There are many misconceptions about fast charging. One being that “the battery should be as cold as possible when fast charging”. I mean, when hooking up the battery to a fast charger, all these fans start to run right? So it must be true. Like so many assumptions, unfortunately it isn’t. Zoe’s Batteries are very, very happy when they are over 25C and actually, when they are colder than that, the BMS will rightfully cap the maximum charging power. 43kW fast charging a pack that has been freezing overnight to 0C would almost certainly damage it beyond recovery.
Having said that, overheating the cells, that will still happily charge at a temperature of over 40C **), is a very, very bad idea. Renault implemented a pretty clever solution for that, installing an extra evaporator of the climate system in the air inlet of the battery compartment, which is why you not only hear the battery fans kicking in, but also the climate control when fast charging.
Which of course leaves the question, why would the car do that when fast charging and the cells are way below that happy temperature of 25C and higher? Well, consider that about 10% of the energy is lost to heat *) when fast charging the battery, that is over 4kW of heat being generated, which is substantial. The car is simply using a pre-emptive strategy, blowing cooled air over the batteries. If that annoys you (those fans can really “take off”), simply put the car in ECO mode before powering it down. The climate system will
not kick in now kick in at a much higher temperature. And of course, the batteries will heat up faster. Which might or might not be a good idea really. Some chargers do not appreciate the interruption of the charging process and ECO mode might avoid it.
*) some say full cycle energy loss in LiPo is 3%. While that can be true under ideal circumstances, 43kW (2C strategy) is not that.
**) Masoto Uriguchi, battery engineer at Renault states the batteries are happy up to 60C, but should not be taken above that.