Right off, I’d say forget about 19 cells.
Use 18 cells.
I’d charge the extra three on the side until 100%, and wait until your bank is charged to 100% and just add them.
Force charging settings so the balancing kicks in for a few hours.
Then
Make the appropriate change to your BMS and charging settings.
New voltage settings @ 18/15 of whatever you were happy with before.
New battery capacity Ah @ 18/15 of whatever you were happy with before.
Then just keep an eye on things for a while.
Why if I may ask?
… and don’t tell me it is an uneven number, bugs me already! 
Two reasons:
You’ll have to use a lower charging voltage, and the cell voltages will translate to the flat part of the curve, which may mean:
A) you stop charging when the batteries are not full, but possibly nowhere near full either.
And you won’t know. So one more cell will cost you battery capacity overall.
B) On the flat part of the curve mV really count. The cell won’t be perfectly balanced even at a 1mV delta. You will likely end up with cells widely differing in SOC.
Balancing has to be done at a higher voltage when the VI curve starts to rise. When this happens to a cell it is already in the high 90% SOC. If balancing is done on the plateau of the curve it is only a few mV for 10% of the cell capacity.
And a third reason for kicks:
It’s not only an odd number it is a prime number, so no 9 by 2 or 6 by 3 arrangements.
Been running my bank at 3.45v … start balancing at 3.40v … always perfect.
Got the idea here:
So today I set the BMS to start balancing at 3.2v … oeps … cells varied a wee bit.
Reason for such a high balancing … the BMS has a “FET Software Locked”. Even with the Balancer for Charge Only, when discharging, it is still balancing.
What I noticed, was the cells that were a little bit behind, when discharging. that Cell/s are not draining as fast as the rest. Nice thing to keep in mind when one wants to not drain the lesser charged cells speed up the balancing.
Ok, 18 cells it is … 18 x 3.45v = 62.1v and I then have a 17.388kWh.
And if I see the volts heading “north” of 64v … will address that then.
The lowest per-cell voltage I’ve seen is for BSL-BAT. 54.5V in a 16-cell battery, that translates to 3.4V per cell.
The thing is, the moment you’re above 3.3V, you are technically off the “flat curve” Phil is talking about. Your cells are giving you a chemical signal that they are filling up. But in my experience, there is a LOT of useful capacity left to be had between 3.4V and 3.45V per cell.
But above 3.45V, there is maybe 1% or 2% left. Diminishing returns and all that.
I think you could probably get away with 19 cells. 19 times 3.4 is like 65V. The thing is, balancing at lower voltages is more tricky, as Phil says, 1mV is quite a lot that low down, and you have calibration differences and temperature drift to deal with as well. I wouldn’t do it unless I’m using a very good BMS. Hence… do it with 18 cells first.
Then it is settled, 18 cells @ 3.45v = 62.1v
My biggest worry is what happens when 66v is reached/exceeded.
Does the smoke leave the unit?
Or does it switch off?
Purely out of interest, cause running at 65v will give me heartburn.
It raises a “High Battery Voltage” alarm, and either switches off or goes into passthru. I don’t know exactly when things starts to go poof, but I think it’s around 75V and up.
Still, there is a little matter of the transformer ratio and the PWM it uses to make a sine wave etc etc… which tends to have a “golden” voltage range where it wants to be.
Awesome, I’m FAR from the possible 75v. Sorted.
Is the 62v close to that range, a good idea or not, in your opinion?
BMZ batteries run around that voltage range and work just fine.
Update:
The bank when connected, as per the above pic.I was so chaffed.
After 24h of use:
Cell 16, there is a trick, was like 17 & 18.
It is being boosted by the EV Peak. As I type this, it is at 3.322v.
Next 17 then 18.
Takes about a day to boost a cell.
Process I followed, with LS as it is:
4 cells on the EV Peak to top balance them. It does not get to 3.5v even, even though the volts are set to 3.65v
Whilst that was happening, took about 3 days, I boosted the 15 cell bank, over days, up to 3.5v using the inverter.
Connected all up.
As it started working, I saw the 3 cells were nowhere near 3.5v.
So I started boosting the cells manually in the running bank.
WHY TTT WHY? (some may ask)
Cause to top balance 18 x 280ah cells, will take months, no no, years(!) I tell you.
I have the App open in front of me, so I constantly keep a wary eye on it all.
Do not do this at home … as they say. 
The power one has over a DIY bank, man, I tell you, is awesome.
Titbit: I now have 4 “warranty cells”. 4 can fail before I’m at 15 cells again.
If 2 cells fail, I will cells the rest as 12/24 banks, whilst importing the next batch.
Because nothing tastes like Fresca?
And 16 is done!
Note: This is also an experiment to see if this can work in the future when one needs to replace a cell.
Now I’m hitting them with Eskom cloudy today with a storm front moving in or whatnot for the next week.
The trick is at 100% SOC … sat down in front of the batts and we had a little chat see, me showing them “The Bulbs” if they don’t behave.
Once all is settled, then install @Louisvdw driver again, disabling the control (the bank runs perfectly under Victron control), to see the 18 cells’ individual voltages so that I can control them remotely.
Last one for today, detailing my progress.
Victron stopped as expected, cells are spot on, bar the 3 new ones.
Cells need some more TLC but the process works … so far.
Is one of these not your solution?
As Plonk points out, it will balance the bank faster, and then be seldom used again.
But yes, I was looking at Andy from Offgrid Garage, which one he now recommends out of interest.
But before I go there, I will first buy a proper charger for top balancing, then the same as I do now, connect it per cell or 8 in parallel.
A penny dropped last night though:
The EV Peaks max is 250000mAH, I need 280000mAH, the core problem.
250aH x 3.66v = 915
280aH x 3.66v = 1024.8
So I’m trying a few things:
The cell the EV Peak is on is now charging much faster.
But o jinne … I better not MOVE from my desk!!!
If you have a 5a active (multi cell) balancer you won’t need a proper charger for top balancing.
You could just set the charged voltage a bit higher for a few days to let the balancer do the work for you. (or am I missing something)
