Actually, 7s is 29.4v fresh off the charger. Granted, the voltage doesn't stay there long, but voltage is what breaks dialectric/insulator materials which then allows current to flow and burn stuff.
Even if you get 7s to work, you will surely blow the TVS device. The TVS helps to shunt high voltage
spikes to an acceptable level during braking. Once that blows, there is no more protection. So then all those brake spikes are free to wreak havoc. A while back I was doing some playing around and recorded
~30v braking spikes when using 4s lipo. On 7s, you could get into the 60v range.