In this Instructable, I’ll show you how you can make a power bank using old mobile phone battery cells.
At the heart of this power bank, are small 3.7V lithium cells that are salvaged out of old Samsung mobile phones. These cells can hold up to 1000 mAh per cell making this a 10 000 mAh power bank as I have 10 of these. The entire pack will be based on wiring all of them in parallel with a single charging board and a boost converter so we can output 5V to be used with any USB based electronics.
TP4056 Charger Module - http://s.click.aliexpress.com/e/bGVTc74k
5V Step Up Module - http://s.click.aliexpress.com/e/bfOizVVO
Alternative charger board - http://s.click.aliexpress.com/e/bdOqOoiG
Soldering Iron - http://s.click.aliexpress.com/e/c4GG9T9I
Solder Wire - http://s.click.aliexpress.com/e/cfqbEV4G
Hot Glue Gun - http://s.click.aliexpress.com/e/cXGtqKRI
Hot Glue Sticks - http://s.click.aliexpress.com/e/Oko4JSC
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Step 1: Prepare the Cells
Before we start connecting the batteries it is crucial that we test the battery voltages of each cell. In my case, there were some cells that were completely empty just from sitting for too long and this can be a problem once we connect them up. Current will start flowing from the full cell into the empty one, in a sort of uncontrolled way until they are both balanced out.
To prevent this, it is best if we get all of the cells to closely the same voltage using a bench power supply. As these batteries do not have holders, I’ve quickly stuck two exposed wires to my bench and used electrical tape to hold them close enough for the contacts to touch them. The cell is then placed pressed to the contacts and a small weight can be added to the cell to hold it in place. This way we can charge the battery up to its nominal voltage without holding it in hands for too long.
Step 2: Connect the Batteries
For the electrical connection of the batteries, I will directly solder the wire to its terminals and for that, I need to strip part of the plastic that sticks out past the terminals. This is easily done with a utility knife but make sure that you are not connecting the terminals and forming a short circuit as this might damage the cells.
Once the terminals are exposed, we can start applying a small amount of solder on the two terminals of the cells and with that, we can later more easily connect the wire that will link them up. To prevent any long exposure to heat, make sure that you only heat the pads for just a few seconds at a time, as otherwise, you risk damaging the cells. To start connecting the cells, I first oriented them so that the same pole connectors are on the same side and using a bare copper wire, I gently pressed the wire on the soldered pads of the cells. By connecting all of the positive pads together with one wire and the negative ones with another, we essentially connected all of the battery cells in parallel where the voltage of the pack will still be the same as one cell but it’s capacity increases. The soldering needs to be repeated for all of the cells while making sure not to touch the two wires together to avoid creating a short circuit.
Step 3: Connect the Charger Board
For charging the pack, I will be using this Lithium battery charging module that is typically used with 18650 cells. The board has a mini USB port and two LEDs, one to indicate charging and another one to indicate that charging is complete. Unfortunately, I somehow thought that this also has a step-up circuit on it so it provides 5v output but I was wrong. I’ve added a USB port that I had salvaged and stuck it to the end of the pack with hot glue.
I’ve connected the battery terminals to the appropriate pads on the charger board and connected the USB port directly to the output. It is then that I realized that the charger board only outputs the battery voltage so I grabbed a step up convertor that I had from another portable charger and wired it to the output of the charger.
Step 4: Connect the Step-up Module
This module has a switch onboard that can turn the output on and off and it provides a regulated 5V output. I stick the board on the side with hot glue and rewired the USB connector to now use the output from it.
As a final test, I connected it to a charger and it draws 1000 mA as per the charger specifications. This is a relatively low charging current for such a pack but it is better for the battery this way as the cells will not get hot while charging.
Step 5: Further Improvements
In the end, with the lack of a proper enclosure, I’ve added electrical tape around the battery pack to isolate its connections and to provide strength on top of the hot glue that keeps everything together. If I had access to a 3d printer then for sure I would have designed an enclosure which would have been a much nicer alternative and would have resulted in a much nicer looking end product.
However, the pack works and will serve me to power many projects in the future. If you have any questions please leave them down in the comments, subscribe to my YouTube channel and until the next one, thanks for reading.