Recycling 18650 batteries: automated tester.

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I have been dreaming of building an electric vehicle for years, but outright buying a new one isn’t really an option for me at this moment. A good alternative is to build one, and the main hurdle is getting enough battery capacity for as little as possible money, so I took up the idea of recycling 18650 cells from used laptop batteries back in 2014.

Every laptop battery contains, on average, six cells. Typically, a “bad” battery has 1 or 2 dead cells, and 4 perfectly functional ones, so I disassemble the packs, cut free the cells, and charge them individually with TP4056 lithium charge boards. This quickly weeds out the bad cells that will not take a charge at all, or that just get really hot, and charges the good cells to 4.2V.

I previously made a proof-of-concept setup where I then discharged them in a tester that discharges the cells over a known resistance to figure out the capacity of the cell. This process takes quite a lot of manual interaction, so further automation is in order.

I have since designed the circuit board for a lengthwise-stackable 4 cell tester based around the atmega168/328 (in the form of the Arduino Pro Mini for ease of purchase) and TP4056 charging chips.
The board includes a 3 color led per cell to give a quick overview of what stage every cell is currently in, a polyfuse-diode combination to guard against wrong polarity, and they will be networked together via serial interface to display the results of any amount of testers on a small LCD screen. I have chosen to omit the NTC resistors in this version, detection of bad cells can also be done on basis of cell voltage while charging in most cases.

Front and back view of the PCB as currently being assembled. On the front is the outline for the battery holder the throughhole polyfuse and the RGB LED. On the back the outlines for the Arduino, power resistors and TP4056 chargers are immediately visible.

Current project status: Writing software, first parts arrived in the mail.

And of course I just now notice 1: a small mistake I made, 2: the Pro Mini’s I got are not either of the two versions I designed the footprint for. For a prototype it won’t matter though, although I’ll have to make a Rev 1.2 board to fix some of these problems. At least the battery holders fit perfectly.