The Reality of Fast Charging
By Noel Adams
Every so often I read articles, like the ones recently about a breakthrough from researchers at MIT, which will allow batteries to charge quickly, in this case, in 2 seconds. The article is often carried by lots of media who will point to this breakthrough as something that has an impact on electric vehicles.
The EV discussion groups take this up and, with huge amounts of optimism, tell us that we will soon have the ability to instantly charge our electric cars at home or on the highway. I have also seen people advertising EVs on eBay who claim 100 mile range from a one hour charge using a standard 110V outlet.
It appears that few people, especially those in the mainstream media, understand the basic physics underlying fast charging, so I though it would be a good idea to review this and its implications for electric vehicle infrastructure.
The basic equation governing the charging of batteries is W = V * A, or to put it into words, Watts equals Volts times Amps. Now, power consumption is usually measured in kilowatt hours and this would be the number of kilowatts (a kilowatt is 1000 watts) consumed by a device, in the case of EV charging that would be the amount of power going into the batteries, in a one hour period.
Let’s consider a simple example, say an EV conversion that has a 120V pack and a range of around 30 miles. A rule of thumb would suggest that a car such as this gets about 5 miles of range for each kilowatt hour of energy taken from the battery. An EV like this would need to store 6KWHrs of power in the batteries if we assume the system is 100% efficient.
Now, to charge this battery we would normally apply about 1.2Kw from a 110V receptacle for about 5 hours. If we want to charge this battery in 2 seconds we would need to put that same 6KWHrs into the battery in 2 seconds which would require us to produce power at a rate of 6 * 3600/2 (six kilowatt hours times 3600 seconds in an our divided by 2 for 2 second duration) or 10,800 KW. Note this value is in KW not KWHrs since it represents an amount of power delivered in a 2 second burst.
Going back to our formulae we will first need to convert our kilowatt value to watts so we have 10,800 * 1000 = 10,800,000 watts needed from the source. If we wanted to recharge this from 110V domestic service we would need to pull 10,800,000/110 = 96,000 amps.
Most houses are set up with a 200amp supply so we already have a bit problem. Even worse, we can’t pull 200 amps from a single outlet. A typical outlet is fused at 15 amps so we can pull about 1.6KW. The best case scenario would be a 20 amp circuit driven by a 120V supply and in this case we can pull 20 * 120 = 2400 or 2.4KW. In reality you would pull less than 20A and the charging circuit is not 100% efficient so you would be lucky to add 2KW to the batteries
Most houses also have 220V service which is used for high power devices. When GM installed chargers for the EV1 they used a 50amp 220V circuit that could pump over 6KW into the batteries. Some industrial sites are even set up with 440V 3 phase service. Chrysler used this for their EPIC Minivan and could charge at a rate of about one mile per minute.
So, it looks like even if we can technically charge the batteries in 2 seconds it isn’t feasible using the supply in our homes; so why am I still so hot on fast charging rather than battery swapping? One of the biggest reasons is that we don’t need to charge batteries in 2 seconds, as useful as that would be. We don’t always need to charge the batteries to full either for that matter.
A visit to the gas station takes minutes not seconds and so the California Air Resource board, when it established fast charging standards, set the goal of charging the batteries to 80% in 10 minutes. Not surprisingly, 80% is the target value that industrial users, like those that utilize electric fork lift trucks, use. They will often keep their vehicles charged between 40% and 80% during the work week with a full charge taking place about once per week to level the batteries.
To accomplish fast charging, even charging to 80% in ten minutes, we still need to push a lot of current through the wires, far more than a domestic outlet can handle. Aerovironment, one of the leading makers off fast charging systems, offers a version of their PossiCharge fast charging system that delivers 600amps DC, the same amount of current as the maximum available to three single family homes. If we look at a 120V system then we can push 120 * 600 = 72000 or 72KWHrs for a one hour charge. That’s equivalent to pushing 12KWHrs into the batteries in a 10 minute period so, assuming the battery pack to take 600amps it would be possible to charge this pack in about 5 minutes with the PossiCharger.
Personally I think that we don’t need to charge the car even in 10 minutes. For most of us a 1 hour charge would be quite acceptable while travelling and most people can manage quite well with a normal overnight charge for day to day driving. I like to use the example of someone driving from Los Angeles to Las Vegas. I like this example because it is a 300 mile drive done by millions of motorists each year. A vehicle with a 200 mile range like the Tesla Roadster would easily be able to do the trip with one recharge. You could leave Los Angeles and drive to Baker, stop and have a meal while the car recharges, and have plenty of juice to make it to your hotel in Vegas. When I drive to Las Vegas I almost always take a break in the trip so the recharge wouldn’t add much to the overall travel time – now if I could only afford a Tesla!
I have never been a big fan of battery swapping. I think that it could work well for fleet applications like taxi companies and car rental firms, and it would also work well in low volume situations like we will see over the next few years where there are a relatively small number of electric vehicles on the roads. The problem will come ten of fifteen years down the road when there are millions of electric cars on the road. The issues with storing, charging and swapping hundreds of battery packs per day, like we would see on a busy stretch of I15 between LA and Las Vegas, are quite daunting.
The ultimate solution of course is to imbed inductive loops in the roadway so that an electric car can drive onto the freeway and pick up current from the inductive loop. The car would then be able to charge as it drove and would be able to exit the freeway at the other end with a full battery pack. This would give the EV almost unlimited range over any freeway system so equipped. The cost of putting these on major freeways would be very high but the technology already exists.
It would also be possible to load your EV onto a high speed train and travel in comfort to your destination. You would then have your EV with you for local travel. They already take cars across the English Channel via the channel tunnel using this method. In the US we don’t have the high speed train network available yet but plans are in the works to start building high speed links between major cities.
To sum up, you are never going to be able to charge your EV in seconds from a standard 110V 15amp receptacle. Fast charging is possible but will require dedicated high current charging units. These could easily be placed on major routes around the country. Battery swapping may work short term but for the long term there are better solutions. We have the technology to implement other solutions to the limited range and long recharge for EVs but they will require a big investment to implement.
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