Poverbank

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.

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.