There are many important specifications for an 18650 battery, including:
- Capacity (mAh)
- Discharge Current (A)
- Charge Current (A)
- Maximum Cycle Life
In this Instructable, I want to talk about maximum cycle life, and how you can use this measure to prolong the life of your batteries.
Step 1: Step 1: Learn About Cycle Life
What is a cycle?
When an 18650 battery is charged and discharged, this is counted as one cycle.
- 18650 lithium-ion batteries are charged up to 4.2V and down to between 2V and 3V depending on the cell's specification for cut-off voltage.
- To be safe, never discharge lower than 3.0V unless you know your cell's specification.
Discharging occurs anytime you use the battery for power. The electricity is drawn from your device, and the battery discharges current. You can also use battery testing equipment to discharge a battery.
- To charge an 18650, you should use a special charger for lithium-ion cells.
How is cycle life calculated?
Cycle life is determined by the difference in capacity of your cell taken from its first-use rating to its present rating. For example, if your cell started at 3000mAh, but now only has 2900mAh. 2900 mAh is 96% of the original capacity.
When this percentage reaches 80%, we say the cycle life has ended (even though you may get a few thousand more cycles out of your battery).
So if we take a 3000mAh cell, when do we say its life has ended?
(80% of 3000 is 2400) so when the cell reaches a capacity of 2400 mAh we say its cycle life is over.
How many cycles does a typical 18650 battery have?
Most modern 18650 batteries have a typical cycle life of 300 - 500 (charge, discharge cycles). When in high-amp or high-drain situations, this can decrease substantially to 200 cycles. If you go over the maximum discharge current limit (A) you can decrease the cycle life all the way down to 50 cycles.
In optimal conditions, your cells may achieve more than 500 cycles.
Some chemistries are able to cycle thousands of times before reaching 80% of their capacity.
Step 2: Optimize Temperature
18650 cells are rated normally at between 20 and 25 degrees Celcius.
Any deviation from this will create a slight loss in efficiency. A 10-degree deviation might account for a loss of 20, or 30mAh.
While the extremes, below zero, or above 70 degrees, will see much faster cell degradation.
Using a cell at suboptimal temperatures will decrease its cycle life.
- Tip: Never charge your 18650 cell below freezing! Eg. in a garage in Winter as it will degrade particularly quick this way
While you are using your cell, if you notice it gets hot, let it rest. Under normaly use your cell should not get hot-to-touch, and should never exceed 60 degrees. If it is getting hot fast, you are over-stressing it.
Step 3: Don't Overcharge (above 4.2V), Consider Undercharging (under 4.0V)
If you are totally concerned about cycle life and not capacity, then you might not even do a full charge.
Instead, you might do a partial charge. This is when you charge your battery only up to say, 3.8V instead of 4.2V.
You will see decreased capacity, but less stress on your cell and an increased cycle life.
Overcharging will see increased capacity but it's dangerous and will reduce your cell's life.
Step 4: Lower Your Charge Current (amps)
Many chargers have the ability to lower charge current. A "fast charge" may be 1 amp or higher. While this charges your cells faster, it will reduce their cycle life.
Notice this graph, the big difference in cycle life that charging affects.
Step 5: Lower Your Discharge Current (amps)
Remember what a discharge is? If not, I recommend going back to Step 1.
When you discharge a cell you can choose its cut-off voltage like we already discussed. But we can also choose the amperage with which we are drawing current. The higher the amperage, the less capacity we will ultimately see.
Higher amperage discharges (called high-drain) will also reduce the cycle life of your cells.
When possible, use less amperage. Official tests by LG, Samsung, Panasonic, Sony, and so forth usually do their discharge tests at just 0.5 or 0.8 amps.
Step 6: Increase Your Cut-off Voltage
On the flip side of a partial charge is a partial discharge.
Rather than a full cycle, this is known as a partial cycle. It has the benefit of reducing stress on your battery and increasing cycle life.
So instead of discharging all the way down to 2.8V or however your battery is rated, you might only discharge to 3.2V.
Step 7: Know Thy Cell Chemistry
Different cell chemistries perform differently.
Many 18650 batteries can reach 1000+ cycles comfortable in the right conditions.
The lithium-ion 18650 cell chemistry with one of the highest cycle lives is the lithium iron phosphate (LiFePO4) battery.