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I am curious about the way Android phones obtain the battery temperature. Android has an API called BatteryManager which provides the value of instant battery temperature, but is there an actual temperature sensor inside the battery? Or this value is purely calculated by a certain equation using the value the current, the voltage and the time? If it is calculated, is there anyway that we can know how hot the phone really is?

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Every mobile battery has an inbuilt thermistor, (which is a transducer which varies its resistance with respect to its temperature) with which the charging circuit constantly measures the battery temperature. It can even prevent charging the battery if the temperature reaches dangerous level and by controlling its temperature, the life of battery is increased.

Mobile battery has more than two terminals, one of which is for this thermistor. See the image in my question asked in this site.

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Thanks a lot, Narayanan! Your answer helped clear my confusion. Do you happen to know whether other components (such as CPU) in the phone have thermistors? Also, do you think the battery temperature can be a sign of showing the temperature of the phone (as a whole)? – spica Sep 15 '12 at 15:23
To my best knowledge no other components of Android phone have any temperature measuring device other than battery. Of course, in some high end Android phone, ambient temperature sensor is available to measure the room temperature (as the word ambient indicates) – Narayanan Sep 15 '12 at 16:09
@spica: Some Android devices do seem to have CPU temperature sensor. However, I guess due to SOC design, and due to the size of a phone being much smaller than a desktop, any heat spreads quickly and makes monitoring each components individually less useful. – Lie Ryan Sep 15 '12 at 18:01
Thanks @LieRyan! I understand that it's impossible for all the components to have a temperature sensor. I just hope there is a way to tell where the heat comes from. Sometimes we makes a long phone call and the phone gets pretty hot, but the battery is not that hot (although the battery temperature is also increasing). I guess in this case, the heat mainly comes from the radio module (because it's using a lot of power). – spica Sep 15 '12 at 20:04
Adding what @LieRyan had said, the rapid heat spread is an intentional aspect, achieved with help of pyrolytic grahphite sheet. – Narayanan Sep 16 '12 at 5:27

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