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.
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