This idea came to me when I built another project
For my needs, I had to have an Android alaways on but I didn't want to let it pluged on a wall with the battery cell inside. A battery that is 24/7 on charge can be damaged and start a fire ! So I wanted to bring power from a wall plug, directly in the phone.
Be careful ! Here we will play with electricity and battery cell. Those elements can be dangerous if you don't know what you're doing. I can't be responsible of any damage caused on your equipmentor on yourself. So please, read every documentation you can found BEFORE starting anything !
Step 1: What Numbers Said
For this project, I use an "old" Samsung Galaxy S3. Numbers I'll give will just be correct with this model. Please check on your device the values you are looking for.
When I removed the battery cell of my device, I found a little label that said the device should be powerd at 3.8V. I found the same number wrote on the battery cell. But when I looked on the power plug, I saw it dispense 5.0V. We will need to reduce this voltage in order to not damage the device.
We can see on the device four little pins. Two of them are the +/- poles to power the device. To know exacly which one does what, you can look on the battery cell. It should be + and - symbols in front of the right ones.
Step 2: Gathering Parts for Assembly
First of all, we will need cable between the power plug and the device. Took an old USB -> micro-USB and remove the micro-USB head.
If you remove the plastic protection you will see multiple wire. We only need two of them : the red one and the black one.
Here comes the hardest part. In order to make a good contact between our wires and the pins on device we will use a part of the battery cell itself.
With thousand precautions, you can start to remove the plastic sheet protection around the battery cell. You will expose the part we need : the one with four contacts. On my model, this part was protected with a little piece of plastic that I removed with a small pliers. With a small blade, you can detach this little part. See pictures to spot it.
Step 3: Welding Parts
Using all those parts, if I measure output voltage I found 4.83V (don't mind the Arduino, it's not even plugged). It's less than the 5.0 wrote on the battery cell but it is still a bit high and it can damaged the device.
You will have to weld the red and black wires on the little battery cell part. The cell itself can be use as reminder to know where plugged which wire.
After all of that, the output voltage is still a bit high but it will do the thing.
Step 4: Power the Device
Ok, now you just have to put your assembly into your device. For my part, I used a small piece of cardboard to hold our it in front of the four pins.
Hold your breath and... Yay ! The device starts \o/ !
Thanks to the Ampere app, you can control the device input voltage. On mine, it displays 4.2V. It's a bit high but I don't think it will damage my device.