What to Expect from DC Fast-Charging

 When I bought my purple Mitsubishi iMiEV, with its tiny 16KWh battery, I skipped the DC-fast charging port option because it really didn’t extend my range by much.  (110 miles is not much further from home than 60 miles, so I kept my gas car for longer trips.)  When I bought the 2017 BMW i3, the fast-charging port was standard.  That coupled with the larger battery and new DC fast-chargers meant that I could get to and from Napa Valley (or Carmel-by-the-Sea) with just a single, quick recharge.

Why is it called “DC fast-charging”?  If I plug in my BMW i3 to a standard wall socket (called “level-1”), I face a 27-hour charge time from empty to full.  (Nobody does this regularly.)  That is because the BMW’s 32KWh battery is being fed electricity at 120v and 1.2KW.  The math (32KWh ÷ 1.2KW = 27h) is straight-forward.  You can decrease the time by increasing the power (kilowatts, or KW).  Switching to a 240v socket and a more powerful charger (called “level-2”), I can charge at 240v and generate 7.2KW feeding into the battery.  The math (32KWh ÷ 7.2KW = 5h) shows how much faster this is.  Still, it takes hours but can be done easily at night or at work.  Both level-1 and level-2 use A/C voltage, which the car converts to D/C voltage for storage into the battery.  A DC fast-charger converts the energy from the electric utility into high-power D/C voltage and sends the D/C energy directly to the car’s battery, allowing greater power transfer rates.  This is called “level-3” charging.

Depending upon when your electric car was designed, your car will have charging capabilities of 50KW to 350KW and more.  Usually, the larger the car’s battery is, the faster it will need to charge.  My BMW’s electronics were designed for 2014 and modest battery sizes, so my car’s fastest charging speed is 50KW.  The fastest charging speed is not always used, though.  I can safely plug my BMW i3 into a 350KW charger because the car and the charger negotiate the best charging rate.  My car says any power level up to 50KW, the charger says any power request up to 350KW.  (I have seen an instance where the electric utility limited the power delivery rate to lower than the charger’s capability and my car was charging at only 25KW.)

Charging rate of various years of BMW i3.

When I connect my car to the charger and start charging, I watch the power level start out low and climb over the course of a minute or two to the highest the car can accept.  As the power flows into the car, the battery will warm up.  The car may direct the charger to reduce power (to protect the battery from excess heat) or increase power (because the warmer battery can now accept more power).  I have seen that my car will maintain the highest charging rate until the battery reaches roughly 85% capacity, at which time it starts to accept less and less power as the battery approaches full.  The last 5% of the charge trickles in no faster than it would using my home charger. 

Charging rate graphs of the Lucid and Porsche electric sedans.

My modest battery behaves differently than most of the newest car batteries.  I have seen the power graphs made by technical folks curious about how fast their cars charge.  In vehicles that charge faster than 100KW, the maximum charging rate is typically achieved between 15% full and 50% full.  After that, the rate usually drops substantially to avoid the battery overheating.  It then finishes out the charge cycle at around half of its maximum power until the last 10% of charging, at which point the power level drops dramatically (just as my car does).  This slow rate of charge at the end can take longer than the fastest rate at the start and only provides 10% of the charge (compared to 40% at the start).  Because of this, it is a courtesy to disconnect from the DC fast-charger when your car has reached 85% full to allow those waiting to charge a chance to get on their way sooner.