Recently, a mobile phone service technician wanted me to build a battery emulator that would enable him to test and run a Li-ion battery-powered mobile phone without using its battery pack. Because he just wanted a simple solution that eliminates the “Nokia BL-5CT” battery, I have designed a “quick and dirty” mobile phone battery emulator with most of the components slipped from my scrap box.
The Three-Legged Battery
A normal battery has only two terminals brought out of its casing, but this battery has three terminals! According to an official Nokia service manual, the third terminal of the battery is the Battery Size Indicator (BSI) point. The battery holds a pull-down resistor (connected to either the ground or –ve terminal) inside the battery casing. My trusty digital LCR meter showed me that it’s an 82-kΩ resistor for the Nokia BL-5CT-type Li-ion battery (3.7 V/1,090 mAh). Thus, I added an 82-kΩ 1% resistor between the second and third terminals of connector J2 in my battery emulator circuit to dupe the mobile phone.
My Battery Emulator Circuit
The typical target voltage of the Li-ion battery is 4.2 V (note that the target voltage is not the same as the nominal voltage, as it is 3.7 V typical). An adjustable voltage regulator is designed as the mobile phone battery emulator. The circuit based on the popular three-pin adjustable voltage regulator LM317T (IC1) receives external dc input of 9 V/2 A through input connector J1. The output voltage available at connector J2 can be adjusted from just below 3 V to just above 4 V with the help of the 500-Ω multi-turn preset pot (RP1). Although most devices powered by Nokia batteries appear to work well at about 4.2 V, a little bit of experimentation with the values of resistors R2-R3-RP1 may be necessary. As stated earlier, the 82-kΩ resistor (R4) is essential to fooling the device into thinking that a real battery is attached at its battery connector.
Note that the LM317T regulator can safely be used to dissipate up to 0.25 W without any external heatsinking. However, in this application, a heatsink would definitely be required, and the TO-220 heatsink should be chosen because its thermal resistance will keep the temperature of the attached regulator well below its maximum tolerable temperature. Finally, try to link connector J2 with a suitable Nokia battery adapter to make the interconnection an easy task.
Lab Note: In the case of an unknown Nokia battery, try to measure the resistance across its negative (–) and BSI (0) points using a precision DMM and substitute R4 with the needed value resistor. Some other resistor values used in Nokia batteries are shown below: