Charging System for Testing 18650 Cells

18650 Charging System

In order to provide the best quality 18650 cells for the buyers on my eBay page, I need some sort of way of verifying the capacity of the “presumably” new 18650 cells from the laptop battery packs that I recycle them from. I already had a hobby-grade IMAX LiPo charger and discharger available, but it would be very time-consuming to test all of the cells individually. Thus, I decided to build this charging system…

My charging/testing method consists of the the following steps:

  1. Bulk charging all available cells
  2. Measuring the voltage of each cell
  3. Leaving the cells overnight
  4. Measuring the voltage of each cell (again)
  5. Fully discharging the 6-9 cells from each battery pack
  6. Measuring the voltage of each cell (to make sure all of the cells were discharged)
  7. Charging all of the cells up again
  8. Measuring the voltage of each cell (to make sure all of the cells were charged)

Any discrepancy in the voltage of the cells that occurs overnight will identify the low-grade or broken cells. The capacity of each cell is calculated/verified by taking the discharge capacity of the cells from each laptop battery, and dividing it by the number of cells. Discharging is typically done at 5 amps, which translates anywhere from 0.5C to 0.25C for each cell. Any group of cells that does not meet or exceed its rated capacity at this discharge rate is rejected.

The charging system itself was easy to build. In order to keep the “operating costs” down, and be able to set a lower price for the cells, I decided to build the bulk-charger using an old pizza box. I reused a lot of the components from my previous projects, like the power connectors from my electric skateboards, and the cell tabs from my old solar panels projects. I have now tested this charging system with a batch of 26 cells, and it works like a charm!

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