Smartphone Without a Battery (2022)

Summary

Smartphone without a battery How to wire and run an old smartphone without a battery. Intro I have an old Samsung Galaxy S5 that I wanted to use to run my 3D printer. There is a great project called octo4a that runs OctoPrint on Android devices. octo4a Using an old smartphone for OctoPrint is a perfect fit - it has USB OTG support to connect to the printer, WiFi to access the controls and upload models, and a camera to monitor the print progress. The only, yet critical, issue is that the kernel for Galaxy S5 does not support charging while it's connected to a USB device. The battery is old and worn, and cannot last through several hours of printing. The approach I took was to build a "fake battery" circuit that emulates the battery, but is powered by 5V via USB. Process First, I removed the battery. Luckily, this smartphone is old enough to have user-serviceable battery compartment. Older batteries might have only two terminals, (+) and (-). Newer batteries might have three or four terminals. I measured voltage and resistance between all terminals to find out what they might be. These are the results: Terminals (+) and (-) are obvious. Generally, Li-ion battery will produce 3.4V when almost-empty and 4.2V when full. The battery also says "CHARGE VOLTAGE 4.4V", so that voltage level at the battery terminal would not cause any issues with the smartphone. The second terminal is the thermistor (T), used to get the approximate temperature of the battery. The resistance between (T) and (-) is around 2350ohm at room temperature. Being a safety feature, my smartphone will not start at all if the thermistor terminal is not connected. The fourth terminal is most likely used for NFC (which is a part of the battery). It's not going to be used. So what I need my "fake battery" circuit to do is: Provide 3.4-4.4V between (+) and (-) battery terminals on the smartphone Create ~2350ohm resistance between (T) and (-) Since I am still powering the smartphone from a 5V USB power supply...