How efficient is charging?

These days everyone is concerned with how much resources do they use. Tracking things down to the penny/gallon/kWh/etc. When you have an ICE vehicle there are no losses from fueling the tank: The amount of gas that went into your tank is exactly equal to the amount of gas that was extracted from the tank in the ground and is equal to the amount of gas that was in the tanker truck, etc. (within reason). When you plug in your EV its not the same: When you charge what happens? The battery and the electronics heat up. That heat is wasted energy. Not all of the electrons going through the charge cable end up in the battery. But how many? Is there a way to measure this?

Many modern charge stations will report out the kWh value they put into the car (Chargepoint, for one), in addition some home stations (like the Juicebox) also report out the kWh value. This gives us one figure, but to figure out the charge efficiency we also need another number: The kWh the car used from the battery. This value is the amount of kWh the car consumed that the charger replaced. Divide these two values into each other and you’ll get a measure of how efficient the charge process is in the FFE.

For example: This morning my commute in to work consumed 3.5 kWh according to Ford’s online application (one of the few values in the trip history that have been accurately reported here). Once at work I used a local Chargepoint station to top it off. Chargepoint reported that the car consumed 4.14 kWh during charging. This would work out to a 85% efficiency while charging (at least for the top 20% of the battery or so which is what these values amount to).

I’ll have to do the math again with a deeper charge and using the Juiceboxes values for a different comparison (I’m not expecting it to be much different but more datapoints is always better).

Update: The commute home consumed 3.3 kWh according to the trip meter and the Juicebox put in 4.2 kWh to charge it back up (about 79% efficient).

Also note that the amount of energy going in also is used to run the TMS (temperature management system) and thus when its running will lower the efficiency.

Update 2: After a deeper discharge (9.6) charging took 11.7 for an efficiency of 82%

Update 3: A normal commute usage (11.1) and overnight charge (13.0) yields: 85%.

Update 4: Another normal (10.3/12.4): 83%

Update 5: 10.5/12.8: 82%


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