Study of degradation of Li-ion batteries as a result of “fast” charging
I have previously posted how I am building a global battery knowledge base. As part of the project, I conducted new research.
Task
Find out how “fast” charging affects the rate of degradation of Li-ion batteries in devices with high current consumption.
As part of the study, I conducted a number of experiments. The experiment consists of charging the battery with a current 4-5 times higher than the rated current. The nominal charge current for the batteries under study is from 2 to 5A, I will use a current of 20A.
The experiment will show how “fast” charging increases the rate of battery degradation.
Li-ion batteries Sanyo NSX and LG 18650HG2 in the 18650 form factor were chosen as test subjects. The choice of form factor is due to the fact that this is the one they are most often used in devices with high current consumption. Namely: cordless power tools, electronic cigarettes, powerful flashlights and some types of electric vehicles.
Process
The capacity of the battery under test is measured.
The battery is installed in a test bench. The stand is completely autonomous and connected to a computer. The stand consists of a load and a charger. Which charge and discharge the battery.
The battery is subjected to 100 charge-discharge cycles with a current of 20A. The charge-charge cycle is restarted automatically by a program on the computer.
After 100 charge-discharge cycles, the battery capacity is re-measured.
Thus, we get the capacity of the new battery and the capacity of the battery after 100 charge-discharge cycles. And we calculate the loss of capacity as a percentage.
Test mode:
– charge current 20A
– discharge current 20A
– DC charging method
– charge voltage 4.2V
– discharge voltage 2.5V
– 100 cycles
Selecting the charge current.
A charge current of 20A is the limit for most Li-ion batteries in the 18650 form factor. Therefore, this value was chosen. In addition, similar current values are used by “fast” charges for cordless tools.
Why exactly 100 cycles?
The capacity loss curve over the battery life is close to linear. Having data for the first 100 cycles, you can extrapolate the process with reasonable accuracy over the entire life of the battery.
Acceleration of degradation is noticeable in the first 15-20 cycles and at the stage when the battery life is coming to an end. Therefore, if in the first 100 cycles the battery lost 8% of its capacity, then in the next 100 cycles it will lose less than 8%. That is, it will no longer be “worse” than measured in the first 100 cycles.
Retreat
On various forums and Youtube channels, I repeatedly come across statements that charging batteries with maximum currents reduces their service life by several times. I also often see advice to avoid fast charging, especially for cordless power tools. It is argued that it is better to charge with “slow” charging than with “fast” charging. Supposedly this will extend the service life significantly. And “fast” charging will quickly kill the battery.
We received data from the stand, what next?
Now we need to compare how the batteries degraded when they were charged with a “standard” current with the degradation of the batteries after using “fast charging”.
I don’t see any point in making measurements with “standard” currents on my own, since the measurement results and graphs for currents of 4-5A are already in the manufacturer’s documentation.
Therefore, we take the graph of capacity loss from the documentation and compare it with the graph that was obtained from the stand.
Comparing “fast” and “standard” charging Sanyo NSX
Manufacturer's data for “standard” charging current 5A
Initial capacity – 2537 mAh.
Capacity after 100 charge-discharge cycles – 2243 mAh.
Capacity loss – 11.4%.
Data from the stand for “fast” charging with a current of 20A
Initial capacity 2461 mAh.
Capacity after 100 charge-discharge cycles 2251 mAh.
Capacity loss 8.5%.
Analyzing the results
From the manufacturer's data it follows that if you charge the battery with a current of 5A, then after 100 charge-discharge cycles the battery capacity will decrease by 11.4%.
The battery tested on the bench lost only 8.5%, which is even less than in the manufacturer’s tests. That is, “fast” charging did not lead to a decrease in battery life relative to standard operating modes.
Compare “fast” and “standard” charging of LG 18650HG2
Manufacturer data for “standard” charging current 4A
Battery capacity 3027 mAh.
Capacity after 100 charge-discharge cycles 2559 mAh.
Capacity loss 15.4%
Measurement results for “fast” charging current 20A
Battery capacity 2848 mAh.
Capacity after 100 charge-discharge cycles 2564 mAh.
Capacity loss 10%
More graphs and measurements for the LG 18650HG2 can be found in knowledge base.
Analyzing the results
For “standard” charging with a current of 4A, the manufacturer declares a loss of capacity of 15.4% after 100 charge-discharge cycles.
In the case of “fast” charging with a current of 20A, the loss of capacity was 10%, which is less than that stated by the manufacturer.
As in the previous experience, the use of “fast” charging did not lead to an increase in the rate of battery degradation.
Pivot table
The table compares the measurement results from the stand with data from the manufacturers' documentation.
conclusions
Using “fast” charging does not cause a multiple reduction in battery life. If the operating modes do not go beyond the permissible limits, then the rate of battery degradation does not exceed the rated ones.
Even in harsh operating conditions, the batteries deliver the promised 300 charge-discharge cycles.
Batteries “die”, quickly degrade and burn for reasons that are not related to “fast” charging. I will write in the next article what is the reason for the rapid “death” and fire of batteries. Some experiments have already been completed.
PS Looking for a new owner for project. Due to a change in the field of activity and a move, I am looking for someone to transfer the rights to research and measuring equipment.
