Update 2018/12/09 on intermittent power:
This bike is impressive, in performance vs. price, features, power, speed. Readers that are thinking on buying a new bike by no means should be discouraged by these relatively minor, solvable problems.
Reference is made to the picture "shorter-pin.jpg" below. The port connector removed from the port assembly is shown. There are two o-rings to make the connector water-tight, one at each end of the threads. I removed both o-rings when I took the picture . The positive pin (the one at the right in the picture, is slightly shorter, not by much). While this is not the root cause, in conjuction with the poor battery-to-port electrical & mechanical engagement when in-the-frame, started the sparking problem. When I reassembled the whole thing, I did not install the o-rings back, what improved the electrical engagement by the width of the compressed o-ring, not much. Without the o-rings in place, water can get into the connection. It seems that this connector is a proprietary design; I could not find a replacement that can be procured on Amazon or other vendors.
In the picture "port-assy-inserted-in-battery-out-of-the-frame.jpg", the port assembly is shown connected to the battery, both out of the frame. The insertion is perfect, the insertion force is very good and the electrical and mechanical fit cannot be better. Unfortunately this is not the situation when the port assy is installed in the frame. In that case the battery is prevented from inserting correctly to the port by the frame wall. This seems to be the root cause.
As stated in my previous post, the port assembly may require a re-design. (I am assuming that my bike is not the only one to have this issue.) An alternative, less desirable but probably less expensive solution would be to make the connector about 4 mm longer; I wouldn't recommend it.
Since I removed the o-rings I could not reproduce the problem by pushing the battery to the left with my right hand. I couldn't ride the bike since then due to the weather conditions; I'll report when weather allows.