Electrical – Identify which bus, mobile phone batteries use


Mobile phone battery bus are usually 3 pin or 4 pin which are usually 2 wire buses (I2C) or a similar type of serial interface for a 1 wire bus. Does anyone know of a method for checking which bus type it is.

I don't believe a thermistor is solely used for Li batteries, that idea came from fast charging of other types of battery.

Best Answer

Mobile phone batteries don't use any "bus". The I2C-like bus is a feature of "smart battery", a different class of multi-cell batteries used in older laptops. These smart batteries need the communication bus because they contain "fuel gauges" and other control electronics inside the battery, so the laptop host can maintain and report the battery health status.

The mobile single-cell batteries can have 3 or 4 terminals. Older small cells (for cameras, other small gadgets, old Nokia phones, Motrorola flip phones) use 3-terminal batteries, with typical middle connection going to a thermistor.

The thermistor is an important part of Li-Ion chargers, to prevent cell overheating in cases of strong cell failure, which can lead to rapid "venting" with possible fire. For example, the entire line of BQxxxx battery chargers from Texas Instruments have a special input which should be connected to a thermistor network. Without having proper resistance (voltage level) on that pin the IC will signal fault condition and won't charge the cell. If someone wants to substitute the battery in some device with AC-DC adapter, this pin has to be emulated with proper resistor to ground (negative) terminal. The value of 10k is typical, but other values were used as well.

In Samsung (and other) phones the battery cell has 4 terminals. In some cases (original phones) these two extra terminals go to a printed near-field antenna that is glued to the body of battery. In replacement batteries this antenna may be omitted, sparkling outcry about "fake batteries".