Hi guys, to start with I really enjoyed this project. I was looking for some sort of battery backup solution for one of my electronic projects. I had two used laptop battery's which were lying around the house. These battery's were from my HP and Thinkpad laptops. I have replaced the HP laptop battery with a new battery as it did not give any battery backup and the laptop was giving warning to replace this battery. The ThinkPad battery was working but the laptop gave backup of only 20min backup on this battery so I had replaced it with a new one. Both these batteries were laying ideal for more than a year and when I thought of starting a new electronic project these batteries came to my mind. Opening the battery was pretty simple affair I just followed the seam with a knife and they opened. Both the battery have 6 cells. These  battery have 3 groups in series with each group having 2 cells connected in parallel. These cells are 18650 Lithium-Ion cells have size of 18.6mm X 65.2mm and have nominal voltage of around 3.7 volts. On testing the HP battery with a multimeter I found 2 dead cells as they showed 0V this explains why the HP laptop was giving zero battery backup.
The grey battery cells came from the HP laptop battery. Serial on these battery cell is LGABB41865 so these battery cells are manufactured by LG. These grey cell have capacity of 2600mAh per cell. The red battery cells are from my Thinkpad R61 laptop there is nothing written on these cells, From whatever information I could gather on these red battery cells I think the manufacturer is Sanyo and have capacity of 2400mAh. The manufacturing date on these battery pack make both of these batters more than 5 year old. These cells are no longer suitable for the high power demands of a Laptop but they still have plenty of life left if used for tasks with less power requirement such as powering few LED or powering my Arduino projects. As a battery ages the internal resistance of the battery increase which makes the battery less efficient for high current draw application but it can still work perfectly where the current draw is minimal. I have initially charged these battery using a DC-DC step down converter with a current limiting functionality as I did not have a 18650 charger. I have charged each cell with 4.2V and limiting the current to around 400mA. It took lot of time to charge all these cells I had to monitor these cells between charging to make sure I do not over charge them. After charging I tested these batters by recording the voltage across the terminals and then checking them after 3 days to see how much voltage had dropped. I also did a simple battery life test by connecting a 1W LED and checking the battery backup time. I did the test for about 2 hours after this I checked the voltage drop across the cell the voltage had dropped to 3.8V from 4.18V which is pretty good.

A Lithium-ion cell should not be over-discharged beyond a point otherwise they might get damaged. That is the reason the laptop batteries have a protection circuit inside the battery housing which prevents them from over-discharging. Since I have taken out these cells they have no protection now and if I directly use them to power any project there is a chance they will over-discharge and get damaged.
To tackle this situation I am using TP4065 charging IC with DW01 protection IC. This small and inexpensive board provides two in one solution it has a charging circuit which is especially designed to charge 18650 cells and prevents overcharging and over-discharge sceneries. This circuit charges the cells to 4.2V and disconnects the cell from the circuit when they are discharged to 2.4V. This board also has a micro USB connector input so that you can charge the cells using USB mobile charger.
For my application I have grouped 3 cells in parallel to create a 3.7V battery with 7800mAh capacity.
I had 10 battery cells so I have created 3 of these battery packs. It is also important not to mix the cells of different types and different age together that's the reason I have not mixed red and grey cells together in a single battery. This charging circuit has 2 LED's to indicate the charging status. A red LED is lit during the charging phase and a green LED is lit when the battery is fully charged. This charging board is designed to charge a single 18650 cell since I am using it to charge 3 cells it take three times the time to charge them. I would recommend that you should recycle the laptop battery as it is environment friendly and a cost effective solution. You can also buy empty power bank casing which are available on ebay and Aliexpress these are quite economic and take the 18650 cells for making your own mobile power bank. I will put these batteries to use shortly stay tuned.
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