Using 20 to 80 Charging Time to Evaluate Electric Vehicles
A common metric used when comparing how fast electric vehicles charge is how long it takes to charge from 20 to 80 percent of the battery capacity.
Let's explore how well this metric may or may not help people understand which electric vehicles are more or less convenient to charge.
Pro: Simple and familiar
Having a metric measured in time is useful to other aspects of life that are already familiar to us.
We all have some concept of how long certain activities take and how it feels when time passes. If we are told a certain vehicle takes 15 minutes to charge, we can immediately compare that to other activities that take about that long, such as going to the restroom or getting a quick bite to eat.
This is simple and easy to conceptualize when being introduced to electric vehicle charging for the first time.
Pro: Easier to compare to other fuels
Some other metrics are measured in kilowatts (kW) and are very specific to electric vehicles. These can be useful when comparing electric vehicles to each other, but not so useful when comparing to other types of fuels.
Since this metric is measured in time, it is not specific to any type of fuel.
Pro: More practical when charging on road trips
This metric attempts to be representative of how people charge while on a road trip.
It is common to arrive to a fast charger when around 20 percent state of charge. Fast charging will also start to slow down around 80 percent, which is usually plenty of range to reach the next charger and thus a great time to stop charging and be on your way.
Con: Range is not taken into account
This metric does not, however, put a vehicle's range into account. Eighty percent capacity of one vehicle could be a very different capacity in another vehicle.
Say two vehicles both take 30 minutes to charge from 20 to 80 percent:
- The first vehicle has 200 miles of range when at 80 percent state of charge
- The second vehicle has 100 miles of range when at 80 percent state of charge
Which vehicle would you rather wait 30 minutes for?
Con: Does not capture the complexity of charging curves
There are many variables that affect how much power an electric vehicle receives at a given time.
Some vehicles have a fairly linear curve, with power being higher at a lower state of charge. Other vehicles have a weird, non-linear curve that is much less predictable. With these different curves, arriving at a charger with the same state of charge could result in very different charging experiences for different vehicles.
In practice, you will not arrive at exactly 20 percent state of charge and leave with exactly 80 percent state of charge every time. Sometimes it will take more time and sometimes it will take less time to actually charge than this metric.