When I would use the supercharger on cold mornings, it would charge at 0mph for 10-15 minutes and then slowly increase. The worst I saw was it stayed at a rate of 25mph for over an hour before I finally had to leave. However, using a regular wall socket has consistently given me 4-5mph even in subfreezing temperatures. For comparison, a supercharger on a warm day gives >300mph.

If you can drive for a bit before charging then it charges much faster. That is why I'm excited about the on-route battery warming!