Thermal management is not only about removing heat. In cold climates, cells must be warmed to a minimum temperature before charging can begin safely, and before the pack can deliver its full power capability. Heating is the other half of the thermal management system, and it is subject to the same physical constraint as cooling: heat has to move through the cell and the pack, and it never moves instantly or uniformly.
Why Cold Cells Need Heating
- Most lithium-ion cells should not be fast-charged below 5°C, and should not be charged at any rate below −10°C (NMC/NCA) or 0°C (LFP) — charging a cold cell risks lithium plating rather than intercalation.
- DCIR increases sharply at low temperature, severely limiting available power at exactly the moment a cold-started vehicle or system needs it.
- Vehicle range at −10°C is typically 20–40% lower than at 20°C, partly from reduced usable battery capacity and partly from increased cabin heating load.
→ Cold Temperature Charge/Discharge — the full electrochemical mechanism behind these limits
The Electrochemistry Behind the Limit
Below 0°C, electrolyte viscosity rises and Li⁺ conductivity falls — the difference in electrolyte conductivity between −20°C and 20°C can be as much as 4×. Combined with slower intercalation kinetics at the anode and a thicker, more resistive SEI layer, cold cells struggle to accept lithium during charge. If the charge rate exceeds what the anode can absorb, lithium plates on the surface instead of intercalating: capacity is lost, and the plated lithium can grow into dendrites capable of penetrating the separator and causing an internal short. This is a safety issue as much as a performance one — see Battery Safety for how cell failure mechanisms connect to thermal runaway.
Heating Methods
- PTC heating elements — positive temperature coefficient resistive heaters embedded in the module or applied to the cooling plate. Self-limiting, since resistance rises with temperature and naturally caps heat output as the cell warms. Simple and safe, but resistive heating is energy-inefficient.
- Heat pump — runs the vehicle’s refrigerant circuit in reverse, extracting heat from the environment and delivering it to the coolant circuit that heats the cells. A coefficient of performance (COP) of 2–4 means significantly less electrical energy is consumed than resistive heating for the same heat delivered.
- AC preconditioning — while still plugged into the grid, the pack is heated (or cooled) before departure or before a scheduled fast-charge session. This preserves the pack’s own energy for driving rather than consuming it on temperature management, which is a meaningful range benefit in cold weather.
- Self-heating (internal AC/pulse heating) — rather than heating from outside, current is switched rapidly through the cell itself to generate heat directly via internal resistance, without an external heater. Vehicle implementations of active self-heating report pack warming rates of up to ~4°C per minute at −30°C — Changan’s Deep Blue SL03 achieves this via high-frequency positive/negative current switching (a technique also covered by an earlier CATL patent), while BYD’s Blade Battery gets a comparable warming effect passively, through Joule heating in its long current-collector foils during charge/discharge (source). Academic work has also explored built-in internal metal-foil heating elements for the same effect. Because the heat is generated inside the cell rather than conducted in from a plate or element, self-heating can warm the cell core faster than external methods — relevant given the internal gradients discussed below.
- Managed discharge — some packs deliberately run a controlled discharge to generate I²R heat and gradually raise cell temperature, a simpler fallback where dedicated heating hardware isn’t available.
| Method | Heat source | Trade-offs |
|---|---|---|
| PTC element | External resistive heater | Simple, safe, self-limiting; energy-inefficient |
| Heat pump | External, via refrigerant circuit | Efficient (COP 2–4); adds system complexity |
| AC preconditioning | Grid power while plugged in | Saves pack energy; only works if time to precondition is available |
| Self-heating (AC/pulse) | Internal, via cell resistance | Fast core heating; requires additional power electronics and control |
| Managed discharge | Internal, via I²R heating | No extra hardware; consumes pack energy, slow |
Heating Introduces the Same Gradient Problem as Cooling
Any time heat is added to or removed from a cell, a gradient forms — this follows directly from Fourier’s Law of Conduction, and it applies to heating exactly as it does to cooling. Preconditioning does not raise a cell’s temperature uniformly: heat has to conduct in from wherever the source is applied, so the point closest to the heater warms first and the rest of the cell lags behind. Measurements on an instrumented NMC 21700 cell have shown a peak core-to-surface temperature difference of up to 9.7°C during just a 1C discharge — the equivalent gradient during aggressive heating can be comparable, meaning parts of a cell can cross the minimum charge temperature well before the rest of it does. The usual target is to keep the differential within a cell to around 2°C to protect capacity and lifetime.
The same effect shows up cell-to-cell across a pack. In a liquid-cooled or liquid-heated system, coolant exchanges heat with each cell in turn along the flow path — by the time it reaches the last cell, a coolant loop that started cold (for cooling) or hot (for heating) has already moved partway toward the cells’ temperature, so the first and last cells in the loop never see identical conditions. See Cell to Cell Temperature Gradient for how charge/discharge rate, cell heat generation, thermal conductivity, coolant temperature and flow rate, and cooling (or heating) circuit design all combine to set the size of this gradient, and how flow path geometry — such as pairing the coldest inlet with the warmest outlet — can reduce it at the cost of introducing a gradient across each individual cell instead.
The practical consequence for heating specifically: a pack cannot safely begin fast charging until its coldest point — which may be the core of one particular cell, or one particular cell in the loop — has crossed the minimum charge temperature threshold. An average pack temperature reading is not sufficient for this decision. Uneven heating also feeds back on itself in the same way uneven cooling does: a cell (or region of a cell) that warms first has lower internal resistance, so once current starts flowing it draws more current and heats further, widening the gap with cells or regions that are still cold. Preconditioning strategy and pack-level thermal design therefore directly set how long it takes, and how much energy it costs, before the pack is actually ready to charge or deliver power.
Control & Integration
The BMS decides when heating is needed and how much to apply, based on the cell temperature sensor array — see Cell Temperature Sensing for why sensor placement matters here: a sensor on the coolant loop or module housing measures coolant or air temperature, not cell temperature, and can significantly under- or over-estimate how close the coldest cell actually is to its charge threshold. Heating and cooling are both managed by the same Battery Thermal Management System (BTMS), which balances pre-conditioning time, energy consumption, and cell-to-cell uniformity as a single control problem rather than treating heating as a separate system bolted on afterwards.
Related Sections
- Thermal — the full thermal management hub, including cooling architectures
- Cold Temperature Charge/Discharge — the electrochemical mechanism behind cold-temperature limits
- Cell Temperature — cell-level temperature limits and behaviour
- Cell Temperature Gradient — why gradients form within a single cell
- Cell to Cell Temperature Gradient — why gradients form across a pack
- Cell Temperature Sensing — sensor placement and what the BMS can and can’t see
- Thermal Management System (BTMS) — the control system that manages heating and cooling together
- Battery Safety — how lithium plating from cold charging connects to cell failure modes