How does the US EPA calculate the "Certified Range" of an EV?

[stopcrazypp]

Okay, so let's see if I've got this right:

According to that page, the EPA rates the S85 at 38KwH/100 miles "combined" which is 0.38KwH/mile

To convert that to miles of range, assuming that we can use all of the 85Kw battery (which of course we are prevented from doing so by the software), divide 85KwH by 0.38KwH/m to get about 224 miles. Which doesn't seem right. What am I missing here?

Thanks!

As you noted before, the EPA efficiency rating (whether 89MPGe or the equivalent 38kWh/100miles) is from wall to wheels. However, the 85kWh number you are using is the pack capacity, not the number of kWh that would be needed from the wall to charge that battery (that number would be higher, probably closer to 100kWh). That is why you can't use the efficiency number and the battery capacity to figure out the range.

What I mean by simple math is that we know that you can scale the range numbers in proportion to the MPGe numbers.
So to figure out the city range, we take 88MPGe city / 89 MPGe combined * 265 miles combined = 262 miles city range.
For highway range: 90MPGe city / 89 MPGe combined * 265 miles combined = 268 miles highway range.

So from the city/combined/hwy MPGe of 88/89/90, you figure a city/combined/hwy range of 262/265/268.

 

[InTheShadows]

When our car is charging on 14-50 at 40amps, the charger is "wasting" 9 of those amps.

I was actually wondering today if there was a way Tesla could make the charger more efficient. Most solar chargers are about 80% efficient where as the one(s) in our cars are 75%. Another 5% charging efficiency would really add up.

Edit: grammar police

 

[ecarfan]

The 85kwh in the battery takes closer to 95 kwh to get so that is why the range does not match the "mileage".

Do you mean that to fully charge the 85kwh battery it takes close to 95 kwh of electricity?

 

[bluetinc]

I missed where the charger is only 75% efficient? Are you lumping all the losses during a charge into the "charger"? A good part of those losses are also chemical in nature.

Peter

When our car is charging on 14-50 at 40amps, the charger is "wasting" 9 of those amps.

I was actually wondering today if there was a way Tesla could make the charger more efficient. Most solar chargers are about 80% efficient where as the one(s) in our cars are 75%. Another 5% charging efficiency would really add up.

Edit: grammar police

 

[David_Cary]

Do you mean that to fully charge the 85kwh battery it takes close to 95 kwh of electricity?

Yes.

 

[InTheShadows]

I missed where the charger is only 75% efficient? Are you lumping all the losses during a charge into the "charger"? A good part of those losses are also chemical in nature.

Peter

When you run visible tesla there is a consistent 9 amps that are being spent for the charger.

I know nothing about the physics of lithium charging. Which might be the the difference between lithium and lead acid/gel charging efficiencies. I know that some of the charging losses are heat. JRP3 would have more qualified insight.

 

[bluetinc]

I'm still a little lost. Do you mean that the current from the wall is not the same as the current to the battery?

Peter

When you run visible tesla there is a consistent 9 amps that are being spent for the charger.

I know nothing about the physics of lithium charging. Which might be the the difference between lithium and lead acid/gel charging efficiencies. I know that some of the charging losses are heat. JRP3 would have more qualified insight.

 

[David_Cary]

If you pull 20 amps, the battery does not get 20 amps so yes they are not the same. Some is lost before the battery and some is lost in the battery (noted as heat - if you heat the battery during charging, then you must be losing "amps"). Overall efficiency is likely somewhere in the high 80s%.

9 amps for the charger sounds like a lot. But 9 amps might represent all of the loses during an average charge.

 

[InTheShadows]

If you pull 20 amps, the battery does not get 20 amps so yes they are not the same. Some is lost before the battery and some is lost in the battery (noted as heat - if you heat the battery during charging, then you must be losing "amps"). Overall efficiency is likely somewhere in the high 80s%.

9 amps for the charger sounds like a lot. But 9 amps might represent all of the loses during an average charge.

Yes, 9 amps is a huge amount, especially when you are talking 240v. That's over 2kwh lost for each hour you are charging. 20kwh for a fully drained battery.

If that loss is in the electronics of the charger then I'm pretty sure Tesla can do better than that, and that would be a great focal point for improving efficiency. If it's lost to changing the chemical state of the batteries and heat then it is what it is.

 

[bluetinc]

Ok, I think I understand the confusion. You can't think of amps as energy directly. You really need to think about things as power. Power is Current times voltage, i.e 40 Amps * 250V = 10,000W (10kW). The battery is running at a different voltage (288-398V for a 85kWh pack). The charger will convert this 10kW into something slight above the the battery voltage, so you will see at ~50% charge a battery voltage of ~360V, so your input will most likely be 40A*240A (9.6kW) and your battery will be charging ideally at 26.6A and 360V (also 9.6kW). This examples drop in current, 40A input to 26.6V sounds to be what you are talking about.

Now there is some loss there. A good conversion would be in the 92%+ range. You can do better (generally) if you have an exactly defined and unchanging input and output but in Tesla's case, they need to be able to handle 50Hz, 60Hz, 85-277V input and 250-405V output. That makes making a more ideal charger difficult.

Peter

Yes, 9 amps is a huge amount, especially when you are talking 240v. That's over 2kwh lost for each hour you are charging. 20kwh for a fully drained battery.

If that loss is in the electronics of the charger then I'm pretty sure Tesla can do better than that, and that would be a great focal point for improving efficiency. If it's lost to changing the chemical state of the batteries and heat then it is what it is.

 

[tezco]

Even the dimunitive 16A Leaf charger generates so much wasted heat that it needs to be water cooled to prevent a melt-down. All that heat = energy that never made it to your battery.

- - - Updated - - -

In my opinion, the current EPA ratings are a disservice to the EV buyer, and results in prospective buyers purchasing an EV that is insufficient for their winter driving needs:

1) The EPA charges the EV to the maximum SOC allowed by the vehicle (which is against most manufacturer's recommended setting for daily use.) The EPA should charge to 80% until such time as higher SOC's are found to be compatible with normal battery life.

2) In order to get a reading on an EV's worst case scenario, an Extended Deep Winter Cycle test needs to be carried out in below 0° temps beginning with a completely cold start, with driving beginning only after the interior temp has reached 70°F. They also need to disqualify any car that cannot maintain an interior temperature of 70° F throughout the Extended Deep Winter Cycle.

3) The results of the Extended Deep Winter Cycle should be reported separately because that is the maximum range EV owners can expect under the most grueling conditions.