This winter, at the GOLEM association we received old laptops to be refurbished. Since they were not brand new, the battery power was practically unusable. In a couple of cases, the batteries were so worn out that they prevented the computers from turning on. As a result, we ended up with about ten batteries to dispose of.

My friend Joseph suggested a while ago to keep them because he had found a project on Thingiverse that showed how to reuse them to build homemade battery packs. Out of pure curiosity, I decided to dismantle a few batteries.

It’s not a simple operation; the packs are intentionally designed not to be easily opened by end users. Lithium cells are delicate objects and have a tendency to explode, so they are enclosed in suitable casings to protect them from impacts and various stresses. Once opened, a battery contains 6-8 cells and a control board. The cells are connected in series to form a string, reaching a total voltage of around ten volts. Multiple strings are connected in parallel to increase the battery capacity, which results in greater computer autonomy.

The cells of a battery pack gradually lose their capacity as they age. Moreover, when left unused for a long time, self-discharge becomes big issue: lithium cells are irreversibly damaged when discharged below a certain voltage, usually around 2V.

In some of the recovered batteries, I found a significant portion of cells with a “quite good” charge level, between 3 and 4.2V, thus recoverable. However, the battery that prevented the computer from turning on had a pair of cells in short circuit. Probably the computer was left unused for a long time, and the cells completely discharged, but some of them managed not to dry out completely.

using my CC-CV power supply I tried recharging all the cells, limiting the charging current to a few hundred mA and the steady voltage to around 3.7V. It’s easy to see if a cell is no longer usable: if the cell voltage jumps from a few volts to steady voltage quickly, there’s little to be done. Instead, the recoverable ones follow a specific charging cycle: a first phase with constant current (CC), during which the cell voltage increases. Once reached the steady voltage value, the cell continues to charge at constant voltage (CV) until fully charged.

A charging battery on a not-so-messy desk

At this point, what can be done with the good batteries? I’ve seen and bought three different circuits on eBay, which I could comment on in a future post if time allows. But the one I liked the most is the so-called “poverbank”: an elegant solution that allows you to create a classic USB power bank from a single “18650-shaped” lithium cell (link to eBay or Amazon). Maybe I’ll avoid recharging my new phone with it; I don’t want it to end up like ElectroBOOM’s devices, but I can power some portable gadgets with it, like a modern computer built on a breadboard.

The poverbank powers a sophisticated 8-bit electronic computer