BEV/PHEVs - Units and Conversions

Quick rule - there is no general rule. Take for example our Leaf. It has a few years on its pack. That means instead of about 21kWh usable capacity - it has 80% left - as of this year. Next year it'll be less. The only true way to measure how much a traction pack holds - is to run it bone dry then use a meter to measure what's going in. Now that you know how much it took to fill it - you STILL don't know exactly how much the pack got, because there's a bit of loss coming in as AC to DC, and then going back to AC during use. And if you charge @ 120v - your loss increases compared to 240v. And for those who use inductive, kiss off another ~ 12% of the amount you pushed pass your inbound breaker.
Contrast to the tesla, on a hot - very hot day will using a lot of juice to assure optimum thermal management / cooling. Same issue on frigid days.
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Well you can always do it the old fashioned way and time your charge. For instance use five minutes and see how much the car registers EV miles. Mine is about 5 minutes/mile with L2. From there you can graph your results and even included outside temperature.

Generally a good source is How Long Does it Take to Charge an Electric Car?

While not perfect, its a good reference for a couple of starting points.

 

Let me suggest this taxonomy for the J1772 plug:

• Level 1 charger - nominally 120 VAC. In North America, typically fused for 15A, NEMA 5-15, or 20A, NEMA 5-20. However the current limit is not hard. There are 30A versions but really rare. However, the vehicle electronics handles conversion to DC and feeding the battery.
  
• Level 2 charger - nominally 208-240 VAC depending upon 3-phase, business, or single-phase wiring (aka., home.) Typical peak circuit wiring is 50A, NEMA 14-50 (RV park), to variation down two 30A, NEMA 6-30 used for home dryers. Again, the vehicle electronics handles conversion to DC and feeding the battery.

The Level 1 or 2 chargers use a fancy plug, J1772, that feeds the AC voltage and current to the car electronics. It also tells the car how much current the circuit can handle so the car electronics limits the load and won't trip the circuit breaker. This the AC interface called an EVSE that we install in our homes. However, built-in converter generates heat and this has to be removed from the car.

Sad to say, the power rating are often treated as obscure specifications, typically in kW. So my BMW i3 is rated at 7.3kW where as a Leaf might be half that or equal depending upn how it was ordered. But the kW doesn't tell how much current and often has to be calculated based upon the voltage:

Current = Watts / Voltage​

The DC chargers are AC-to-DC converter electronics located outside of the car. They set their voltage to what the car electronics tells them is needed. Using out-of-the-car electronics means more power to charge the battery than the car converters can handle and relocates the waste heat outside the car. But there are two, competing standards:

• DC Chargers x00 volts, CHAdeMO, a large, circular plug provides AC-to-DC conversion outside of the car so it does not have to deal with the conversion losses.
• DC Chargers x00 volts, SAE Combined Charging System (CCS) - a smaller two-part plug using J1772 and a DC connector. It also has higher power than the Level 1 and Level 2 because the working electronics are external to the car.

DC charging is limited by the car batteries and battery controller and the DC charger follows what is asked. The smallest DC chargers are in the 25-50kW range but can be more. I did not mention the Tesla supercharger which is proprietary.

Bob Wilson

 

Now for the fun part, power. The basic unit of electrical power is the kilowatt hour (kWh) which you can find on your utility bill. The meter reads the voltage and current and counts up how much goes into the house. So in Huntsville, we have two, residential electrical rates:

• $0.09220 / kWh - first 1,400 kWh
• $0.10034 / kWh - over 1,400 kWh

So I use $0.10 / kWh, the highest rate, to calculate the cost of charging my BMW i3. But the capacity of the battery is a technological 'mine field.':

• battery capacity - this is the absolute rating and sad to say, does NOT reflect what can be charged or used to power the traction motor.
  
• usable battery capacity - this is the battery with 'head room' above and below the operating range. A battery consists of lots of cells in series. During charging and discharging some cells will reach their chemical-electrical limits before the others. If a single cell gets abused by over or under charging, it becomes a dead, resistor generating heat that kills the other cells. This is what the battery management electronics works to avoid.
• battery overhead - NEVER documented. Heat is the enemy and charging and discharging the battery generates heat. Some in the power electronics but the real enemy is heat inside each cell. This is where some cars use air-cooling and others liquid. Air cooled, lower weight and simpler but suffers 'hot spots.' Liquid cooling, more complex but all but eliminates 'hot spots.'

The easiest approach is to measure the charging kWh with a smart Level 1/2 charger or public ones that report kWh. The charging kWh incorporates all overhead. It is the owner's cost to operate the car. I use my home electrical rate and the miles driven to figure out the cost per mile. For our gas Prius, I use the MPG and $/gal to figure out the cost per mile. Of course market prices vary but in Huntsville, AL the summer of 2016:

• ~$0.027 / mile - BMW i3-REx
  
• ~$0.038 / mile - 2010 Prius

To simplify the math, I tell folks that around town:

• ~$0.25 / 10 miles - BMW
  
• ~$0.35 / 10 miles - 52 MPG Prius

Going back to battery capacity, BMW reports two numbers: 22 kWh (total) and 18.7 kWh (usable). Given I'm typically getting about 4 miles per kWh, we can estimate the usable range as 4 * 20 kWh ~= 80 miles (EPA lists EV range as 72 miles.)

Bob Wilson

 

boy, it doesn't take long for electric vehicle charging rate charts to become antiquated. Already a bunch of the model X folks are opting for the 72 amp on-board Chargers - which aren't included on the chart. Time Marches On.

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