36V * 12AH = 432WH, good for about 35-40 miles.
the battery has lock and keys, quite safe in public places.
You can remove it to charge indoors or charge it in situ, the charging port is covered by a rubber flap.
I also have a new 20" folding bike which folds smaller than the Zephyr, at sea at the moment. The first shipment is painted black though, same battery capacity (36V * 12AH):
None of them folds as neatly as the Brompton though.
I will have a kit specifically made for the Brompton in the new year.