The most important thing to remember is that heat is a li-ion cell’s worst enemy. It ages the cell faster and if the cell gets hot enough it can start a series of chemical reactions that can eventually cause the cell to vent with flames or even burst (“thermal runaway”).
Low temperatures can be a problem too as they significantly affect the performance of a cell. The colder it is the slower the chemical reactions are and this creates more voltage sag. A cell that runs at a lower voltage delivers less power and will reach the low voltage cutoff point faster.
Let’s look at the full range of temperatures for a cell and what happens at different points:
-40°C to -20°C / -40°F to -4°F
This is typically the lowest temperature you can store or operate a lithium-ion cell at. Some cells can be stored to -40°C but only used down to -20°C but others can be both used and stored down to -40°C. If you are operating in a very cold area then check the datasheet as some cells and li-ion chemistries are better than others for low temperature use.
This refers to the cell’s temperature, not the air temperature. If you use a cell in -40°C weather but the cell was in your pocket until it was used then that cell is not cold.
The performance will be terrible if the cell is actually cold. The chemical reactions are slower and you will get a lot more voltage sag. Use the cell at a reduced power level for a while until you are sure it has warmed up to near room temperature. Then you can use it normally.
Never charge a cell that is this cold! The lithium ions cannot move around properly and will bunch up and this forces them to start plating lithium metal onto internal parts of the cell. If this continues for long enough the cell can short circuit internally. This might just slowly discharge the cell down to zero volts or it could cause the cell to catch fire and burst. Since we can’t know what will happen to any particular cell it’s just best to never charge a cell when it’s below room temperature.
-20°C to 0°C / -4°F to 32°F
Performance will still be terrible but most cells can be stored and operated at these temperatures. It’s still too cold for charging any li-ion cell.
0°C / 32°F
Stay above this temperature for any li-ion cell that is being charged.
0°C to 25°C / 32°F to 77°F
As the temperature rises up through this range the performance of a cell will get better and better. You can charge at any temperature in this range.
If you are storing cells for a long period this is a good temperature range to stay within. Do not freeze them and keep them at room temperature or a little lower. I haven’t seen any good data on whether keeping them in a refrigerator can actually make a noticeable difference but if you completely seal the cells (to prevent condensation when you take them out) then it probably won’t hurt.
25°C / 77°F
This is the temperature used for setting many of the cell ratings and is where most cells will perform well with just the normal amount of aging occurring for each cycle you put the cells through.
It is a good temperature for storing, charging, and using cells.
25°C to 45°C / 77°F to 113°F
As the temperature rises up through this range the performance of many cells will actually become a little better. This is because the internal resistance goes down as the cell gets hotter, resulting in less voltage sag.
But the cell ages faster and faster as it gets hotter so there is a tradeoff for this slight performance improvement. This also includes when you are storing cells. Cells are best stored at room temperature or a little below.
45°C / 113°F
This is the highest temperature I recommend letting a cell regularly reach if decent cycle life is important to you. At above (roughly) this temperature the aging starts speeding up. Most people would probably call this “very warm” for a cell firmly grasped in their hand.
45°C to 60°C / 113°F to 140°F
For some cells there will continue to be tiny performance improvements as the cell gets hotter and hotter. But it is aging faster and faster and I do not recommend this for any cells you want to last for a while. Do not charge a cell that is this hot. Let it come to room temperature first.
60°C / 140°F
This is the hottest I recommend ever letting a cell reach. It is about the point where you can no longer hold a cell in your hand. Never use or charge a cell that is this hot!
Continued in Part 6B to be posted soon.
