13.2 How long does charging cars take?

Charging electric cars can take a long time. Charging was free for up to four hours, after that there was a $1 per hour fee. It was also recommended to staff that they try not to connect for more than two hours at a time. Figure 13.8 shows the distribution of charging times with the two hour and four hour limits marked with dotted red guidelines. One outlying charging time of over 55 hours has been excluded as have 55 sessions with a total charge of zero. Either cars were fully charged within 4 hours or many people did not want to pay any fee. There appears to be no drop at 2 hours. There are a few very short times.

Figure 13.8: Distribution of charging time in hours

Plotting charge against charging time produces a surprising graphic, Figure 13.9. An alpha value of 0.2 has been used (i.e., a point is coloured black where there are at least 5 cases). For a lot of the data there is a rising linear pattern from 0 to 2 hours, which then flattens off. There are plenty of individual values, some where charging has been more effective, some where charging has been less effective, even some where it seems to have barely worked at all. The apparent horizontal boundary across the plot from 2 hours on could be due to cars being completely charged but not having been disconnected from the charger. There are a few cases of high charges in a very short period that might be errors. All but one of the cases above 10 kWh are due to 6 of 85 users, as can be seen in Figure 13.10. Possibly they had bigger cars.

Figure 13.9: Energy charge in kilowatt-hours by charging time in hours

Figure 13.10: Energy charge in kilowatt-hours by charging time in hours, where charges higher than 10 kWh have been coloured by the user ID (charges less than 10 kWh for those users have not been coloured)