charging efficiency

charging efficiency

I noticed that it takes more kWh (according to the electric meter) than is what is actually stored in the battery. Example: I drove ~ 75 miles on a particular day. According to the trip meter this required 24.4 kWh battery power. After charging overnight with the Tesla HPWC connected to a dedicated meter, it required ~ 32 kWh to replenish the battery to the same level as before the 75 mile trip. The charging current was set to the default 60 A setting, and I let it charge to the default 85% level. The temperature in my garage was normal (about 65 degrees). Thus, it required ~ 7 kWh more power to charge the battery than the actual power that was deposited in the battery.

Does anybody have experience if it is more efficient to charge the battery more slowly, i.e., set the current to less than 60 A?

axel.brunger | 12. Mai 2013

Is there a difference in charging efficiency when charging at a lower amperage for longer? In other words, what is the optimum current when using a HPWC in order to maximize charging efficiency, assuming there are no time constraints?

ghillair | 12. Mai 2013


Two separate issues, charging efficiency and vampire drain.

Charging efficiency should be in the 85 to 90% range. There is some loss in the cable and converting from AC to DC is not 100%.

The current vampire load is reported to be 10 or 12 miles per day, this should improve with future software updates.

axel.brunger | 12. Mai 2013

So, charging the battery more slowly (by reducing the current) should not improve charging efficiency, but it may have a small effect on vampire drain?

campsalvage | 12. Mai 2013

It wont effect the loss of kwh from drain. Drain is caused because the car's computers are on all the tme. With a slow charge you will just start the day with more stored power.

petochok | 12. Mai 2013

Is it possible that the battery conditioning system has to work harder to keep the pack from getting hot when charging at higher rates? If so, it would make sense for charging efficiency to suffer from increased parasitic power drain required for battery conditioning system to operate.

Brian H | 12. Mai 2013

Slow charging will prevent vampire drain from lowering the charge remaining in the battery. In effect, it puts the vampires on "shore power". But the energy (overall) is "wasted" just the same, from the POV of your electric bill.

EVTripPlanner | 12. Mai 2013

see my spreadsheet/tables at for overall efficiency including vampire losses, real-world wh/mile, etc.