I have several battery build projects that I have been wanting to build for a long time. Previous batteries that I've built used 18650 cells sourced from old laptops. I was able to salvage these cells for free, but they were very old. As a result, the batteries I built out of them didn't have optimal capacity and longevity. In my new battery packs, I faced a dilemna: purchase new cells and pay a premium, or use scrap cells and deal with subpar performance. After some research however, I found a solution that satisfies both: modem batteries. Modem batteries are a fantastic source for cheap, barely used 18650 cells suitable for high-capacity battery types.
Many modems contain a built-in UPS, or uninterruptible power supply, which can provide power to the unit in the event of a power outage. Depending on the operating conditions of the modems, its unlikely that these batteries were routinely discharged. Rather, they were probably kept at full charge most of the time, only being discharged in the relatively rare event of a blackout or brownout. This means that the cells inside have likely seen little use, preserving their original capacity. Furthermore, these packs can be obtained in bulk for very cheap from sources like eBay. I purchased 10 of them for about $35 with free shipping. As each pack is in a 2s2p configuration, there are 4 cells in each, totalling 40. That comes out to about 90 cents each 18650 cell, including shipping! Much cheaper than the cost of buying them new. There are even cheaper sources, such as Jag35. Unfortunately, they did not ship to my location, so I cannot attest to their quality.
The battery packs I purchased were the BP044S model, containing the ICR18650-22F Samsung cell. An excellent page detailing the electrical characteristics of this cell can be found here. These cells are rated for 2200 mAh and 300 cycles. It is a decent cell, but there are much better options available. However, given the price, it is a fantastic option for the budget-minded builder.
I opened these packs by snipping the seam of one corner with some side cutters. Then, using a flat head screwdriver, I carefully pried apart the two casings to reveal four Samsung 18650 cells. I then snipped the leads to the internal BMS and used the side cutters to cut the nickel strip connecting the cells. This process is DANGEROUS! Accidentally puncturing a cell could cause a fire, and you can easily slice your hand open on a sharp nickel strip. Only do this if you know what you're doing, because I am not liable for any damage you cause. If you do decide to do this, I'd recommend gloves. Thank you for reading this short article about my favorite method of obtaining cheap 18650 cells for battery projects. If you want to read more about batteries, stick around because I will be using this cells in future projects.