I have been dreaming of building an electric car for years, but outright buying a new one isn’t really an option for me. 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.
Discharging an individual cell.
To test the capacity of each individual cell, I’m building an automated cell discharger, which calculates the capacity from the time it takes to discharge over a known resistor.
Proof of concept setup, using an Arduino Nano, 16×2 LCD screen, and 7 channels with 10 Ohm resistors and IRFZ44n mosfets.
The proof of concept setup discharges the cells with a fixed resistance, but variable current.
Planned upgrades for the next design include a constant current load, built in charger, and the option to charge to a pre-set limit for cell storage instead of ‘full ‘.
Additionally, perhaps a NTC resistor per cell to detect the ‘hot’ cells,
Additionally, a mechanism to prevent damage from reverse polarity.