Balance wire AWG is more about your BMS's ability to balance. A BMS capable of 50mA balance current doesn't need 16 AWG wire when 28 will do fine. So the answer to this question is highly dependent on the BMS balance current.
Yes that is true when everything is working nicely. Thickness based on balancing current.
But, we are all taking safety measurements for the times when stuff is failing.
If you have a bad cell, it will completely sag under load.
Lets say the vehicle is accelerating from standstill up to a high speed which takes several seconds at least.
Under that load, a bad cell will collapse in voltage, which will recover a bit, but the state of charge of that cell will maybe drop by 50% where the state of charge of the good cell that is parallel to it, will only drop like 10% or something. (just examples).
During this high system load, and also after that, I think there will be very high currents over that balancing wire.
Now you could put a fuse on that wire, like somebody suggested here, but after that blows, that bad cell will just become invisible to the BMS. And it will go downhill from there.
Lots of us are on a budget. Today, I can afford to spend $2400 on a battery solution. This gets me 10 miles range, but it's enough to get the EV on the road. However in 6 months I can afford another $2400 for more batteries. So I add them to the first batch and double my range.
It all seems a bit expensive to me. I also built a scooter, based on a broken Chinese Vespa clone/ripoff. I spent 400 Euros on some cells from a Mitsubishi Outlander PHEV, and another 150 euro on the BMS. That gave me 40km of driving range, driving faster than 50km/h a lot of times.
And yes the Outlander PHEV aren't the most compact cells, but it fit nicely in original lead-acid battery location.
..monitor them individually... Tesla does this.
No, Tesla puts them all together, treats them as one big cell in terms of monitoring. But it has a fuse wire to each cell, if it burns the cell is no longer part of the circuit, no longer used. A very different, but also safe approach.
CANBUS can be really useful. Most modern BMS's...even the aliexpress options from sources like ICGOGOGO, often times support or run from CANBUS. An arduino nano has enough CPU power to monitor quite a lot more than 3 or 4 BMS's to make sure they are all following set rules. Programming for arduino is pretty easy. Grade school kids figure it out. Quite a lot of LCD's talk CANBUS. I've seen a few implementations for an LCD and BMS's on CANBUS. A touch LCD adds some coding, but it can allow you to use the LCD as your control interface instead of just a dashboard displaying status. That's of course means more programming effort...
Well I am an embedded software developer, that is not really the problem. The problem lies in sky high quotes to a customer, because instead of buying a BMS, I'm also writing a bunch of code that needs proper testing, which means lots of time to spend on it, leading to that high price.
In the case of independent BMS's "doing their own thing", this is an issue, but a manageable one. In my scooter with 3 chemistries and each on an independent BMS, this was a problem I dealt with. The 3 packs had different capacities and different discharge rates. I spent some time tweaking BMS settings to account for this so they each discharged at more or less the same rates. This can be workable without the need for clever management on some other device or programming.
You could tweak some settings to have it behave better. But the main problem lies in when you will reconnect a string. You can only reconnect the string if the voltage of the other operational string is very similar. Otherwise you have huge power surges. If this reconnecting of strings occurs often, I am pretty sure you're going to physically damage the batteries.
I think it is easier and safer to just have the strings connected at all times through a fuse at each string end, and just shutdown the whole battery by a BMS that is monitoring every cell. (such as the Dilithium BMS example)