Thanks. I was hit in the head pretty hard as a kid. That's my latest excuse for being an occasional ass.
On the forum linked, it seems when referring to top balancing, they are actually talking about active BMS system (specifically a "Vampire" board) but bottom balancing they are referring to initial balancing just once with no active measures. Same thing on the first web page, which was part of my initial confusion where he was talking about active balancing on top, then going to claim bottom balancing was better. (I thought he was talking about active bottom balancing). I was assuming there was cell voltage monitoring in both cases.
The things that stand out to me which sound accurate and appropriate are:
Most who use LFP batteries in EV's and solar use either the Motor Controller LVD in EV's, and the LVD built into the Inverter Both those use the total pack voltage to protect the battery.
and
A voltage that allows you to go close to full discharge under heavy load will not trip until it is too late when discharging under very light load
These seem to be pretty obvious issues. Of course you can't rely on a "Vampire board" to save you from overdischarging your battery if it has nothing to do with this. And relying on the minimum voltage which your vehicle's motor controller will accept before it stops functioning? It seems perfectly reasonable that a manufacturer of an EEV will design the motor controller to accept a minimum voltage which MIGHT still be safe for the S's and chemistry of the expected battery. It's not up to the motor controller to limit your mileage, arbitrarily. Unless it talks to a battery management system on the battery, of course it doesn't do this job.
If bottom balancing and simply running until the motor controller shuts down work for you, then great. I don't understand why you want to run out of battery in an EV, though. Seems like you'd be stranded. I would rather know when the battery is dangerously low and have the option to kill a cell if I really wanted to.
Then there is a lot of explanation that is less realistic.
Take a complaint that the Vampire board allows overcharging of cells.
First and the lessor of the issue they do not prevent you from over charging the battery. On my 100 AH batteries in my EV, I have a 50 amp charger. When that first Vampire Board turns on it shunts only 1 amp around the battery, while the remaining 49 amps continues to flow through the fully charged cell. Second, third cell and so one turns on until the last battery finally reaches 100% and terminates the charge. So in my 16S pack I have 15 overcharged cells cells, and one happy cell.
This is specific to the Vampire board and the specific battery charger and battery being charged, firstly. If 1 amp shunt isn't enough to prevent overcharge, that is potentially bad design of the Vampire board or it is maybe user error, using the board for something out of its specification. But he is probably not aware that his battery/charger is going to hit the constant voltage phase well before the first cell reaches max voltage, and the current will have probably dropped well below 50A by that time. But maybe 1 A isn't enough. This is a fault of the overall system and/or user error.
There is also the repeated notion that a cell dies because of reverse polarity, when good cells push current through a dead cell. This is news to me. With regular li ion, the cell doesn't need any help to be killed by overdischarge. You have to actively stop using them before this happens whether they are all balanced or not. So I know LiFePO4 are more forgiving, but I would personally be doing something a little more, here, that just balancing them on the bottom and running them dry.
Only the weakest cell ever reaches 3.5 volts or roughly 90% SOC. No cell will ever see 100% SOC. All others will be slightly lower.
This explains what happens on the top end. So after bottom balancing, you have to cut charging when the weakest cell reaches full charge (or your target goal, 90% or whatnot). Perhaps this is easier to implement than monitoring voltage at the bottom for many applications. In charging a batt in your home under controlled conditions that can't leave you stranded. I can see the advantage, and you could even simply set a charger to a conservative voltage and then physically do a voltage check on your cells (assuming you have access to all the terminals).
But all cells will have the same capacity in a Bottom Balanced system
Same amp hours, slightly lower energy capacity, though.
But I really like the idea of bottom balancing, esp if this is enough to do the job on LiFePO4 batts. Esp because the weakest cell ironically will take the longest to charge when actively top balancing, at least with Li ion. IME, it's not ever a matter of one cell having slightly less capacity than the others because it didn't get as much of the good stuff in there... it has always been found, IME, that one cell is slightly defective when there's a significant difference in capacity. By bottom initial/pack-balancing, you will get the weakest cell to finish charging first or close to first, and this is very simple and elegant and the balance should remain stable for a very long time. But I think eventually, the stronger cells are going to generally charge up a little more and discharge ever so slightly less, and you end up out of balance. But starting from bottom balanced might essentially give you a head start and a longer time between manual pack balancing (compared to top pack-only balancing, of course). I might have to try it, but it is a lot of work to do this.... I have a circuit for draining cells, somewhere.