@juanmijm - that discussion link is good, but quite frequently gets confusing to some when voltage, SOC, and TIME get mixed up. What triggers the confusion is the amount of charge current you are using initially.
Here is the major rule you are looking for starting with some facts:
1) ANY cv voltage from 3.45v to 3.65v for LFP and allowed to continue charging as the "tail current" (aka absorb sometimes known) falls will result in a cell that is 100% charged.
2) That 100% SOC will be achieved faster at 3.65v. It will take much longer at 3.45v, but it will happen.
3) Using a charge source that is < 0.1C will result in cells becoming 100% charged, albeit slowly by the time they reach 3.45v. Essentially, at <0.1C, you are
already in low "tail current" charge levels starting out at the beginning of the process! This sometimes manifests itself as an inability to go much higher than 3.5v no matter how long your charge is. (bad)
4) If you use a charge current on a relatively discharged bank of < 0.1C, and you let the charger reach 3.65v, then you are actually exceeding 100% SOC with an overcharge!
(In my old EV days when doing this at slightly higher voltages and low currents, we were actually trying to balance based on secondary-reactions! [overcharge voltages] Really bad!)
So there is your rule:
If you use a commonly recommended CV for top balancing of 3.6 to 3.65v, then your charge source should be
at least 0.1C capable when starting from a relatively normal discharged state.
Ie, a 10ah programmable bench charger, should not be used on a battery larger than 100ah.
This is best practice. For those that DO care about spending the least amount of TIME when charging or balancing, then the 0.1C minimum charge rate for a 3.65 CV to get the charge over and done with quickly, is prudent to follow.
(What I and they are saying is essentially you can do more harm spent at low voltages, and long times, than you can at a higher cv voltage, and a short time spent getting there. To get to 3.65v without actually overcharging past 100% SOC and entering secondary-reaction territory, you need a minimum of 0.1C to start)
The BIG ISSUE!
So what happens when you try to balance 8 paralelled 280ah cells (now 2240ah !). Not many of us have our minimum 224ah charger handy.
So you simply do the best you can. And limit your initial balance to a one-time only event. Larger current is better, but may not be practical to have. Just do your best, but don't do it often with a 5ah charger!
A solution to < 0.1C chargers - stepped balance.
The solution when using < 0.1C is to do a "stepped balance". For some this is too much, but if you bought yourself a programmable bench supply, why not make the most of it?
* You START your CV set to 3.4v. Don't touch it again! Take a reasonably long time if you like until tail current falls way off, since overcharge
generally doesn't happen at 3.4v.
* DISCONNECT the bench charger, reset the CV voltage to 3.5v (and leave it alone!) until tail current falls off again.
* DISCONNECT the bench charger, reset the CV to 3.65v (and leave that alone!) until the tail current falls off. This gets you high in the knee, where balancing does the most good.
If you are impatient, and just start swinging and changing your voltages around to push more current because you are impatient, then all it can take is one distracting phone call when you have set to 3.65v, and return to find the actual value MUCH higher! Ie, once you set your CV, don't touch it!