LiFePO4 charging - the overcharging question

[ptourin]

I figured I'd start another thread because I still don't understand the overcharging discussion. I think it got discussed in Wincrasher's thread, but that's gotten so big I thought I'd branch off from it.

I'm going to put down a charging scenario - if I get it wrong, flame away <g>...

So let's say I'm down around 50% SOC. I turn my charger on and it puts out lotsa amps - my battery voltage creeps up - 13.1 - 13.2 - 13.3.....
Sooner or later I get to the point in the charging curve where I'm near full charge and the voltage starts rising quickly. Now the charger ends up putting out 14.6 volts and as the battery climbs to meet that voltage, the charge current starts dropping. Finally the battery is up around 14.6v and the charge current drops to around zero. I say, "I'm fully charged now" - I disconnect the charger.

Let's say I don't have any load pulling the battery down. So it sits for some hours, or a day or two (I understand it takes a long time for the lith phosphate batteries to settle) - and the voltage slowly drops down to around 13.4v. I measure that, and I say, "it really worked right - I actually AM fully charged" - and I go set my monitor so that 13.4 is what it considers 100% SOC.

In a perfect battery world, where my battery is perfectly balanced, would I be perfectly safe to leave the charger on? All banks are at exactly 14.6, they're perfectly balanced and no cell draws any current - my charger current is really zero - right?

So now we step away from the perfect world into the real world. If I have one bank that's weak, so that it's not quite up to 14.6v, I assume the charger will try to lift it up to 14.6 - so there's some charger current, and all the banks are seeing it. At this point I start getting vague about what happens if there's no BMS to decide what to do. I assume that the other banks don't just say, "I'm already at 14.6, so I don't need anything" - if they did, we wouldn't need balancing circuitry. What's happening here, and why is it likely to damage the battery? I can't picture yet what's happening because of the imbalance, whether the other cells start going higher than 14.6 in the charger's effort to get the weak cell up. Does anybody have a good enough grasp of this to describe it? I'm of course trying to understand where we get to the point where the battery starts to sustain damage.