Why charging speed is more important than range

When shopping for a new (or a new to you) EV, range appears to be top of mind for many shoppers. However, looking at long road trips, charging speed is much more important in determining how long it will take to go from here to there. Eventually your battery will run low and thus some form of charging is required to get you where you need to go. Further looking across models in our used EV survey, to estimate industry cost of adding battery pack capacity and charging speeds, charging speed is a much better bang for your buck.

We begin by looking at the impact of charging speed and battery size on a mathematical road-trip. Assuming efficiency of 6 km/kWh, a drive speed of 90 km/hr, and a 50 kW charge speed, we look at the percentage of time spent charging as we go from 200 km leg all the way to 1000 km. We do this for an EV with 40 kWh, 60 kWh and 80 kWh pack size. We assume that the time spent driving is identical in all three cases.

As we can see, the largest pack has the shortest charging time, but for longer journeys, the three pack sizes come within a few percent of one another. Interesting, since one often pays dearly for larger battery packs. Even at 50 kW charging speed (common around Toronto, but kinda slow these days), we are spending less than 20 % of our total time charging. Next lets see what happens as we compare charging speeds for the mid-case, a 60 kWh pack size. We compare 25 kW, 50 kW and 100 kW, using the same assumptions as above.

We now see a much wider range of charging times. A 25 kW (common for first generation Nissan Leaf’s), we are spending a lot more time charging than with a 50 kW speed, and even more so if we could tap into a new Leaf with a 100 kW speed. Indeed, the percentage of time spent charging is greatly reduced the faster we can charge, now for our 1000 km journey, we are seeing values that are 5-10 % apart compared to the “few-percent” we saw earlier when comparing battery pack sizes.

On the average, how much do you pay for either faster charging speeds, or pack size? Lets peer into our EV survey from a previous post, and charting price vs either battery capacity or charging speed.

Adding a trend-line reveals some interesting tidbits. On the average, mathematically we get about 1.4 Wh of battery capacity for every dollar that we spend, however, we get 4.1 W of charging speed for every dollar that we spend. Lets put these two numbers to the test. Suppose we had a 40 kWh pack size car, with a charging rate of 50 kW, and $10,000 to spend, which is better, paying for battery capacity or charging speed? Well lets chart percentage of time charging. With our $10k, we can get either a 54 kWh pack, or a 91 kW charging speed, lets see which gets us there faster.

Indeed, seems that adding to our charging speed wins the race for anything longer than 400 km or so.

Of course life is complicated. Weight for one does impact efficiency Adding charging speed may not add much weight to the EV, after all, even a 100 kW charging speed, corresponds to power levels my car is likely to see anyways during normal driving, albeit briefly as I accelerate to merge onto the highway, or use regen braking to stop at a stop-light. Hence not much may be required in terms of cabling, relays etc to support faster charging speeds. Batteries however are heavy, in our example the 14 kWh we added would have weighed about 100 kG (assuming 40 kWh weigh 300 kG source). 100 kG is enough to negatively affect the car’s efficiency, perhaps to a point where the added range may not get you there any faster.

The 14 kWh pack addition, is close enough to the Leaf + to make some interesting comparisons. As we saw in a previous post, the base Leaf gets 6.1 km/kWh, while the Leaf+ gets 5.6 km/kWh. This will impact travel times, as on a 50 kW DC fast charger, we add 305 km/hr on the base Leaf, but 280 km/hr on the Leaf +. All this adds to a longer trip time for the Leaf+ when your trip is longer than 1500 km.

Yet, the cost of adding fast charging shows that it is a very cost effective way to get you where you need to go a little faster. Adding pack capacity impacts weight, to a point where the added benefit in range, may be not be worth the loss of efficiency.