquid is therefore not a pure efficiency question. It is about costs, maintenance, behaviour in the event of failures, safety evidence and the operating profiles that the storage system is to fulfil over the long term.

Some systems use refrigerant circuits, heat pump functions or cold plates with direct flow through them. Passive elements also count: thermal pads, phase change materials, insulation, module spacing and housing materials help determine whether heat spreads evenly or accumulates at individual points. Passive measures dampen temperature peaks. However, they do not replace control if the system has to run at high power for a long time.

The power consumption of thermal management is sometimes underestimated in profitability calculations. Fans, pumps, refrigeration units and heaters need auxiliary energy. This lowers the net efficiency of the installation, i.e. the round-trip efficiency at system level. A storage system that absorbs a lot of photovoltaic power during the day in summer has to remove the resulting heat precisely when it is hot outside. The costs then arise not only once for the cooling technology, but also continuously as auxiliary energy and possibly as a power limitation.

Ageing, warranty and economic operation

Thermal management is an economic factor because degradation determines the value of a storage system. This means the ageing of the battery: usable capacity falls and internal resistance rises. With lower capacity, the storage system can shift less energy in time. With higher internal resistance, more losses and more heat arise at the same power. A vicious circle can result: ageing produces more heat, and more heat accelerates ageing if the operating strategy is not adapted. The uniformity of temperature is also important. In a modelling study, a temperature gradient of only 3 °C within a cell accelerated ageing by 300 percent in the case examined. This is a simulated individual case, but it shows how sensitively ageing can react to uneven temperatures.

Warranties for battery storage systems therefore often contain conditions on temperature, state of charge, number of cycles, C-rate, operating windows and ambient conditions. An operator thus does not simply buy megawatt-hours, but the right to use the storage system within defined limits. Whether these limits are observed also depends on thermal management. Anyone seeking high revenues through intensive use must factor in the additional costs from heat and ageing. Otherwise a business model looks better on paper than in real operation.

In large projects, it is also a question of how the risk is distributed between cell manufacturer, integrator, operator, insurer and maintenance service provider. If an installation ages in a thermally conspicuous way, it must be clarified whether cell quality, container design, control strategy, operating profile or location is the cause. Without reliable temperature data, this can hardly be answered. Sensors, logging and access to operating data decide whether problems can be detected early and allocated cleanly under the contract.

Safety and thermal runaway

Thermal runaway is a self-reinforcing process in a battery cell. If a cell is heated strongly enough by an internal short circuit, mechanical damage, overcharging, external heating or other faults, reactions begin that themselves release heat. If this heat is generated faster than it can be removed, the temperature keeps rising. Gases can escape, pressure can build up and the gases can ignite. Neighbouring cells are heated and can likewise go into runaway. This is called thermal propagation.

Thermal management reduces the probability of critical temperatures in normal operation and can make the spread of a thermal event more difficult. However, it does not prevent every cell fault, because an internal defect can also occur in a well-cooled system. Battery safety therefore needs several layers: cell quality, electrical protection functions, temperature monitoring, mechanical separation, fire protection, gas routing, shutdown concepts and response planning. Cooling is one of these layers, not the entire safety architecture.

A common misunderstanding is to treat thermal runaway like ordinary overheating. An overloaded storage system heats up and can be throttled by control. Thermal runaway, by contrast, is a fault case with its own dynamics. Once the reactions are running in a cell, normal cooling is often no longer enough to stop them in that cell. The safety question is therefore not only how a system removes heat in normal operation. It is also how it detects faults, limits them and prevents them from spreading to further modules or racks.

Legal framework in the EU

In the EU, batteries are governed by the Battery Regulation (EU) 2023/1542 of 12 July 2023. It applies directly and does not have to be transposed into national law. Article 12 requires that stationary battery energy storage systems placed on the market or put into service be safe under normal operation and intended use. Since 18 August 2024, the technical documentation must demonstrate that these systems have been tested, in line with the state of the art, against the safety parameters in Annex V. In addition, they must assess hazards that Annex V does not cover.

Annex V lists eleven safety parameters. These include tests for external short circuit, overcharging, heat propagation in the event of thermal runaway of a cell, and a fire test. The parameters are to be applied only if a corresponding hazard exists under the conditions of use intended by the manufacturer. The regulation does not prescribe a particular cooling concept. It requires safety evidence for the product, and thermal management is an essential part of it. For construction, permitting and fire protection at the site, national and regional rules as well as technical standards apply in addition, which differ depending on the country and the size of the installation.

Typical misinterpretations

  • "If the battery is within the permissible temperature range, everything is fine." This statement overlooks temperature differences. Warmer cells age faster, colder cells are limited more strongly during charging. In groups of cells connected in series, the least favourable cell often determines how the entire system may be operated. Differences between the cells thus become a problem for performance and service life.

  • "Cooling is only important in hot countries." Cold can severely restrict charging ability. In electric vehicles, this shows up as lower fast-charging power and weaker regenerative braking. Stationary storage systems may have to be heated before certain periods of use or can participate in the market only to a limited extent in the cold.

  • "A lot of cooling capacity means good design." A powerful refrigeration unit can mask planning errors, but costs more in acquisition, auxiliary energy and maintenance. A better design absorbs heat where it arises, guides flows evenly, places sensors at meaningful locations and couples operation to the real state of the cells.

  • "The location determines the thermal load." Two identical storage systems at the same site can be stressed very differently if they are operated differently. The outside temperature sets the framework, but the internal heat load arises from the operating profile.

Significance for the power system

With more renewable generation, the need for short-term flexibility rises. Battery storage systems can absorb feed-in peaks, reduce load peaks, supply balancing energy and ease grid bottlenecks. However, this works only if they are available and actually deliver the promised power. If a storage system has to throttle in heat, cannot charge quickly enough in the cold or loses power earlier through thermal ageing, it contributes less to security of supply and to flexibility markets.

Storage systems are increasingly planned as part of larger portfolios: aggregators bundle many installations, project developers combine storage with solar and wind farms, companies use it to cut load peaks. In all cases, the promised power is reliable only if the thermal limits are known and feed into forecasts. A battery can be technically very flexible, but how much flexibility it may actually offer is thermally limited. Frequent changes of direction, large power jumps and short pauses make heat removal and temperature equalisation more difficult. Anyone assessing storage flexibility should therefore look not only at installed megawatts and megawatt-hours, but also at continuous power, temperature windows, cooling reserve and ageing costs.

Planning, operation and responsibilities

Thermal management must be incorporated into planning early. It affects site selection, container spacing, air intake, shading, noise protection, maintenance access, fire protection and the electrical design. If fans blow warm exhaust air into the intake zone of the neighbouring container, the installations heat each other up. If a container stands in full sun without good airflow guidance, the auxiliary energy demand rises. Filters can become clogged by dust or pollen and reduce the air throughput. This sounds like construction topics, but it directly affects cell temperatures and revenues.

In operation, thermal management is a control task. The system decides when to cool or heat, when to limit power and which cell temperatures are decisive for this. The control can be cautious and keep ageing low, or aim more strongly at short-term revenues. A storage system that often runs at its thermal limits can turn over more, but ages faster. This trade-off is economically and contractually relevant.

Many actors are involved in thermal management. Manufacturers set the cell limits. Integrators design modules, racks and containers. Operators determine the duty profiles. Maintenance service providers check sensors, filters, coolant and control. Insurers and permitting authorities assess fire and failure risks. Fire brigades need information on shutdown, access, gas release and possible event sequences. If these responsibilities do not mesh, gaps arise. A system can be delivered in compliance with standards and still age unfavourably under a particular operating profile or raise questions in the permitting procedure about the limitation of major incidents.

What the term makes visible

Battery thermal management shows that the performance of a battery storage system does not follow from cell chemistry, capacity and inverter power alone. Between the cell and the usable contribution in the power system lie heat paths, control algorithms, protection functions and operating limits. They decide whether a storage system permanently reaches its nominal values, how strongly it ages, how much auxiliary energy it needs and how robustly it responds to faults.

The term alone, however, does not say whether a particular system is good or bad. For that, cell chemistry, module design, cooling concept, sensors, operating profile, location, maintenance and safety evidence must be considered together. Liquid cooling is not superior per se, an LFP installation is not automatically risk-free, and a high rated power says little about continuous thermal capability. Battery thermal management is the interface between electrochemistry, plant engineering, safety and economic viability. It translates heat into concrete operating decisions: charge, discharge, limit, cool, heat, shut down or continue running.

Sources

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Edited by eCMS on 11.11.2026 15:14

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