Revisiting the 85/60 decision with regard to efficient use of superchargers

Revisiting the 85/60 decision with regard to efficient use of superchargers

Looking at the account of the cross-country travel is informative. A number of superchargers were skipped in the interests of minimizing the total charging time. The time taken to deliver a kWhr increases as the battery reaches its capacity. It's better to start with a battery that's lower, and charge it only with enough to ensure making the next station.

Tesla's aim is to place stations every 150 miles or so. In the cross-country trip there were a few intervals of over 200 miles. They were probably the result of skipping stations. I'm going to assume that pretty soon there will be stations every 150 miles on major routes.

Lets say cars are only charged to 90% of their capacity as a day-to-day maximum and use the EPA range (208 235 for the 60 and 85KWhr battery respectively. After a 150 mile trip, the 60 kWhr car will be down to 32 miles, 18% capacity. That's close enough for me, let's use that 32 miles as a buffer and restore the battery to 90% capacity.

The 85Kwhr battery will retain 40% of it's capacity after 150 mile trip starting at 90%. to give the same buffer of 32 miles at the end of the next leg, we'll have to start with 182 miles, 69% of capacity.
At the end of the second leg, both cars will be down to 32 miles. The 60 kWhr battery will have to be charged from 15% to 90% whereas the 85 KWhr will only need to be charged from 12% to 69%. What's the difference in these two times?

My guess is that it will take longer to put 150 miles in the 65 than the 85 kWhr battery when both are pretty low, but I don't know how much longer. Some might prefer to use the bigger battery to gain a bugger buffer. Sometimes the bigger battery could be used to skip chargers. The average charging time for the cross-country route was 38 minutes. I'd rather not do that every 150 miles - every 2 hours in good conditions.

Yes, I know I have stated elsewhere that I need a city car, not a GT. But it's hard to spend $70K on a car that dictates frequent long breaks on any substantial trips. Maybe it's worth the extra $10K to reduce the length and frequency of such breaks.

riceuguy | February 4, 2014

So far this is about my only regret with the 60...I seem to max out around 100kw at superchargers, and then only very, very briefly before the taper begins.

Nu2Ecar | February 4, 2014

Ricuguy, how long does it take you to increase your range by 150 miles? Thanks.

Docrob | February 5, 2014

I think conceptually speaking they will end up designing the supercharger network so that 85kwh cars have to stop every 2nd supercharger and 60kWh cars have to stop at every supercharger. There will probably be exceptions to this based on geographic realities and on the busiest routes it may even be more frequent to allow even 60s to leapfrog some sites if they are fully occupied however that will be the general idea imo.

slipdrive | February 5, 2014

An important consideration is the flexibility an 85kwh gives. And if and when degradation shows itself years from now, it will still be for example a 75 when the 60kwH is a 50....... In my estimation, an uphill headwind in winter could take a 60kwH out of the game for a few of the Supercharger gaps, depending on direction, and hopefully there won't be any mis-calculations. We would hate to see pictures of the Model S being yanked up onto a tow truck in the middle of nowhere ....

bp | February 5, 2014

Travel time is a combination of driving time (speed vs. distance) and charging time (which slows down considerably above 80%).

Driving at or slightly above the posted highway speeds will reduce range by 20-30%.

85's should be able to make it to the next supercharger without having to reduce speed, and likely need only 80-90% charge to reach the next SC (assuming they are typically 150 miles apart).

60's may have to reduce speed some and may require charging 90-100%.

Hopefully we'll get some trip planning tools (online or in the car) that can help map out road trips, and provide assistance in planning for speed, charging times and estimated travel times.

kenj | February 5, 2014

Really depends on where you start out supercharging. on my S60 I have the screen shot from the iPhone app 310 mi/hr, 322v 270a

I think it is going to depend on what you plan to do. Going at the speed limit 65-75 will get you to the next SC. I was at the NYC gallery for the LA to NY reception and JP said the goal for SCs is to have them within 100 miles. So it will be just as convenient for a 60 or 85.

SarahsDad | February 6, 2014

Currently if you want to drive to Florida from anywhere on the east coast you have to pass through the Savannah, GA and St. Augustine, FL superchargers which are 170 miles apart. The interstate's speed limit in between is mostly 70MPH and traffic usually moves at 75MPH. I burned 370 Wh/mi with cruise control set at 77MPH between those two chargers, dropping my 233 rated miles to 12 miles. No wind, no weather, temp 68 degrees. No way to make that in a 60 unless you slow down or do a true max charge (which takes a lot longer than a 90% charge).

SarahsDad | February 6, 2014

To add, according to this Supercharger graph (the red line),

it takes almost 30 minutes more to charge from 90% (235mi) to 100% (265mi) (85kW battery).

jjaeger | February 6, 2014

Try it in a 60 and then complain...

Pungoteague_Dave | February 6, 2014

I think that most people focused on absolute range when selecting battery capacity. Many of us didn't realize that it would take longer to use a 60 for a road trip because of quicker ramp to taper time at Superchargers. Assuming optimal SC placement for a 60, the cross country trip will take longer because the 85 will not need a full charge in circumstances where the 60 would require a range charge. Unintended benefit of an 85 (or cost of 60, depending on your perspective).

riceuguy | February 6, 2014

Nu2ecar, I'd say from near 0 to 150 would be about 35-40 min, while 50 to 200 is more like 45 min. due to the taper.

PD, for others (like me), when I ordered back when I didn't even think SCs would be an option where I am in Texas. They announced SCs around the time I finalized I think. And the map sure didn't look like it does today!

jbunn | February 6, 2014

I'm still cool with my 60. It's a bit lighter and sips power a bit slower than the 85. My range is very accurate in all driving conditions. I think my range is a bit better than 60/85ths of an 85.

(but I would love the extra range. Just did not have the scratch for my itch. Don't hate)

ChasF | February 7, 2014

+1 @jbunn

Brian H | February 8, 2014

Even for the same charge, say 40kWh, the 60 will take it in slower than the 85. I.e., charge time per mile is greater for the 60 in all parallel circumstances.

Nu2Ecar | February 8, 2014

Thanks for your help. The graph from SarahsDad was particularly useful. If I assume that the 60kWhr batery performs the same way when expressed as % of capacity, then I estimate the difference between putting 150 miles into the two batteries is only 35 versus 30 minutes. But riceuguy's empirical estimates for the 60 suggest a longer time.

A 60 kWhr battery would be more than enough for 98% of my driving. It doesn't make sense for me to pay another $10K (or $8K)to make possible long trips which are very rare for me. On the otherhand, over $70K is a lot of money for car a that I would think of as hobbled... Residual value will always be small for me - I tend to keep cars for ten years or so, then donate to charity and claim a deduction.