I've been dabbling with my own chargers and BMS (battery management systems) for Lithium Ion 18650 packs for a while now and decided that I needed to do more than just blindly build.
I see four main areas of expertise required to safely work with and build my own battery packs:
- battery management (BMS boards)
- chargers (really just power supplies for BMS boards)
- construction (3D printing safe battery holders, welding nickle conductor plates)
- testing second hand / recovered cells
This project covers the last item in the list: testing your recovered cells to find the best ones and to create a balanced set (similar capacity cells).
ConstructionThere is a popular and freely available 18650 battery capacity tester circuit, the ZB2L3. It comes with the two discharge resistors and some spacers.
Hooking up the circuit is about as trivial an electronic project as possible. If your 18650 battery holder came with leads, then you don't even need to use a soldering iron (although I prefer neatly soldering the resistors rather than just scrunching their leads up).
The 3D printed case is entirely optional. If you are only testing a few cells then probably not needed. However, if you want a decent tool to use over the long-term, highly recommended. I'd recommend a filament other than PLA based on how warm we expect the device to get. I used ABS because that's what I had on hand.
The orientation of the cell is important - reversing the polarity will most likely damage the device. Mark the positive side of the battery holder red.
Note: if using the 3D printed case, make sure you mount your two resistors so they are free in space and not touching the sides of the case. They get very hot.
Hook up a 5V USB power supply. Insert the first 18650 cell to test. Keep in mind that the cell needs to already be fully charged in order to discharge it to measure its capacity.
The minimum / cutoff voltage can be adjusted ([+] and [-] buttons) but the defaults work fine for me. Simply press OK to start the discharging.
The device will cycle through three numeric readouts:
- mAh : capacity in milliampere hours measured so far
- A : current cell current
- V : current cell voltage
Once the cutoff voltage is reached the discharging stops and the device will flash the capacity of the cell in mAh. Press OK to hold.
To start the next test, insert the next cell and press OK.
Comments