Question 2 – How long does it take to charge?

Chargepoint Public Charger


“How long does it take to charge?”  This is always the second question we get at car shows right after “how far will it go”.  The answer really isn’t a simple number.  It depends on a few factors, but it can easily be calculated.

Factor number one is how much energy does the pack hold?  (See question number one for range vs pack size.) Electric car battery packs are rated in kilowatt hours.  What this means is how many hours the pack will last if it has a one kilowatt load on it.  A kilowatt is one thousand watts, so it is equal to ten 100 watt light bulbs burning.  So a 30kwh battery pack could power 300 of these light bulbs for an hour, or 30 light bulbs for 10 hours.  To recharge you need to put the same about of energy back into the pack.  So if you want to charge your pack in an hour you are going to need the energy that would power 300 light bulbs for an hour.

The second factor is how many kilowatt hours of energy can our charging source supply?  Remember a watt is amps x volts.  So lets look at a standard home outlet.  The standard home outlet is 110 volts.  Most circuits are rated at about 15 amps unless its in a garage or shop.  What that means is the maximum amps it can take before poping the circuit breaker is 15 amps, it doesn’t mean you can use 15 amps.  You can only safely use about 80% of that continuously.  So thats 12 amps.   You also better not have anything else hooked up to that circuit such as a light, refrigerator, curling iron, etc.  So for easy math sake, lets say you can pull 10 amps.  That means that your standard home outlet can charge our car at 1.1 kw per hour.  (10amps x 110volts =1100 watts or 1.1kilowatts)  That means it is going to take about 28 hours to charge our 30kwh pack if it is completely discharged.

Now 28 hours is a long time.  But remember that was just a plain old standard plug that was designed for just lighting up a few lights and a tv.  We need something bigger. Luck is with us though because most homes have two other sources…the oven and a dryer.  A standard dryer outlet is 220 volts at 30 amps.  This means if we plug into one of these circuits or one like it, we can charge at 5.2kilowatts.  (220 volts x (.8 x 30))=5280 watts or 5.28 kilowatts.)  This means we can now charge our 30kwh pack from empty to full in 6 hours.  This is more like it.  This means even if I get home at midnight to my home, my car is fully charged and ready to go when I get up in the morning at 6am.

So we are looking at about 6 hours to charge a 30kwh pack.  But we need to consider that most people don’t drive enough in one day to discharge a 30kwh pack.  (See question 1)  This means that you are not charging a fully discharged pack.  You only put into the pack what you have taken out.  On average I only use about one third of my pack a day.  That means it only takes about 2 hours to charge it back up.  If I am at work and all they have is a 110 volt outlet, in most cases I can fully charge my car back up in 8 hours, about the time I am at work.

The standard J-1772 charging plug can handle 220 volts at 50 amps.  This is what most public chargers are setup to charge at.  This means that a it can charge about 11 kilowatts in an hour.  So from a full discharge, we can charge our pack in less than 3 hours.  Time enough for a nice diner and some shopping.

The next level is what they can quick charging.  These are chargers that can charge at over over 100 amps.   It has been found that most newer lithium battery packs can be charged to 80% capacity at these high rates without creating enough heat to cause damage.  Heat is what damages a pack.  With these new chargers we may soon be able to charge the car in as little as 15 minutes to a 80% charge.  These won’t be available in your home, but may soon be available in many parking garages or shopping areas.  Run into the grocery and get your car charged while you are shopping.  Stop at a rest area and use the bathroom and grab a soda and hop back in a charged car.  This is where the near future is going.

Next week I will take a look at the third question everyone asks – “How much does it cost?”

Written by

Andrew is a designer and inventor with experience in many fields. He is also the CEO and Co-host of Car Show Television which airs across the country and on Roku and Amazon FireTV.

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