18650 Lithium Battery Overview and Testing

Overview:

Lithium batteries have incredible performance and capacity compared to NiCd (nickle cadmium) or NiMh ( nickel metal hydride ) batteries. They don’t have the drastic self discharge or “memory effect” that Ni-Cd or NiMh do have. Which is why we are seeing more battery powered products being made with Lithium cells.

Another big difference between Li-Ion and NiCd / NiMh is the better
energy-to-weight ratios you get with Li-Ion cells. A NiCd or NiMh cell will be a nominal 1.2V where as a Li-Ion cell will be 3.6V nominal. So, you would need 6 NiCd or NiMh cells to build a 7.2V battery pack, where you could get the same voltage from only 2 Li-ion cells.

A common type of Li-ion cell is the 18650. This style of battery gets is name from its shape, which is 18mm in diameter and 65.0mm in length. I say that this is a common type because it is found in today’s laptop battery packs, cordless power tools and a many other products that previously may have come with NiCd or NiMh batteries.

By comparison, and 18650 cell is larger than a regular AA battery. As you can see in the picture below where the 18650 is on the left and the AA on the right.

18650 size compared to common AA battery

Test equipment needed:

New 18650 batteries can be purchased online (eBay, Amazon, etc.) or they can be harvested from used laptop battery packs. However you obtain your individual 18650 cells, you will want to safely test them and check their capacity. To do this you will need a way to charge and discharge your battery. What I have been using to charge and test my batteries is a SkyRC iMAX B6 Lipo NiMh Battery Digital Balance Charger. This charger can charge / discharge : NiCd, NiMH, Li-ion/Fe/Polymer and even Lead acid (ie: the small SLA types).

SkyRC iMAX B6 Lipo NiMh Battery Digital Balance Charger

This charger was designed for the RC (radio control) market, where model planes, cars, trucks need to be quickly charged in the field or on a workbench. It can be set to charge or discharge up to 6 Lithium cells in series or 15 NiCd/NiMh cells in series. This makes this charger ideally suited for bench testing Li-ion battery packs and individual cells.

Testing procedure:

To test an individual 18650 cell you will can use a single cell holder with leads and use the banana plug cable with alligator that was included with the charger. The photo below shows my setup.

Single cell test setp

Here is a closer look at the cell holder. In case you are wondering: I had to put a screw in the top of the holder so that it would provide a better connection.

Battery Holder

Here is the sequence I go through when I get a new set of batteries that I want to test. If you have a lot of batteries to test, you can gang charge them in parallel , but unfortunately you will have to do the discharge test to each cell individually in order to find the cell’s capacity. Here are the steps I take:

  • With a new or unknown cell , do a discharge first to get it down to a known state ( ie: down to 3V)
  • Fully charge the cell. Once it is fully charged let it “rest” for 24 hours. This is to ensure that the cell is retaining its voltage. Measure the cell voltage after 24 hours to see if it with still 4.2V +/- 0.1V.
  • Once satisfied that the cell retains voltage the discharge test can be done. Attached the cell back onto the ShyRC charger and this time do a “discharge” run. Once the cell discharges down to 3V the charger will stop and beep. Take note on the display of the mAH shown in the bottom right of the display. The photo below shows what I mean:
Cell capacity shown as 1142mA (1.2A)

Conclusion:

And that’s it! If you have more cells to test, just repeat the process. You will want to mark each cell and record its capacity. Keep a spreadsheet for all your cells, so that when you go to build a battery pack you can use cells that all have a similar capacity. This will be important to maintain good cell balance within your battery pack.

If you are interested in getting the same SkyRC iMax B6 charger that I use, its available on Amazon: https://www.amazon.ca/ORIGINAL-Charger-Balancer-Lithium-Battery/dp/B00HED90RU