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Discussion Starter · #1 ·
This is the second in a series of posts designed to help new and prospective EV owners decide the best Home Charging solution for their needs.

Part 1 explored acronyms new EV owner may wish to understand to begin their exploration of Home Charging alternatives.
Part 2 covers some math, or how to calculate how powerful of an EVSE you may need (or not need).
Part 3 covers some characteristics of a variety of EVSE choices in the market (Smart vs Dumb, Portable vs Wall Mounted, Hardwired vs Plug in).

EVSEs are rated on the amount of energy in kW that they are capable of delivering safely to an EV. Most EVs come with a cord that can be plugged into a standard 120V outlet in your home. This may be enough for some owners for most of the time if their daily commute is short. The following chart was prepared for a Chevy Bolt discussion, but lends itself well to the general discussion. For instance, the Bolt's 60kWh pack and average efficiency of 4 mi/kWh were used in the calculations.

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This chart highlights a couple of important topics.
  • The last 3 columns indicate roughly how many miles of range would be added per hour, or how long to charge a 60kWh pack to 100% based on the kW capacity of the EVSE being used. For example, an L1 EVSE at 12A will add about 6 miles of range per hour, and take 20 hours to fill a 60kWh battery pack from 50% to 100%, or 42 hours from 0% to 100%.
  • Second, it highlights the Breaker or circuit capacity requirements for a given level of charging. Notice that per the national electric code, sustained loads of no more than 80% of the circuit capacity are recommended (read that required) for safety reasons. So, a 32A (7.68kW) EVSE would require a 40A circuit and breaker to operate safely.
Using this chart, an EV owner with a 50 mile daily driving routine could estimate that in a 10 hour overnight charging session on a 1.44kW L1 12A cord would typically see up to 60 miles of range added. That may be sufficient for daily use, and the few occasions when more driving is required, the owner might opt to use public chargers to catch up with the occasional deficit.

An EV owner with a 100 mile daily routine may opt for a higher powered unit, say a 12A or 16A EVSE and 240V circuit which would add 120 - 150 miles in a 10 hour overnight charging session.

One more thing to consider, utility rate plans. An EV owner may prefer a higher powered EVSE to reduce the amount of time the EV charges in order to take advantage of lower utility rates. Many utilities offer a Time of Use (TOU) rate structure which offers lower rates for late night use. So, a faster EVSE that can compress the time energy is being used could offset the higher cost of the EVSE and electrical circuit with lower utility bills.

What is TOU, and why do utilities offer this? And, is this worth considering even if your home is not on a TOU rate plan?

Electricity prices follow a traditional supply and demand market pricing mechanism. Those who have taken basic economics classes in high school or college are familiar with the idea that as demand exceeds supply, prices rise. Conversely, as demand drops below supply levels, prices drop.

In CO, typically the highest energy use period (referred to as Peak) is 5PM to 9 or 10PM. This is when people typically get home from work, turn on the Air Conditioning or heating, cook, do laundry, turn on lights, computers, TVs, and charge their EVs, etc. Come 9-10PM, people go to sleep, turn off most electrical appliances, and the electricity demand drops (referred to as Off-Peak).

Utilities build their generating equipment to supply enough power to meet average Peak demand, but some of the equipment sits idle for part of the day when demand is low, and it is costly to start generating power from this equipment when demand rises (think of it like a train, it takes a long time and a lot of energy to get up to speed, and to slow down). The grid suppliers must accommodate almost instant changes in supply and demand to keep the network optimized. Generating too much energy means some will go to waste. Generating too little and lights flicker or outages happen. To supplement the occasional spikes in demand, they will often buy excess energy from other grid suppliers in an auction like market. As you can imagine, when widespread demand spikes occur such as during a heat wave, the prices the utilities pay for power can soar.

As EVs use far more energy than most appliances, the single most effective way to reduce demand (and keep costs down for all) is to avoid charging an EV during Peak times. Thus, even if your utility rates are on a flat rate plan (no Peak vs Off-Peak variances), it is a responsible thing to delay charging until Off-Peak times if possible. Since this can be programmed once and forgotten on most EVs, it is a simple thing to do to help your community.
 

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More useful information. Would be interesting if anyone has figured out how much loss(heat) for all the different charging methods. 8amp vs 12amp vs 16amp vs 32amp, etc. Level 1 vs Level 2 vs DC fast charging. If anyone can give some insight on this, I'd like to see it.
 

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2018 Chevy Bolt EV
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Discussion Starter · #4 ·
More useful information. Would be interesting if anyone has figured out how much loss(heat) for all the different charging methods. 8amp vs 12amp vs 16amp vs 32amp, etc. Level 1 vs Level 2 vs DC fast charging. If anyone can give some insight on this, I'd like to see it.
Vermont did a study several years ago, the purpose was to determine whether to push for 120V or 240V charging for government fleets as I recall. I found this using Google, but it is getting harder to find now, probably more clutter on the internet to sort through?

The study identified that L2 (240V) used less energy overall than L1 (120V). The difference was only a few percent, and came down to a couple of variables. First, higher voltages and amps results in a slightly lower net loss due to resistance. Second, because Thermal Battery Management Systems (TBMS) in EVs often use grid power to run heat or cooling to keep battery temps optimal for charging, the less time it takes at higher powered L2 charging means less time the TBMS has to run to manage the temps. In hot or cold periods, the efficiency difference reached as much as 8% better on L2 as I recall.

Heat is the result of resistance, so the above essentially answers your question. However, the study didn't focus on the kW rates of various L1 or L2 EVSEs.

Reading between the lines, one could assume higher currents (32A vs 16A for instance) would in fact generate more heat in shorter periods despite the lower impact resistance has to energy used. However, except for extremely hot conditions (like desert areas), the heat generated by 32A charging is not going to cause the need for much TBMS to cool the pack. Regardless, pack health is probably not even a consideration as AC charging rates cap out at 80A/19.2kW (J1772 standard limit). DCFC generates considerably more heat due to higher current rates (exceeding 500A in some instances).

My observation with a Chevy Bolt is cooling does tend to kick in at times while charging at 32A, but this is probably well within the TBMS capabilities to manage as compared to the 150A limit on DCFC.
 

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That makes sense. I have mostly charged the bolt at L2. Think that is the best way forward, other than road trips when DCFC is essential.
 

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2018 Chevy Bolt EV
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Discussion Starter · #6 ·
That makes sense. I have mostly charged the bolt at L2. Think that is the best way forward, other than road trips when DCFC is essential.
That is the conventional wisdom, use DCFC sparingly. I would love to see every hotel/motel along travel routes have L2 charging. I would even be willing to pay a reasonable fee for it.
 
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