Charging Speed  

 

   


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Sunday June 16, 2019 Charging Speed One question that I get asked all the time is about how long it takes to charge an EV.  There appears to be a lot of confusion around this issue and I have found that giving some examples can give people a pretty good idea of how much time they would need to spend at a charger to complete their trip.

 

Back in 2012 when the Tesla model S was first starting to ship I talked to a doctor who had just placed his order.  He was quite happy because the car could charge in about 40 minutes.  The problem is he thought that it could do that with the supplied 110V charger, but this sort of speed requires a visit to a Tesla supercharger, and even then you would drive away at only  an 80% charge. 

 

There are three levels of chargers known as L1, L2 and L3.  Technically these are not chargers but Electric Vehicle Servicing Equipment (EVSE), the actual charger is installed in the car.  The EVSE is there to provide the correct level of current to the onboard charger in a safe manner. 

 

L1 is the slowest charge level and is accomplished by using a standard 110V outlet.  All plug-in cars currently sold in the US come with a 110V charger as standard.  L2 is the next quickest charger and this uses a 220V supply to provide charging power up to  around 7.2 KW.  The actual amount that an L2 charger can provide varies depending on the manufacturer so they may not be able to charge at full rate the car's charger can accommodate.  L3 charging is also sometimes known as DC fast charging.  In this case the charger supplies DC current to the car at various rates.  The higher the current the faster the car will charge.

 

For some people L1 charging is quite adequate for their daily needs.  A 110V outlet is capable of providing up to 1.65 KW of power but that means running the charger at maximum current which, over a long period of time may lead to a tripped breaker.  Typically the EVSE is going to limit current to about 12 amps which will provide about 1.3KW of power.  Let's say you plug in a car at 10pm and leave it charging until 7am, that's 9 hours of charging each night which will give you 11.7 KWh of power into the battery.  There will be some losses in the system so lets say we get 11KWh of actually charge into the battery.  Depending on the driver, a car will typically get between 3 and 4 miles per KWh.  As they say on the EPA sticker your mileage may vary.  At the low end the car will get 33 miles of range from this charge and at the high end it will get 44 miles of range. 

 

If you have a typical 32 mile round trip commute you can just make it with a 9 hour charge but if you are only getting 3 miles of range per KWh then you are going to be cutting it very fine.  You could easily build a safety margin by plugging in earlier.  Plugging in at 8pm would add another 2.6KW to the battery which would be good for about 8 more miles of range.  If you get closer to 4 miles per KWh then you could easily manage the 32 mile round trip. 

 

Level 2 charging is a bit more complicated since different EVs offer different charging speeds and some EVSEs do not pass the maximum current that the internal charger on some cars can take.  Level 2 chargers will typically supply either 32 amps of 40 amps although some of the cheaper chargers are limited to 16 amps.  A 16 amp charge can only provide about 3.5 KW of power.  This is good for most plug-in hybrids, such as the Chevy Volt, that have an internal charger that runs at 3.3 KW.  At this rate the car can get about 29 KWh into the battery in 9 hours.  This is more than enough to top up any plug-in hybrid and should offer between 87 and 116 miles of range in something like a Chevy Bolt.  Most public charging stations offer 6.6 KW of charge capacity, but keep in mind that something like a Chevy Volt with a 3.3 KW charger is still only going to charge at 3.3 KW.  Some newer chargers are now able to charge at up to 7.2 KW and more recent EVs such as the Chevy Bolt and Honda Clarity are able to support this rate of charging.  At 7.2 KW a Chevy Bolt will fully charge in 9.3 hours and offers an EPA estimated range of 238 miles on a charge. 

 

Level 3 charging, also known as DC fast charging is usually offered as an option on most fully electic cars but is usually not available on Plug-in Hybrids.  DC fast charging is best used for longer trips that go beyond the range of the car.  For example if you want to drive the 400 miles from San Francisco to Los Angeles you would not be able to do that on a single charge in any car currently on the market.  While L1 and L2 charging are compatible for all manufacturers except Tesla, following the SAE J1772 standard, there are 3 different standards for L3 charging.

 

The Japanese manufacturers use the CHAdeMO standard that requires a separate charge port to be installed in the car.  Most European and American manufacturers use the SAE J1772 standard which does not require a separate charge port but it should be noted that the port which supports both L2 and L3 charging is different from the one that supports just L2 charging.  Korean manufactures originally supported the CHAdeMO standard but have recently moved to the SAE J1772 standard.  As with L1  and L3 charging Tesla has a proprietary system know as the supercharger.

 

The purpose of the charger in the car is to take AC current at a given voltage and turn it into DC current which is required to charge the batteries.  With L2 charging the car is supplied with high current DC which can be applied directly to the batteries allowing them to charge much quicker than with an AC feed.  The amount of current supplied by the DC fast charger is dependent on the specific equipment and can have a good deal of variation.  Typically DC fast charging is aimed at charging the car to about 80% of capacity in 40 minutes to 1 hour.  The current rate of DC fast charging varies depending on the state of charge of the batteries and once the battery gets above 80% the last 20% of charge is typically about the same as it would take on an L3 charger.  Fast chargers are useful if you are on a road trip as you can get pretty good range in the time it takes to get a meal and take a bathroom break.   DC Fast Charging is rough on the batteries so it is not recommended for daily charging.

 

Here is something else to remember, most charge times given by the manufacturer are when charging from 0 to full.  If you manage the EV correctly you will never be charging from empty.  Overnight charging means that you will have sufficient charge the following day to do your daily driving, and have some left over at the end of the day.  With modern Lithium-ion batteries you don't need to charge the battery to 100% every time you charge and you don't need to wait until the battery is empty to start charging.  It's not a bad idea to fully charge the batteries once in a while as the finish charge is needed to keep the batteries at around the same voltage, known as leveling, but it is actually a bad idea to fully discharge the batteries.

 

Selecting the correct charging level for your needs is key to successful adoption of an EV into your family fleet.  Once you have become familiar with  how long it takes to charge and how many miles you get per KWh you will find that the EV becomes the car of choice within the fleet for any trip that the EV will accommodate.  Range anxiety becomes a thing of the past and you will really enjoy driving electric.

 

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