Watt hours per mile from the wall?

[JRP3]

I've been reading that some owners need to replace 25% or more of the energy used to drive the Roadster because of the battery pack management and cooling needs. This would seem to be a drawback in Tesla's battery pack technology if it is indeed the norm. Has anyone else seen similar numbers? What is the average charge efficiency?

Example:

Due to cooling and other losses in charging, filling from empty takes about 68 kWh, or 26% more than 54 kWh the battery holds. This 68 kWh is the seminal amount; it quantifies how much truly is needed.

http://www.teslamotors.com/blog/pvev-we’re-getting-72-miles-day-sunlight-or-72-mps

 

[JRP3]

Also do those of you in colder climes see similar losses from heating the pack?

 

[PopSmith]

This would seem to be a drawback in Tesla's battery pack technology if it is indeed the norm.

I think the benefits (i.e. longer overall battery life, more consistent performance) outweigh the "negative" of having to use extra electricity to cool or warm the pack before charging. Remember that Tesla designed the pack to only warm/cool itself before charging if necessary, AFAIK.

If the car is plugged in with the battery already at it's optimal charging temperature it will start charging immediately.

 

[Doug_G]

Also do those of you in colder climes see similar losses from heating the pack?

I currently have my car in an unheated but insulated garage, sitting in storage mode (I've decided not to drive it during the worst part of the winter). A rough estimate of the power loss based on the mileage display is about 300 Wh per day, which means if I store it for three months I'll have to replace about $3.50 of lost electricity. Obviously there is no heating of the battery pack happening.

 

[JRP3]

I think the benefits (i.e. longer overall battery life, more consistent performance) outweigh the "negative" of having to use extra electricity to cool or warm the pack before charging. Remember that Tesla designed the pack to only warm/cool itself before charging if necessary, AFAIK.

I'm comparing it to other technologies that don't need such aggressive amounts of temperature control. Certainly Tesla felt that much control was necessary, but if their battery technology makes the vehicle 20%+ less efficient than another technology it may inhibit the adoption of Tesla's technology.

 

[tomsax]

I'm comparing it to other technologies that don't need such aggressive amounts of temperature control. Certainly Tesla felt that much control was necessary, but if their battery technology makes the vehicle 20%+ less efficient than another technology it may inhibit the adoption of Tesla's technology.

From what I've heard from people who have been building their own EVs and chargers for years/decades is that 80% efficiency is expected from a well-designed system. By that, I mean that for every kWh you pull from the wall charging the battery, you get 0.8 kWh back out of the battery to push the car down the road. Maybe the charging overhead is a little larger when the ambient temperature is especially high or low, but you just can't change energy from one form to another without giving some up.

Do you know of any EV that does substantially better than that?

However much it varies between EVs, it's small potatoes when you're comparing the efficiency of charging to the inefficiency of an gas-burner that throws away 80% of the energy in a gallon of gas as waste heat.

 

[JRP3]

Good points. I've never actually checked my efficiency from the wall.

 

[JRP3]

Searched a thread from someone using the same charger as mine and same brand LiFePO4 cells, he shows 94% charge efficiency.
http://www.diyelectriccar.com/forums/showpost.php?p=195066&postcount=568

 

[tomsax]

Searched a thread from someone using the same charger as mine and same brand LiFePO4 cells, he shows 94% charge efficiency.
http://www.diyelectriccar.com/forums/showpost.php?p=195066&postcount=568

It would be helpful to know how the assumed 94% efficiency was measured. I can imagine several ways to do it. The method I've used is energy taken from the battery pack (as reported by the car) divided by energy taken from the wall which accounts for many different losses: the coolant pump, any active heating/cooling, transformer losses, heat losses, battery self-discharge losses, etc.

 

[JRP3]

LiFePO4 doesn't need active cooling, some people do use heating in the winter to help with range, usually preheating from the wall, but I think his numbers were from this summer. LiFePO4 has no self discharge losses to speak of. Those numbers are simply the charger efficiency numbers replacing the energy used in driving, measured through an EKM meter at the wall compared to reported energy use of the vehicle.