I haven't fully read the following posts, but I Disagree with the OP on several points
- How Resting Voltage works, vs the way a cell is treated
- The overall concept of why true (CC/CV) vs emulated CC/CV is better
Lets start with some battery stuff.
Termination voltage. All chargers should be trying to aim for 4.20V finish, without Ever exceeding this voltage (we all agree here). A true CC/CV charger will have voltage measurement systems in place to monitor this voltage, and will always stay below this. Consequently, achieving a cell resting voltage of 4.20V is very difficult when you cannot exceed this. This is because the cell has an internal resistance (which as the old points out, applies in both charging and discharging states).
Charging - CC/CV technique.
The CC part is where a cell can be charged at its recommended charging current, without exceeding the max charge current. In this part of the cycle, the voltage rises. Until which time it reaches 4.20V per cell, then the charger will reduce current to the cell to maintain 4.20V. This is all governed by the internal resistance of the battery.
SO, if we applied 1.0A charging current to a cell at 3.00V, its internal resistance of 500mOhms will result in the voltage of the cell to Rise under charging load. Based on Ohms Law, this setup will result in a 0.5V increase, and the cell voltage rises to (3.00 + 0.50 = 3.50V). Once we remove the charge current the voltage will drop back down.
Lets say this cell now reaches a resting voltage of 3.70V. The same 1.0A charging current, will give the same 0.50V increase, so the voltage of the cell under the charging current will now reach (3.70 + 0.50 = 4.20V)
Beyond this, the resting voltage of the cell will continue to increase past 3.70V, but the actual voltage at the cell due to the charging current will continue to rise past 4.20V. So we must now Reduce the current to maintain a maximum cell voltage of 4.20V. At this point, we say the charger transitions from the CC mode to the CV mode.
If the cell now has a resting voltage of 4.00V, there is a certain amount of current that we can apply that will not exceed the charging voltage of 4.20V. A charge current of 0.40A will cause a voltage increase of 0.20V, and so at this instant, we can only be applying 0.40A charge. Our charger is slowing down so as to not exceed 4.20V, but accordingly will be give the cell less and less energy.
The key to this is --> as the charging current drops, the difference between cell resting voltage and cell charging voltage will get closer and closer, but will take longer and longer because were giving the cell less charging power.
SO... To achieve a resting voltage of 4.20V, if we extrapolated the graphs, we will see that as voltage difference tends to 0, charging current also tends to 0. What does this mean? - to get a resting voltage of 4.20V, we need our charger to charge for an infinite amount of time.
So again, lets put this down simply.
Because of the Internal Resistance of the battery, The ONLY way to achieve a resting voltage of 4.20V is to
a) While holding a charge voltage of 4.20V, apply a current that tends to 0mA, i.e. charge for an infinitely long time
b) Allow the charge voltage to go ABOVE 4.20V and terminate charge at a given current so that the cell resting voltage drops down to 4.20V
The number one rule of a True CC/CV charger is that it Never exceeds 4.20V, and because it usually terminates at a current above 0mA, (usually 1/10 charge rate), it CANNOT EVER give you a resting voltage of 4.20V.
So how does our budget a charger achieve a resting voltage of 4.20V??? Simple - It either charges for infinite amount of time, or it has allowed the cell voltage to exceed 4.20V
(remember, our cell protection circuits protect for over-voltage between 4.25 to 4.35V. It can stop huge over charging due to wrong chargers etc.. but it won't stop regular insidious overcharging with less accurately regulated chargers)
2) CC/CV charging profiles are the Ideal charging profile for lithium. Lithium cells that we commonly use, charge to a fully charged voltage of 4.20V. A cell at this voltage is safe, and this voltage Must not be exceeded at any time.
The CC part is where a cell can be charged at its recommended charging current, without exceeding the max charge current. In this part of the cycle, the voltage rises. Until which time it reaches 4.20V per cell, then the charger will reduce current to the cell to maintain 4.20V. This is all governed by the internal resistance of the battery.
*A budget charger doesn't have a true CC/CV profile, it only roughly emulates it, reducing current much earlier than necessary before the cell has even reached 4.20V. This is effectively means during the time in which a charger Can be very safely delivering charge power, the budget charger is already backing off. This period is also where charging time could be significantly reduced.
At the last part of charging, the CV phase, where the overcharging is done, the budget charger keeps going and as demonstrated above, Must be allowing a slight over-charge to achieve a 4.20V resting voltage.
So what does the budget charger give us? A Slower charge because its backing off too early, but an over-charged cell because its not truly regulating to a maximum 4.20V.
A resting voltage of 4.17 is therefor MUCH healthier than a resting voltage of 4.20V.
Lets have another example,
It is true that this voltage differential (and voltage settling) will occur with hobby chargers, simply because they usually terminate charge at 1/10 charge rate. A 0.5A charge will terminate at 50mA, and accordingly with a internal resistance of 500mOhms, this equates to a voltage increase of 0.02V.Therefore a charge that terminates at 4.20V, 0.05mA, will give a resting voltage of 4.18V.
If your budget charger that terminates at 50mA, gives a resting voltage of 4.20V, it must have terminated at 4.22V.