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What is Fast Charging?

Clarifying the confusion behind what "fast charging" actually is

What is "Fast Charging"?

Google defines it as: by which it can feed more juice more quickly into your phone, for a quick burst of energy.

Most references to fast charging actually refer to the mobile phone industry, but the same technology, and speeds, are also relevant when it comes to BEV (battery electric vehicles).

Background

To understand charging speeds on a BEV or a mobile phone, one has to understand how charge speed is rated. Charging speed is actually based on a ratio of how much is being put into the battery vs the battery size, which is called a "C" rating.

1C means that the power being put in to the battery pack is equivalent to the size of the battery pack.

If you have a 100wh portable mobile phone charger, and you were charging it at 100w, it would take you 1 hour (theoretically) to charge that charger and you would be charging at 100w/100wh = 1.0C

PS. That is very, very fast to charge a mobile phone charger, and would probably damage wires, connectors or the battery itself if it was charging that fast.

Most USB chargers can output only 10w of power, so if you were to charge your 100wh portable mobile phone charger, that would take you 10 hours and you would be charging at 10w/100wh = 0.1C

PS. This is very typical when it comes to charging; it's slow, but safe for the wiring, battery, and will help maintain capacity over its lifetime.

Now Over to Electric Vehicles

So now that same charging time references can be had with electric cars, electric motorcycles, basically anything with batteries.

The Evoke Urban Classic has an 7,800wh battery pack and an onboard charger that outputs 1,800w. 7,800wh/1,800w = 4.3 hours to full charge (theoretical) and about 3.0 hours to 80% and is charging at 1,800w/7,800wh = 0.23C

With the optional onboard dual charger the Evoke Urban Classic can output 3,000w, which reduces the full time charge to 2.6 hours or about 2.0 hours to 80% and is charging at 0.38C or 75km per hour of charge.

Taking a look at a Tesla Model S, their onboard charger at 110v puts out 1,600w and will take about 52 hours to charge their 85,000w battery pack to full, charging at 0.02C. At 240v the same onboard charger can now output about 7,000w and takes it down to about 10 hours to a full charge, or 0.1C

Now, their Supercharger Stations can output much more than their onboard chargers at a whopping 120kw, or 120,000w of power. That bumps up their C rating to 120,000/85,000 = 1.4C and less than 40 mins to 80% maximum charge.

So What is Fast Charging?

So the whole point of the original article was to define what Fast Charging really is, and the easiest answer is: "It depends". Different people are going to have different expectations of what "fast" really is, but from a technical point of view, charging anything over 0.3C+ puts a lot of strain and heat on the battery pack. Without adequate cooling:

  1. the battery pack will have a significantly shortened lifespan,  
  2. the charger may deliberately slow down to reduce the amount of heat, or 
  3. the battery pack will catch fire. (every company has safety and control measure to prevent this from happening)

So, to hopefully not confusing things anymore, anything above a 0.3C charge rate is pretty much deemed as fast charging from a technical point of view. Which is about 1 - 2 hours to 80% full charge from zero.

PS. Just remember that almost all the time, driving in an electric car or electric motorcycle that you're never going to be at completely empty on the battery pack, so more than likely, charging times are going to be much shorter if you follow an ABC (Always Be Charging) pattern, topping up whenever or wherever you can.

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