Designing and Building a DIY 18650 Battery Pack

Spring 2019, I found the YouTube Channel "K R A L Y N 3D" and his numerous projects involving custom 18650 battery packs.

While I was working as a student worker at Kaskaskia College IT, I had access to several recycled laptop batteries. Laptop batteries typically go bad once an individual cell in the pack goes bad, so one can recover quite a few quality cells from a recycled laptop battery pack. These cells can be used to make custom Lithium-Ion battery packs for numerous DIY Electrical projects.

Which Cells Can Be Used?

When selecting recycled cells to build a battery pack, several factors need to be considered:

Each cell needs to be able to hold a charge of 4.2 V and not drain significantly over time

Each cell may vary in capacity, which can depend on the cell's model and age

A healthy Li-ion may have an internal resistance of 20 mOhms - 80 mOhms, anything higher is a damaged cell

Above shows my battery charger/tester which I can use to check voltage, capacity, and cell resistance

Notice that the first cell on the left was tested at 121 mOhms. The cell was also measured at 69 mOhms last time I tested it

This cell must have been stored with a low voltage, which likely damaged it as shown in the higher resistance.

What Kind Of Battery Packs Can Be Built?

Larger battery packs are built from different configurations of single Lio-ion cells. Since a healthy cell holds 4.2 V, adding them in series (+ -> -) would only add their voltages, proven by Kirchhoff's Voltage Law. Adding cells in parallel (+ -> +) will add their capacities (Ah) together, proven by Kirchhoff's Current Law.

Battery packs are classified as _s_p with blanks representing the number of cells in series (s) and in parallel (p). The two diagrams below, made by the courtesy of app.diagrams.net, demonstrate the difference between a 6s2p pack and a 2s6p pack.

This 2s6p configuration will output a voltage of 4.2 * 2 = 8.4 V

Adding six 3000 mAh cells together, gives a total capacity of 18000 mAh

This 2s6p configuration will output a voltage of 4.2 * 2 = 8.4 V

Adding six 3000 mAh cells together, gives a total capacity of 18000 mAh