Li Ion Battery Charging - Preliminary Test Results

Recently I blogged some guidelines for Li Ion battery use. I have ignored the "Store at 50% charge" folklore I read about everywhere because I store in the charger at a terminal voltage around 4v per cell. No research appears to exist on how harmful this is. But then I couldn't find research that supports storing at 50% compared with say 90%. 

Page 10 in this document has graphs which suggest charging to 4.0V represents a battery capacity of around 80% and a substantial increase in the number of charge discharge cycles available.

Still, I wondered if there was any degradation. To see if this could actually be tested I ran a trial on a few rubbish cells to see if anything happened. The trial is not as rigorous as an academic paper, but reflects my real world use of these batteries.

The method is:

• Each morning remove a battery and discharge to a fixed voltage. 
• Record the capacity
• Put battery back into charger in the same slot.
• No attempt to control temperature or other variables. 
• Some batteries do not get tested every day simply due to time constraints. These sit in the charger at the nominal terminal voltage of 4V.

I say 4.0V but I actually adjusted the charger circuit to be 3.95V for Battery A and 4.05V for the Battery B.

As I said - rubbish cells. Brand new 18650 from China but had been sitting in storage a while. Over the course of the trials I decided to reduced the discharge current since 450ma was more than the capacity of the cells being tested. I also learned there is relatively little energy between 3.1 and 3.0 volts so I adjusted the threshold to 3.1V after 18 days.

 

Battery B initially appeared defective. It seemed to terminate discharge early and I twice had to re-start the trial on that day. With the drop in discharge current the battery magically had reduced variation in each trial and each test ran to the terminal voltage.

Since I haven't seen evidence of degradation after 24 days I will in future check less often. In between trials the battery will remain in the CC/CV charger at the nominal 4.0V.