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Workflow of recycling 18650 battery cells

Recently I started involving in a project which requires me to explore the idea of building battery packs for battery powered sensor node (another story for another day). For cost reasons, I decided to explore the feasibility of reusing 18650 lithium ion cells from old laptop battery. Today, I’d like to briefly share the workflow I’m practicing to test the cells.

The first thing I do when ripping the 18650 cells out of the old laptop battery pack is to note down (sharpie on the cell itself) the sitting voltage it was recovered at. This can range from close to 0v and up to 4.2v. All the cells will be charged (as long as they are above 0.01v) to 4.2v and accordingly based on the recovered voltage:

  • <= 2v  : charge them at 50mA max until reaching 2v, then proceed to next charging stage
  • >2v, <3v  : charge them at 100mA max until reaching 3v
  • >=3v  : charge them normally, 500mA

Charging at 50mA and 100mA is done on my bench-top power supply unit. Charging to 4.2V is done using a generic 18650 charger. At this stage, all the cells should be around 4.1-4.2v. There are 3 “metrics” I utilise to categorise the cell – decide if the cell should be tossed, used in low drain or high drain device.

  • Available capacity – they could range anywhere from few hundred mAh to probably 3000+ mAh. The cells I recover are normally between 1000-2300 mAh.
  • Internal resistance – below 50mR can be used in higher drain devices (such as e-bikes, although I’m not working on one at the moment) and the rest are to be use in the sensor nodes or as custom power banks.
  • Self-discharge rate –

I use the Opus BT-C3100 charger to process the cells. Set the mode to “charge test”, the charger will charge-drain-charge the cells and after 12-24 hours I know the available capacity. Then, switch the mode to “quick test” and note down the internal resistance.

Finally, let the cell sit on the rack for at least 2-3 weeks. If the voltage drops below 3.95v, it’s good for the trash can or may also be used in less important sensor node – but definitely not a good idea to use it in battery pack as excessively self-discharging battery will throw the battery pack out of balance.

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