Baby it's cold outside: Is your cell phone battery pack smart enough to know it?

PowerSource reader BatteryGuy commented that concerns about charging a cell-phone in subfreezing temps are unwarranted because cell-phones shut down their charging circuitry in sub-freezing temperatures. Definitely a good feature for cell phones to have – but is this always the case? How does the charging circuitry sense the temperature, and how can you tell if a phone has the capability?

So I talked to the battery charging experts at Maxim/Dallas Semiconductor. Maxim is one of the major suppliers of battery charging ICs and fuel gauges. They said, yes, the majority – but not all – of cell phones have a thermistor in the battery pack that tells the charger when the temperature is below freezing and prevents the battery pack from charging. They estimated that 80 - 90% of cell phones have this capability. A good way to tell is by looking at the contacts on the battery pack: Two contacts means no temperature sensing capability, a third contact is probably for the thermistor and indicates the charging indeed shuts down at sub-freezing temps. (A fourth contact is probably for a memory device.)