BATTERY ENGINEERING · OUTDOOR IoT CASE STUDY
A battery heating film can help an outdoor IoT device resume charging in cold weather by warming the cells before charging is enabled.
The complete solution also needs suitable cells, temperature sensing, a controlled heater circuit and a charging profile matched to the cell specification. In this Swedish tracker project, Yilai Power combined a thin PTC heating film with battery management system control to address winter charging limitations.
A Swedish customer reported that an outdoor IoT tracker struggled to start in winter and experienced a 30–40% reduction in runtime. The deployment environment could reach approximately −30°C, creating difficulties for both battery operation and recharging.
The initial response was to evaluate cells designed for better low-temperature performance. However, the proposed battery still could not support the requested 1C charging rate while cold. Improved discharge performance alone did not resolve the charging constraint, so the development team added controlled battery heating.
Test boundary: the laboratory warm-up result described below was obtained at −10°C. It should not be interpreted as verification of the same performance at −30°C.
Charging and discharging have different temperature limits. During cold charging, lithium can deposit on the anode rather than being stored as intended. High charging current can increase this risk. A pack that powers a device below freezing may still require charging to be blocked until its cells warm up.
Charge-current limits and temperature thresholds must follow the selected cell manufacturer's requirements. Reducing current is appropriate only within an approved charging profile; it does not automatically make charging below the permitted temperature acceptable. Texas Instruments explains this temperature-dependent approach in its Li-ion charging guidance for JEITA compliance.
For OEM projects, evaluate low-temperature battery options against separate discharge and charging requirements.
For this project, Yilai Power customized a heating film with a reported thickness of 0.15 mm and a power rating of 2 W. The thin construction allowed the heater to be integrated around the battery body within the available space.
PTC stands for positive temperature coefficient. As a PTC heating element warms through its regulating region, its resistance increases. At a fixed supply voltage, this reduces electrical power and limits further heating. This behavior supports thermal control, but the final temperature still depends on the heater, supply voltage, insulation and heat transfer into the battery.
PTC self-regulation is not a precise on/off thermostat. Heater-surface temperature also does not prove that every cell has reached a suitable charging temperature. Temperature sensors and a controllable heater power path remain part of the system design.
Thermal contact, electrical insulation and sensor placement should be checked in the assembled pack. Heating should not obstruct cell vents, damage cell insulation or create local hot spots.
The battery management system, or BMS, monitored temperature through an NTC thermistor and also checked cell voltage. The case design used a heating trigger below 0°C when the estimated state of charge was above 20%. At the configured temperature setpoint, the BMS switched off the heater supply.
The 20% threshold was selected to reserve energy for the equipment and reduce the risk of excessive depletion during heating. It is a project-specific setting. Cell-voltage protection is still needed because a state-of-charge estimate alone cannot protect against over-discharge.
For this type of system, the charging and heating controls should work together as follows:
1. Check the pack before heating. Verify temperature readings, cell voltages, available energy and the permitted operating range.
2. Preheat while cold charging remains blocked. Enable the heater only when the energy reserve and protection conditions permit.
3. Release charging after temperature qualification. Follow the approved current and voltage profile and continue monitoring temperature during charging.
4. Stop or restrict operation when a fault appears. Evaluate sensor faults, excessive heater temperature, insufficient warm-up and low cell voltage during design validation.
The need for an independent thermal cutoff or secondary protection should be assessed for the complete assembly. It should not be dismissed solely because the heater uses PTC material.
Yilai Power completed heater customization and sample assembly in approximately two weeks and delivered samples in Q4 2024. The customer then evaluated the battery in outdoor conditions. The figures below describe this project rather than guaranteed specifications for every heated battery pack.
| Evaluation item | Project observation |
|---|---|
| Heating-film specification | Reported thickness of 0.15 mm and power rating of 2 W. |
| Laboratory warm-up | Approximately 15 minutes to reach the project's target temperature during testing at −10°C. |
| Charging after warm-up | The evaluated pack supported a 1C charging rate after preheating under the reported test conditions. |
| 72-hour connectivity monitoring | The tracker maintained 4G connectivity apart from one brief reconnection, which the customer attributed to network coverage. |
| Extended field evaluation | Three months of field testing, with heating activated when required. |
Warm-up time depends on battery mass, starting temperature, insulation, wind exposure and heater contact. The connectivity result describes device operation; it does not establish a universal battery runtime or cycle-life rating.
Heating consumes part of the available energy, so the heater must be included in the device's power budget. As a simple estimate, a constant 2 W load operating for 15 minutes would use:
Heating energy = 2 W × 0.25 h = 0.5 Wh
This is an illustrative calculation, not a measured energy figure for the prototype. PTC power changes with temperature, and total system demand also includes control electronics, conversion losses and any repeat heating cycles.
If the battery supplies the heater, the energy reserve must cover both warm-up and continued device operation. If an external supply powers preheating, its available output and the power path to the cells must be evaluated separately.
The same approach may be evaluated for asset trackers, remote sensors and environmental monitoring equipment. Heater size and control thresholds need to be recalculated for each enclosure and operating profile.
A useful starting specification includes:
1. Battery voltage, capacity, cell model and approved charge/discharge limits.
2. Minimum ambient temperature and expected duration of cold exposure.
3. Enclosure dimensions, insulation and space available for the heater.
4. Device standby consumption and peak current during radio transmission.
5. Charging source, target charging time and acceptable warm-up delay.
6. Required energy reserve, temperature logging and fault-response behavior.
Yilai Power's custom lithium battery pack development can evaluate cell selection, BMS functions, heating integration and mechanical fit together.
Many conventional lithium-ion cells prohibit charging below 0°C. Some specialized cells have different limits. Follow the exact cell specification and confirm actual cell temperature before allowing charging.
No. A PTC heater limits its own power as resistance rises, but it does not monitor cell voltage, state of charge or charging permissions. The pack still needs suitable monitoring and protection.
No. The reported time relates to this prototype tested at −10°C. A different battery size, enclosure, starting temperature or heater layout can produce a different result.
The controller should avoid draining the pack through repeated heating attempts. Depending on the design, recovery may require an approved external heating supply or warming the equipment within its permitted conditions before charging.
Send your device load profile, operating temperature, battery space and charging requirements. Yilai Power can review whether low-temperature cells, controlled heating or a combination suits your application.
Request a Battery Engineering ReviewAbout the author: Engineer Sun, BMS Design Lead at Yilai Power, has more than 20 years of experience in the battery industry.
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