LiPo Charging: IC Thermal Foldback and Battery Temperature Limits

Which temperature does the charger regulate?

A linear charger dissipates power as it drops input voltage to the battery voltage. As a first approximation, charger dissipation rises with charge current and that voltage difference. The package, PCB copper and enclosure determine how much the die warms. Some ICs reduce current as junction temperature approaches an internal threshold. Others expose different controls or status indications. Read the exact part and revision before assigning a response.

The Texas Instruments BQ24064 is a single-cell charger example. TI lists IC thermal regulation and a separate battery temperature thermistor function. Its junction loop reduces current under high die temperature, while its battery temperature input separately qualifies whether charging is permitted. The Analog Devices LTC4054 datasheet is another example of junction-temperature feedback. Neither source specifies the charger inside a Jetray product or qualifies a multi-cell pack.

Test matrix for separating charge IC and battery temperature responses

What should the pouch temperature sensor measure?

Choose a repeatable sensor position with the cell or pack supplier and document the contact material, attachment method, cable routing and measurement uncertainty. Nearby displays or power electronics may make the board and pouch follow different temperature curves. Validate the sensor’s response and lag in the assembled product at the declared charge rate and worst credible thermal conditions. Distinguish a board temperature used for system control from the battery temperature measurement used to qualify charging.

Specify the permitted charge window, who enforces it, the action at each boundary and the recovery rule. If the design uses a thermistor divider, verify the actual sensor curve and tolerances together with the charger input thresholds. If the pack or host sends a charge-inhibit signal, record which device controls the charger and what happens when the signal is missing. Limits and fault behavior must come from the selected cell, pack and charger documentation.

How should the lab diagnose reduced current?

Capture input voltage, battery voltage, current, charger state and independent board and pack temperature traces on one time base. A lower current alone cannot identify junction regulation, a hot battery, source-current limiting or the normal constant-voltage tail. Reproduce one condition at a time within the approved test plan. Use a suitable electrical fixture for fault simulation and avoid inducing battery faults by heating or modifying a live pouch.

Record the enclosure revision, firmware, sensor position, input source, initial state of charge and ambient temperature so another reviewer can repeat the run. Reopen verification after moving the sensor or changing the charger, PCB, enclosure, battery configuration or nearby heat source.

What belongs in the engineering handoff?

Send the selected charger and pack references, a marked sensor drawing, charge current and voltage conditions, temperature-source definitions, and representative logs showing both thermal paths. Use Jetray catalogue entries as starting points for dimensions and configuration. Request controlled specifications and verify fit for the exact product before release.