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Automotive Electric Vehicle Notes to self

Musings on EV Charging

Introduction

We are considering switching from our current plug-in hybrid (PHEV) to a fully battery electric vehicle (BEV or EV). The ones we are looking at typically have about a 75 kWh battery.

With range, “your mileage may vary”. Hugely.

Some of the candidate vehicles do not yet have officially published figures, some only have a WLTP (“Worldwide Harmonized Light Vehicle Test Procedures”) range, some have both a WLTP and EPA range published. And there are some Internet reviewers who do a range at 70 MPH test with little control for weather and traffic conditions. All the numbers are different, often by large amounts and not always in the same direction. The error bars are quite large on this specification.

The vehicles we are looking at typically have a 310 mile or so WLTP rating which I think translates into something closer to 250 miles of “real world” range.

My working assumption is that we should plan around an EV with a 75 kWh battery getting 250 miles of range or about 3.3 miles/kWh.

The worst case scenario considered is arriving home with 10% charge and then charging the car to 90%. In the cars we are considering, that is adding about 60 kWh to the battery.

At Home Charging

If possible, most charging should be done at home. This is for both cost and convenience reasons.

Time-Of-Use Electrical

Electricity costs on our current electrical service varies with the time of day and day of week. This is called “time of use” (TOU) billing and it is really beneficial to avoid peak hours. The tariff we are currently on has the following characteristics:

  • Peak Hours – 4 PM to 9 PM.
  • Off Peak Hours
    • Weekdays 6 AM to 4 PM.
    • Weekend and holidays 2 PM to 4 PM.
    • 9 PM to midnight every day.
  • Super off peak hours
    • Weekdays midnight to 6 AM.
    • Weekend and holidays midnight to 2 PM.

The amount charged depends on time of year. Current rates are:

  • Winter
    • Peak – $0.29/kWh
    • Off peak – $0.28/kWh
    • Super off peak – $0.09/kWh
  • Summer
    • Peak – $0.60/kWh
    • Off peak – $0.33/kWh
    • Super off peak – $0.09/kWh

Clearly it is desirable to do as much charging as possible during the “super off peak” hours and to avoid the summer peak hours altogether.

Chargers

Home charging in the US fall into two camps: “Level 1” and “Level 2”.

One should realize that the home Level 1 and Level 2 “chargers” are not really chargers. They are “Electric Vehicle Service Equipment” (EVSE). The EVSE communicates with the car letting it know how much current the car can draw and it performs a number of safety functions. The maximum charging rate for a vehicle connected to a Level 1 or Level 2 EVSE is the smaller of the amount the EVSE can provide and the size of the actual charger built into the car.

Level 1 Charging

A Level 1 EVSE plugs into a standard 120v 15 amp rated receptacle and draw 12 amps. (Code requires continuous loads be no more than 80% of the circuit rating thus the 12 amp draw on a 15 amp circuit.) This limits Level 1 charging to 1.44 kW.

Charging a car that gets about 3.3 miles/kWh at a rate of 1.44 kW means you can add about 4.8 miles for every hour you are plugged in. In our worst case scenario, we are looking at nearly 42 hours to charge the car from 10% to 90% using a Level 1 EVSE.

Setting up Level 1 charging is basically free if you have any code compliant receptacle in the garage as the car manufacturer provides a Level 1 cable with the car.

Level 2 Charging

A Level 2 EVSE provides 240v power to the car. The amount of current depends on the 240v circuit powering the EVSE. Very few cars have a charger capable of using more than 40 amps at 240 volts which can be provided by a 50 amp rated circuit.

The vehicle charging speed can vary considerably on a Level 2 EVSE based on the car’s charger and house circuit. But it will generally be several times faster than the 4 to 5 miles/hour of a Level 1 EVSE. If the car can take the full 40 amps at 240v then you are looking at about 32 miles/hour or 6.5 hours to charge from 10% to 90%.

Some garages in the US may have an existing 240v outlet available. Ours does not. So a new circuit would have to be installed. Depending on how fastidious the electrician is with following permitting requirements and how much unused capacity there is in the main electrical panel this can be a long and expensive process. Apparently our local utility can take several months to their part of upgrading our home to a higher amperage service.

In our case a fully permitted service up grade, new main panel and lots of electrical conduit would need to be run around the perimeter of the house leading to cost estimates close to $5,000. At the low end, overloading the main panel and avoiding permits would be around $1200. These are the costs for the labor, and materials excluding the actual EVSE which runs several hundred dollars.

Is It Worth Installing Level 2 EVSE?

There are two ways to look at this. The first is having the capacity to fully charge the vehicle in a single night. The second it to look at how much you will need to charge in the normal use case.

If you have regular need to travel long distances every day there is no getting around installing a Level 2 EVSE.

But that is not our use case. The question becomes, with our historical vehicle use, can we get by with a Level 1 EVSE?

Our Vehicle Use

Having compulsively kept records, I know that we have averaged 31 miles per day in our current car. If everyday was an average day, then we could recharge an EV using a Level 1 EVSE in about 9.3 hours. If the car starts charging at midnight it would be fully charged by 9:30 AM which, being retired, is early enough.

But very few days are average days. Sometimes we don’t drive at all. Other days we drive considerably farther than 31 miles. And some days we want to leave the house by 7 AM.

Unfortunately, I am not compulsive enough to have recorded daily mileage. Or maybe that is fortunately.

We can make some educated guesses about what our driving in the “worst typical week” is. The thinking is that if we cover 99% of our use with home charging, including our “worst typical week”, we still have a reasonable back up by visiting a local DC Fast Charger (DCFC) for the other 1%.

Worst Typical Week

About one weekend a month we perform volunteer service a considerable distance from home. Based on sample vehicles, A Better Route Planner indicates we would likely return home with about 30% battery remaining having fully charged the vehicle during one stop on the trip.

At least in non pandemic times, we often have a group activity on Wednesdays. Distances vary but are generally within 35 miles of home (70 miles round trip).

While most of our local shopping and other errands are walking distance, we drive to some local destinations a couple of times a week with a total mileage of around 8 miles per trip.

So our “worst typical week” looks something like:

  • Arrive home Sunday evening with 30% battery. (Need to add 60% or 45 kWh).
  • Errand on Monday for 8 miles. (Use 2.4 kWh)
  • Group activity on Wednesday for 70 miles (Use 21.2 kWh).
  • Errand on Thursday for 8 miles. (Use 2.4 kWh)
  • Want car at 90% by Friday afternoon for possible long road trip.
  • Total home charging during week: 71 kWh. (50 hours using Level 1.)

Looking at this, we shouldn’t be stranded. By the time we start our errands on Monday (or Thursday) we would have 30 or so miles added to the battery. Enough range regardless of how low we were when we returned home.

Level 1, Level 2 Cost Comparison

Since there are only 30 hours of “super off peak” during the week, it is not possible to keep the car charged using a Level 1 EVSE using only the cheapest power. We will need to use at least 20 hours “off peak”
electricity which, in summer, costs $0.24/kWh more. This works out to about $4.80/week in electrical costs that could be avoided by installing a Level 2 EVSE.

Not every week is a “worst typical week”, but assume they all are. Then our penalty for staying with Level 1 charging is $4.80/week or just under $250/year.

It would take between 5 years (unpermitted, dicey install) to 20 years (fully permitted install with upgraded main panel) to recover the cost of installing a Level 2 EVSE.

In our case, it looks like using a Level 1 EVSE makes sense.