Not long ago, a mobile phone service technician approached me with a request to create a battery emulator. This emulator would allow him to test and operate a Li-ion battery-powered mobile phone without the need for its battery pack. As he sought a straightforward solution to bypass the “Nokia BL-5CT” battery, I devised a makeshift mobile phone battery emulator using various components I had salvaged from my collection of spare parts.
The Three-Legged Battery
A standard battery typically features only two terminals exposed from its casing. However, this particular battery stands out with its three terminals. As indicated in an official Nokia service manual, the third terminal in the battery serves as the Battery Size Indicator (BSI) point. Within the battery casing, there is a pull-down resistor, which is connected to either the ground or negative (-ve) terminal. My reliable digital LCR meter confirmed that for the Nokia BL-5CT-type Li-ion battery (3.7 V/1,090 mAh), this resistor measures 82 kΩ with a tolerance of 1%. Consequently, I incorporated an 82 kΩ resistor with a 1% tolerance between the second and third terminals of connector J2 in my battery emulator circuit to mimic the mobile phone’s requirements.
My Battery Emulator Circuit
The standard target voltage for a Li-ion battery is 4.2 V (it’s important to distinguish this from the nominal voltage, which is typically 3.7 V). To emulate a mobile phone battery, we have designed an adjustable voltage regulator circuit. This circuit is built around the well-known three-pin adjustable voltage regulator, the LM317T (IC1). It receives an external DC input of 9 V/2 A through the input connector J1. The output voltage, available at connector J2, can be fine-tuned within the range of slightly below 3 V to just above 4 V by employing a 500-Ω multi-turn preset potentiometer (RP1).
While most devices powered by Nokia batteries tend to function well at around 4.2 V, some experimentation with the values of resistors R2, R3, and RP1 might be required for precise calibration. As mentioned earlier, it’s crucial to include an 82-kΩ resistor (R4) in the circuit to trick the device into believing that a genuine battery is connected at its battery connector.
Please take into consideration that the LM317T regulator can handle power dissipation of up to 0.25 W without needing external heatsinking. Nevertheless, for this specific application, a heatsink is strongly recommended. Opt for a TO-220 heatsink, as its thermal resistance will effectively maintain the regulator’s temperature well below its maximum allowable level.
Additionally, it’s advisable to establish a convenient interconnection by connecting connector J2 to a suitable Nokia battery adapter, simplifying the overall setup.
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:
- BLB-2 Li-ion 650 mAh: 68 kΩ
- BLC-2 Li-ion 900 mAh: 75 kΩ
- BL-4U Li-ion 1,110 mAh: 91 kΩ