I've read on batteryuniversity.com that the constant voltage (saturation) stage of Li-ion charging adds approximately 10% of SOC compared to charging with only the constant current (CC) charging phase. For example when charging only with CC to 4.1 volt you get approximately 80% SOC, but with full adsorption approximately 90% state of charge can be achieved. Link: https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
I have not yet been able to find a good visual representation of the saturation stage of the charging and how it is different from the constant current stage of charging. What is a good visual representation of this?
There is clearly some current flowing into the battery while in the constant voltage (CV) charging stage, meaning electrons must be flowing. Wouldn't that mean the amount of free electrons trapped in the graphite layer would increase and thus the voltage would (slightly) have to increase? Or is my assumption that during the CV charging stage electrons are moving in the same way as they do during the constant current charging stage - albeit at a much slower speed - incorrect?
If my assumption is incorrect and the amount of electrons trapped in the graphite layer does not increase during CV charging, then what happens inside the battery that causes the SOC to increase during this charging phase?
Another question (regarding C-rates and saturation time): The same page on batteryuniversity.com mentioned that faster charging (higher C-rate) of Li-ion cells results in reaching a relatively high SOC (for example, 85% SOC) quicker than when charging with lower C-rate, but mentions that, when charging with this higher C-rate, the saturation stage of charging will take longer. What is the reason behind this? Does some of the 'saturation' already occur during the CC stage of charging? And because, when charging with a higher C-rate, there is less time spent in this CC stage of charging for a full charge, so will there be less time for the 'saturation' to happen? Does this (my) theory make any sense? What is the approximate difference in 'saturation time' when charging at different C-rates (for example, when charging at 0.5c vs. at 1c vs. at 2c)?
Even though saturation takes longer with higher C-rates, I would expect the total charge time to be lower since the average power going into the battery is higher (accounting for extra losses due to charging inefficiencies being higher with higher C-rates), unless the saturation stage is very significantly slower for higher C-rates, is this assumption (higher C-rate always equals faster total charge time) correct?
Last question (regarding image from batteryuniversity.com):
The text under the image at the top of my post reads "Adding full saturation at the set voltage boosts the capacity by about 10 percent but adds stress due to high voltage"
I don't understand why this would add extra stress when according to the charging graphs the voltage during the CV charge cycle is the same as the charger cutoff voltage. If for example charging to 4.1 volt (a voltage which is well within the specs for most Li-ion cells) and charging the cell to full saturation, would this prolonged charging at 4.1 volt (until the cell is fully saturated) be harmful to the cell?
Second example: Let's say we're charging to 4.2 volt, would this prolonged charging at 4.2 volt (to reach full saturation) be any more harmful to the cell than the CC charging up to this level (4.2 volt)? If the maximum voltage for CC and CV charging is the same, why would CV charging be more harmful to the cell? Because the average charging voltage is higher? Am I missing something?