It’s nice to envision that recharging an electric vehicle is as straightforward as filling up an internal combustion vehicle, but it’s not: there are multiple charging, power, and connector options as well as limitations.
You’re driving your internal-combustion (IC) car, and the gauge indicates it’s time to get “gas” (of course, it’s actually gasoline). So you pull into a nearby filling station, pull up to a pump, and insert the filler hose; within a few seconds, you’re pumping about 3 gallons/min into the tank. At a non-ethanol gasoline energy density of about 45 megajoules per kilogram (MJ/kg) or 9500 watt-hours/liter under nominal conditions, that’s a fairly high rate of energy transferred. A modest 10-gallon (about 38 liters) tank holds nearly 400 kW-hr of energy; most mid-size vehicles have 12-to-18 gallon (45 to 68 liter) tank.
In a few minutes, you’re done and filled up with enough fuel to go at least several hundred miles; your mental metric is “miles/gallon” (or km/liter). No matter what make/model car or what octane you need, the refueling hose and filler fits, works quick, and there’s no hassle. You don’t even give the process a second thought, it’s all so straightforward and standardized.
Now, change the scenario: you are driving your pure electric vehicle (EV), the console meter says the batteries are getting low, and perhaps you are not in your usual travel path or area. You need to find and get to the nearest EV-charging station, but you need to do more than that: you need to make sure the charger set-up is compatible with your car. Further, even if there’s a compatible charger (and it is not in use), you need to decide on a tradeoff: how much time to spend charging versus how much range you’ll get in that time. It’s a function of the charger, your car, and several other variables. The mental metric you’re using is also simple to express: “miles (or km)/minute charging.”
The charging reality for EVs is a very different and more complex than that it is for gas- or even diesel-powered IC vehicles (Figure 1). This FAQ will look at some of the considerations and issues when charging an EV, beginning with the numbers.
Q: Beyond the obvious, what’s another difference between fueling with gasoline and fueling an EV?
A: Wit gasoline, the fueling rate is fairly independent of the source fuel reservoir. Whether you are filling up at a gas station, or from a hand-carried five-gallon container, you’ll be filling at a high rate on the order of several gallons/min. With EVs, there are two fill-rate factors: how much fuel (electricity) is available, and how fast you can deliver it. These are somewhat independent factors.
Q: EV fueling seems complicated — are there standards which apply here?
A: Absolutely Two standards are most important: IEC 62196, ”Plugs, socket-outlets, vehicle connectors and vehicle inlets – Conductive charging of electric vehicles” is a series of international standards that define requirements and tests for plugs, socket-outlets, vehicle connectors and vehicle inlets for conductive charging electric vehicles. The similar SAE J1772, “Surface Vehicle Recommended Practice, SAE Electric Vehicle Conductive Charge Coupler,” which covers the general physical, electrical, communication protocol, and performance requirements for the electric-vehicle conductive-charge system and coupler.
Q: What’s the energy capacity of the battery pack in an EV?
A: The capacity varies with the vehicle type, of course, but 25 to 50 kW-hr covers most of EV battery packs. The Tesla Model S claims about 100 kW-hr capacity.
Q: Can you charge an EV from a standard 115 VAC/15A line (roughly equivalent to 1500 W)?
A: Certainly; it just will take a very long time. Each hour of charging could deliver 1500 W-hr (1.5 kW-hr) of energy (assuming no loss in the power-delivery subsystem). In practice, though, 10 A and not 15 A is the realistic maximum current for safety reasons. Either way, are looking at tens of hours for a full charge: just do the basic math.
Q: What about using a standard 220/240 V line, which is available in many homes?
A: The higher voltage will charge about 25% faster. This is not because the voltage itself is higher, but because the 220/240 VAC line more easily delivers more current, about 20 A maximum.
Q: I am getting confused here with voltage, current, amp-hours, and more; can you clarify?
A: Unless you’re a software-only EE, you should be a familiar with these concepts. Just keep in mind these facts: energy (joules or amp-hours) is the ability to do work, while power (watts) is the rate at which work is being done. Power is thus the time-derivative of energy (amp-hours); energy is the time-integral of power. You charge the batteries with energy at whatever rate you can from a trickle to tens of amps, while you expend this acquired energy as power at the value needed to move the vehicle.
Higher voltages can deliver more current with fewer losses and greater efficiency and thus fill the battery with energy more quickly. This is explained by basic electrical engineering principles, and there are many sites which explore the concepts. Note that engineers often are “sloppy” about using words such as “energy and “power” in casual conversion (the context usually makes clear what is really meant), but they are distinctly different physical entities with different implications.
Q: What are the basics of residential and some commercial charging?
A: The SAE J1772 standard provides for 240 V AC charging (known as Level 2 charging) as well as what is called “DC Fast Charge” via a 500 V DC high-current charging arrangement. A Level 2 charging station can be installed at home (usually around a thousand dollars); public charging stations at businesses provide Level 2 and DC Fast Charge.
Q: What are the defined charging modes?
A: IEC 62196 defines four modes (levels) of battery charging:
- Mode 1 – slow charging from a regular electrical socket (single- or three-phase)
- Mode 2 – also slow charging from a regular socket, but with some EV specific protection arrangement (known as the Park & Charge or PARVE systems)
- Mode 3 – slow or fast charging using a specific EV multi-pin socket with control and protection functions (defined via SAE J1772 and IEC 62196)
- Mode 4 – fast charging using some unique charger technology such as CHAdeMO, which is used by some Japanese and European car makers
Q: So, am I limited to charging using just the 120 VAC or a 220/240 VAC line?
A: Absolutely not. In fact, high- energy proprietary charging systems such as the Tesla-specific one for their EVs can cut the charging time to the one-hour or less for substantial range.
Q: Are the EV charging standards universal?
A: Yes, and no. Since cars usually do not venture outside their continent, there can be different preferred modes for North America, Europe, and Asia.
Q: Where is the charger itself located?
A: The charger which transforms the AC or DC source voltage to the voltage and current needed to actually charge the batteries, and also manages the transfer, is a key element of the EV system. In some EVs, the battery chargers are external, but others have onboard chargers buried in the car. Built-in chargers take home AC power and recharge the car’s battery. The internal battery chargers use a wall-mounted box that supplies 240 VAC to the car’s charger, and the entire box, cord, and plug. This assembly is called the Electric Vehicle Service Equipment (EVSE) (Figure 2).
Part 2 of this FAQ will look at charging “cases,” connectors, and circuitry.
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- What are the different methods of charging an electric vehicle?