Theory and Practice

A lot of this I'm using sloppy theoretical numbers and generalizations. Here are more details. "In theory there is no difference between theory and practice, while in practice there is." A lot of the time, I'm using sloppy theoretical numbers and generalizations. This goes deeper into the actual numbers/details. ℹ️ Info           ~ Aristotle Sabouni Created: 2021-07-04

❝ In theory there is no difference between theory and practice, while in practice there is. ❞ ~ Benjamin Brewster ℹ️ The Yale Literary Magazine; February 1882

The manufacturers and advocates give you all these specs -- and they are good for relative measurement -- in that bigger is faster (or less time to charge), and that's better. However, they do NOT hold up in the real world in absolute numbers. Just add in some slop and don't worry about it. But if you want to know the math/why? Here's some answers.

A couple things are at play:

1. With electricity, there's rated amount, and actual amount. We generally run things about 10-20% below maximum rated amount, to keep from overheating. So a 15A circuit is best to run continuously at about 13A, just to prevent overheating the cable or tripping the circuit. We talk rated, we run actual.
2. There are losses. For example, the cable/batteries getting warm? That's electricity getting converted to heat and getting lost. Chargers, inverters, cables, just operating the cars computers, cooling (cabin or batteries), is all a little bit of waste (loss) on charging. Figure a few percent total.
3. Charging Curves (Peak versus continuous) exist. When charging at home/destination, it doesn't matter much. When SuperCharging, it matters a lot. The specs usually talk about the best case, and not the average case. As the batteries are charged, they start to heat, and the car slows the charge rate to regulate that heat/wear. Thus how fast you are charging varies by how far into the charge/heating you are -- and it is the car that controls this (based on battery temp) more than the Charger.

Rated versus actual

• Losses Generally, the charging losses are only 2-3 percent. On top of that, your battery doesn't have 100% usable space. So if it's a 78 kWh battery, probably more like 74 kWh is usable, and that means the losses in charging efficiency, and losses in usable capacity, kinda cancel each other out. And the numbers are low enough to usually ignore as noise. But you might notice discrepencies between what the Charger says and the Car says on charging; that difference is usually just disipated heat, or other losses.

Level 1 charging

You might think 120v @ 20A is 2.4 kW... but in truth, a 20A rated circuit breaker, will be throttled by the charger/car to about 16A to leave headroom (for heat and not tripping by accident). Theory is 2.4 kW. Reality is 1.9 kW.

Some people will drop it on a 15A breaker, and only get 12A. New Reality is 1.4 kW. Thus by their math? They can charge their 72 kWh battery in 30 hours (2.4 kW x 30 hours is 72 kWh). In practice? 1.9 kW * 38 hours is 72 kWh.

Level 2 changing

240v @ 80A is 19 kW.... but only Ford uses an 80A home charger. Most use 60A, throttled to 50A, or about 12 kW. Thus the 4 hour charge time becomes 6 in the real world.

Some cars are worse than others -- a Chevy Volt and Spark only take about 3.3 kW on their level 2 circuitry, the Nissan Leaf does about 6.6 kW, but a Tesla can do like 20 kW (even if your charger only puts out 12 kW). So a 12 kW charger only runs at 3.3 kW on a Volt. But the Volt's tiny battery still charges pretty quickly.

Charging Curves

Supercharging gets more dramatic: Batteries heat up and wear as you charge them. The slower you charge, the better for the battery. Charging an empty battery can take full load, say 350 kW, for a short period, but they quickly start to heat, and thus they drop down draw, and by the end of a full session are down to 50 kW. Loosely 30 minutes will get you from 10-75% or so, and it will take you another 30 minutes to get to 100%, because of these charging/heating curves.

Right now, cars are not really limited by how much power you can send -- it's more about battery cooling and wear. So you can send 350kW, but the battery will heat up, and keep dropping how much power it can take over time (to prevent overheating/fire). By the end of a charge session, it might only be taking 50kW. This is called the "Charging Curve". Active cooling batteries, software, new chemistries, will improve this over time. Level 1 (120v) and Level 2 (240v) AC chargers (in homes/Grocery Stores/malls) is not going to overheat the batteries and increase charging time much. But SuperCharging will. So you'll only get near the theoretical peak performance for a few minutes, then it starts throttling (slowing) the charge. This is why it's easier to say good enough at 70-80% and drive off rather that having to wait to the extra 30 minutes just to get to 100%. This is different than ICE cars (where it doesn't take much more time to top off the tank), but the advantage of EV mapping your next charging stop for you, and once you learn to stop at 80% makes it only a little worse than an ICE. Thus, most people SuperCharging on road trips, use "Splash and Dash", they take the car as low as they can (batteries will gobble power), and they only fill it up enough to get to their charging destination or final destination (with a little bufffer) or about to 70-80%, before the curve really slows down.

V2G

Some chargers and EVs offer V2G (Vehicle to Grid). Basically, that you can drain your car battery and sell it back to the Power Company, during peak, and make some money back. It's a solution that nobody really wants, thus it doesn't really exist in the real world, and you don't probably want it if it did.

In theory, it V2G sounds good. But in practice? CHAdeMO, CCS, Tesla connector all have the theoretical capability. Tesla car inverters haven't implemented it (despite it being low cost), Ford has it on their charger, and Nissan implemented the capability, but no Power Company did anything but a minor pilot with it. The reason for the lack of interest is people's cars are out during the day (peak). And who wants to wear out their car's batteries faster, or give up range they might need/want, all to make/save a few dollars on electricity? Solar Systems (with Battery Backups) have more capacity and better battery chemistries for it. So using your EV in a power outtage for a little extra power is popular. V2G's economics (value) doesn't seem to work in the real world.

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Charging Charging has a lot of confusing specs and tradeoffs. They aren't difficult, it's just that you're unfamiliar with them.

Tags: Electric Vehicles/Charging