When the battery gets low on charge the Roadster gives a "Reduced power" notification. But recently I got reduced power without this warning. When this happened today, the battery charge was around 50 % (but the battery was -4 C cold, and -8 C ambient temperature). When I noticed this before, the charge has been lower than 50 %. I think I found the reason: Brick #28 reached as low as 2.91 volts, it seems like a brick cannot go below this voltage limit when accelerating.