Guess what? I changed the PTC... At least this time I knew exactly what I was looking for ?
I wanted a 3 A recovery current (used to charge the battery when in under-voltage protection mode) but that's just not gonna happen without big and costly power resistors (shared with the precharge circuit), I had to limit it to 2 A (and even then I had to use 4 10 W resistors).
I added the recovery and precharge circuit and the temperature sensors so there's only the power section of the main disconnect left to be finished.
But,
I've done some calculation about what kind of power the TVS protecting the mosfets will have to handle... the answer is almost 100 kW... but that's not the worst (I mean 10x 10 kW TVS can do the job), the real problem is that the clamping voltage would be 103 V for a 64 V TVS (lowest we can put on a 16s pack) and that's like more than 25 % over the voltage rating of the mosfets... I checked quickly and 100 V mosfets (still not ideal but I can manage the 3 V) would be 1.5 mOhm instead of 1.1 mOhm of Rdson (which would be somewhat ok actually) but they also are 30 % more expensive...
So yeah, 100 V mosfets aren't an option, so TVS aren't either. What I'm thinking about is some active circuit but I need something able to handle 10 kA at 65 to 80 V for about 1.2 ms.
All of this is based on a 10 kA current flowing into 2 m of 10 mm diameter cable (about 2.4 µH of inductance). That's 120 J stored in the cable.
Now that I think about it, seeing the time figure: if it takes 1.2 ms to unstore the energy it takes the same time to store it (more or less) and the BMS is capable to detect and interrupt an over current under 5 µs (actually 4.46 µs worst case calculated, need to confirm in real world of course) which is about 250 times faster, so that might just have saved my butt actually. I need to look more into that (arg, more maths, ...) but if the inductance doesn't have time to store a lot of energy before the current is interrupted then we're fine.
Any thoughts?