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Is the NTC thermistor used in the Pinephone battery a special type or does it correspond to typical characteristics of a 3kΩ@25°C type?
Battery as supplied with UBPorts edition (1.2) Pinephone (mid 2020) Serial # not given, Sensor seems located towards the side facing the display (inward)
Temperature reference Pt100 RTD probe calibrated traceable to national standards on AMR Almemo 2890-9 total uncertainty < 0.1K
Resistance measurement FLUKE 289 true RMS Multimeter
Setup: Battery NTC and Pt100 probe tied together side by side, sandwiched between thermal capacity (gel packs) heated to ~45 °C, all wrapped in insulation material of much higher thermal resistance than expected within the battery/sensor package. Apparatus slowly cooling down (~3K/h at the beginning) readings were taken at intervals. Later on cooled gel packs were used in order to expand the range of readings below room temperature
Findings seem to be largely in line with table http://www.mamacsys.com/pdf/Sensors/C_Sensor10.pdf as represented by orange dots in the plot.
While the match is not ideal, particularly for low temperatures it is found in very good agreement for the 30…45°C range. This suggests that the thermistor is quite likely of typical characteristics.
Further study in higher temperature would be beneficial but was not done on this healthy precious battery. If anyone could salvage a thermistor from a dead battery I will be happy to investigate further.
Measured data and LUT from pdf data sheet, including R/R0 ratio are available here shortly.