Models for EV Charging


Electric Vehicles represent a challenge for the future. This blog post imagines an EV future and what it may mean. I’m an outsider to the EV space, having had a used 2017 Nissan Leaf for just over 4 months.

I’m also going to look beyond the irrational and skewed EV world that we live in currently, with free Supercharging and heavily workplace subsidized EV charging. In this future, people won’t be sitting in their cars for 30 minutes on a Friday evening to fill their car for free. Neither will we be getting juiced at work for free, having never charged your car at home or at a EV charging station. You have to pay for electricity in this world, one way or another, grounding this post in a mostly rational economic world.

Charging Options

In this post I will explore the two fundamentally two types of EV charging available today. Although there are others in the future, and have been available in the past, the two most common forms available now in the US are 6.6kW Level 2 (L2), or DC Fast Charging (DCFC).

Charging at home – L1 or L2

In this first scenario, which is mostly a real world scenario now, homes are equipped with a L2 chargers. You install the number of chargers that you need for your car. L2 chargers provide around 6.6kW, or about 25 miles per hour.

In a simplistic scenario, you plug in when you get home, and unplug when you leave for work. This would give around 12 hours of charging per day, or about 300 miles of charging overnight PG&E, my local power supplier, has an EV rate which provides a heavily reduced rate from 11 PM to 7 AM, leaving about 8 hours or 200 miles of charge each night. That’s sufficient for a very large commute or a reasonably sized car.

For small commutes, L1 also works, but at 3.3kW, you will only be able to recharge less than 100 miles overnight which places you at range anxiety if you can’t charge above your previous day’s charge. Electricity rates that make charging very cheap between 11 pm and 7 am, put the consumer in a difficult “do I pay more” situation. Don’t underestimate the psychological barriers to paying double the off-peak rate, even if it is much cheaper than the retail rates at public stations.

Charging on the Road – DC Fast Charging

The across the country road trip in a Tesla is a clear possibility with the Supercharger stations. These DCFC chargers range from 20 kW all the way up to 350kW. The common rules of thumb range from 1 hr/100 miles, up to 1 hr/300 miles for 150 kW and 20 minutes at 350 kW.

Note that most of these DCFC systems will charge quickly up to 80% and then move to trickle charging for the last 20% (see https://use-cases.org/2019/05/05/real-ev-charging-rates-from-the-bay-area/). So the math becomes complicated unless you discount that last 80%.

Most of the DCFC in the bay area charge you per minute, rather than per kWh, so there is a strong disadvantage to go beyond the 80%, in which case you are starting to pay for parking rather than charging. DCFC that charge per kWh usually charge a premium, likely balancing the time of use with the charge they deliver.

Either way, the DCFC is the closest that we will get to the gas station experience unless we go down the battery charging or start pushing up to high hundreds of kW charging. Which when you think about how electricity behaves at high power levels, the safety interlocks, cooling and so on will make high kW charging a challenge to install, maintain and keep safe.

Charging Scenarios

At this stage, I can’t see too many other fundamental options, cheap and slow, or fast and expensive. Time may prove me wrong, but unlikely in the next 5 or so years.

Home Charging

Home charging with L2 charging is likely to be the most popular and predictable charging experience. Your car is charged when you get up in the morning, you do your driving during the day, come home and plug your car back in and it’s ready fully charged by the next day. If your daily commute and errands sit below the night time charge rate of around 100 miles, you should be fine to continue with this model for as long as you want.

Road Trip Charging

The Tesla Supercharger or the Electrify America network are good example of road trip charging networks. The DCFC chargers on these networks vary between 150 kW to 350 kW. Most of the reports from people using these networks indicate that in most cases the trip stops (typically after 4-5 hours of driving) are completely reasonable to charge while having a short break. Most online articles (mainly Tesla) indicate that the the car is typically charged before the driver has finished their break.

Destination Charging

Destination charging covers the scenarios where you go somewhere and you plug the car in at the destination. These destinations are typically work, business or other activities. Most of these are L2 chargers for a typical commute of 10-20 miles, a car is recharged from the trip in 1-2 hours.

Work Charging

Work charging is an interesting challenge, the car is going to be at the office for about 8 or 9 hours. If the car is charged at night is above 80% when destination charging starts, it may take 2-3 hours to complete the charge. At many workplaces there are 2 or 3 shifts of people shuffling cars and coordinating with each other through corporate chat systems. I usually go for a car spot that doesn’t have enough available charger, relying on colleagues to move the charger over when they finish the first shift, leaving me the option of moving the car early afternoon or if there are spots open – not at all.

Shopping or Leisure Charging

Destination charging at a shopping center or leisure location is a slightly different scenario. Although most destination chargers are L2, the amount of time spent at the location may be measured in a few hours. For the purposes of this post, I’ll assume that the average distance to shopping or leisure will be around the same 20-40 miles from home.

In most cases the amount of time that a car is parked and charging will be slightly or considerably less than the amount of time needed to get the car fully charged.

Future Charging Directions

Charging at scale will present some critical challenges for electrical infrastructure. A large number of DCFC charging status can easily overwhelm the available power within a particular area. Commercial installations for power charging will invest to support what is needed. Destination charging may struggle with the electrical installation requirements for charging at scale.

Battery Smoothing

For home L2 charging, solar and house batteries like the Tesla Powerwall will be extremely useful for ensuring spare charging capacity is available. Although the Powerwall 2 is only around 13.5 kW, much less than the capacity of the EV battery, it does help reduce the dependence on the network for charging.

Another benefit of energy storage is that it allows short term peak consumption of energy above what would be available from the utility.

Adaptive Charging

As described above, most of the destination charging has a difference between the amount of time that a car will be charging and the amount of charge that will be required.

Allowing cars to have a target time and adjusting the charge rate to ensure that the car is fully charged at the time that the driver is ready to depart. If a car needs 10 kWh of energy, it can charge at a very low rate for 8 hours. Being fully charged by the end of the day. Likewise at a shopping center, declaring a departure time allows slower but consistent charging to get a car fully charged by the time that a customer is ready to leave.

This has two benefits:

  • Allowing more charge points to be installed
  • Reducing the electrical infrastructure required to support the peak load from the number of charge points installed.

Powerflex offers such a system, having installed some of these systems in the Los Altos/Mountain View school districts. The powerflex system is captured well with the graphs below.

Obviously, adaptive charging will not be suitable for road-trip style charging where the starting.

The Future – Energy Storage and Adaptive Charging

Ultimately, the charging future I see is a blend of energy story and adaptive charging. As the EV revolution continues, with places in Europe expecting to be weaned off internal combustion engines by around 2025. I look forward to driving into a car park and be presented 50 to 100 L2 charging stations and then selecting when I think I’ll be leaving the car park, knowing full well that the car will be mostly charged by the time I get back.

What price should we expect to pay for this sort of charging? It will likely be per kWh plus a small space usage rate. I’d be happy paying a 50% to 100% premium on the best charge rate that I’d be able to get at home.

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