I have just changed the connectors and measured the voltages.

The NIU flash charger has 5 output pins: 2 charging pins and 3 data pins. When in idle, the voltage between the charging pins is 0.69 V. The voltage on one of the data pins is 4.5 V, the other data pins have 0 V. When in idle mode, the charger LED flashes red-green which means "please connect battery". I do not know how the charger detects a connected battery, if it uses the data pins or just waits for a high voltage on the charging pins. I hope it does not expect a sophisticated protocol

Tomorrow I will try to charge the Super Soco battery with it.

Oh interesting. My fast charger is the same. I had to plug it onto the battery, then remove it again before the main connector read the actual output voltage. Maybe your one is the same? 

Also the BMS will protect your battery if it senses anything more than about 71v. The battery shuts off, and when you plug the battery into the bike, the battery symbol flashes red and you get fault 92 (over voltage detection) 

it seems to disappear once you start riding and then everything is fine! 

This also means we are unable to charge the standard battery to 4.2v per cell (71.4v) for a little bit more range ?

Bad news: The NIU flash charger charges the Super Soco TC battery with 5A (300 W) only. It seems that it does not recognise the battery. Also short-circuiting the data cables (white, grey, black) with each other does not help the charger to go into fast charge mode. It just charges with 5A, not 15A. It seems that the charger is expecting some kind of identification of a fast-charging-enabled battery (like those for NIU N-GT or N-Plus)