State of Charge: Percentage or Distance?

[mspohr]

A long time friend who has a Founder S told me that his rule of thumb is really simple and takes into account the worst case which is winter... He banks on 2 real miles per percentage of charge. This gives him ample inter Superchaarger range of 200 miles which is pretty much very good. It also simplifies the estimation by doubling the percent to get your range.... So 80% = 160 miles of worst case range.

I suppose winter in the western mountains could break this method but here on the East Coast it seems to work and is simple to do and understand and estimate.

Why go through the mental gymnastics when the car will do it for you?
The nav system displays miles to the destination. It's easy to compare that to the rated miles (and include a fudge factor for weather,etc).
Computers should work, people should think.

 

[ArtInCT]

Why go through the mental gymnastics when the car will do it for you?
The nav system displays miles to the destination. It's easy to compare that to the rated miles (and include a fudge factor for weather,etc).
Computers should work, people should think.

I think we are talking about the same thing really....
In your second sentence... there is an implication of mental activity going on....

In the way I described, you look at the miles needed to destination on the NAV and then quickly look at your % charge and double it.
You then see if you are good to go...

Example:

NAV = 45 miles to destination

Center % Display = 80

80 X 2 = 160 (mental activity)

160 is way greater than 45... we are good to go...

Pretty simple....

BTW, this method works for 85 and 90 packs.... dunno about the smaller or larger 100 pack.

 

[rhumbliner]

Definitely percentage. I agree that SoC is only an estimate, but it's a good estimate and Rated Miles is an estimate based upon an estimate, with other variables, like average Wh/m tossed in for good measure. I just don't trust RM.

 

[SDRick]

The comments in this thread make it clear that the proper approach is for the display to show as much or as little information as the
user desires. I would prefer both percentage and miles. Why not give us what we want and make it selectable?

 

[AWDtsla]

Definitely percentage. I agree that SoC is only an estimate, but it's a good estimate and Rated Miles is an estimate based upon an estimate, with other variables, like average Wh/m tossed in for good measure. I just don't trust RM.

RM is just SoC, in different units.

 

[rhumbliner]

RM is just SoC, in different units.

That's a very misleading statement.

You have to pick a Wh/mile to compute RM and, as so many of the threads here and on Tesla Forums have pointed out, it could be a fixed value or a value based on your past driving or something else. The Owners Manual states that its based on EPA range which doesn't have a lot of credibility. There doesn't seem to be a consensus on how Tesla computes RM.

I'll stick with SoC.

 

[AWDtsla]

That's a very misleading statement.

You have to pick a Wh/mile to compute RM and, as so many of the threads here and on Tesla Forums have pointed out, it could be a fixed value or a value based on your past driving or something else. The Owners Manual states that its based on EPA range which doesn't have a lot of credibility. There doesn't seem to be a consensus on how Tesla computes RM.

I'll stick with SoC.

No. Rated miles ARE a fixed conversion from SoC. It's almost trivial to confirm it yourself. Divide RM by SoC. Do it again at at a different SoC, and again, and again, and again. It is always the same number, plus or minus precision of your measurement of either input. The EPA has absolutely nothing to do with this part.

But, it IS based on EPA mileage. It is the EPA mileage with the charging efficiency and expected vampire loss with 12k miles per year subtracted from it. It is less trivial to derive but still some very basic algebra.

 

[TexasEV]

That's a very misleading statement.

You have to pick a Wh/mile to compute RM and, as so many of the threads here and on Tesla Forums have pointed out, it could be a fixed value or a value based on your past driving or something else. The Owners Manual states that its based on EPA range which doesn't have a lot of credibility. There doesn't seem to be a consensus on how Tesla computes RM.

I'll stick with SoC.

The threads you're referring to saying that rated miles is based on past driving are always corrected by people who know better. The "rated" in rated miles refers to the EPA rating. It's the miles the car can be driven in the EPA test cycle at that state of charge. What do you mean by the owners manual not having a lot of credibility? Do you believe what some random misinformed person writes on an Internet forum over what the owners manual said?

Neither a percentage state of charge or rated miles are exact because the battery state of charge can not be measured directly, however one is convertible to the other by taking a percentage of whatever rated miles your 100% charge displays as.

 

[bob_p]

At least with 7.1 (don't have 8.0 yet), the user interface allows shows a combination of charging level - you can select rated or ideal range, and in some cases, the interface only shows % charge.

And what driver's typically want to know is the "practical" range of the car - the projected range, based on expected driving conditions (elevation changes, winds, traffic congestion, likely speed, ...) - and because the software doesn't provide this number, we all have to make the calculations ourselves.

Most ICEs have a cruder percentage of tank display and may have an estimated mileage for the remaining fuel (based on different algorithms). These numbers are not as critical for an ICE, since fueling takes less time - and their are fueling stops in many locations.

With an EV, the practical range is more critical - and while the trip planner is a good first step in providing the driver assistance in energy management for trips, the current algorithm appears to be too optimistic - and will hopefully continue to improve in 8.x and subsequent releases.

 

[tliving]

Recently I switched my settings from rated miles to percentage, and I like it. Instead of thinking about how my rated miles convert to actual miles, I find that I can plan on using 30% for my daily commute. What setting works best for you?

I used rated as a new owner for about a year then I switched to percentage and haven't looked back. I much prefer the percentage approach.

 

[rhumbliner]

Wow, I created a firestorm there. First off, I didn't mean to imply the Owners Manual was lacking credibility, I said the EPA numbers lack credibility. I wouldn't quote a document that I didn't find to be credible.

Second, I stated that RM may be based on a fixed value for Wh/mile. What I missed were the rebuttals discrediting the notion that it was based on anything other than a fixed value. For that I apologize and stand corrected.

 

[mspohr]

Wow, I created a firestorm there. First off, I didn't mean to imply the Owners Manual was lacking credibility, I said the EPA numbers lack credibility. I wouldn't quote a document that I didn't find to be credible.

Second, I stated that RM may be based on a fixed value for Wh/mile. What I missed were the rebuttals discrediting the notion that it was based on anything other than a fixed value. For that I apologize and stand corrected.

It's interesting that this subject has created so much heat (and very little light). A lot of people seem to have very adamant opinions on the subject.
The EPA numbers are generated under a standard protocol so you can compare cars. They are certainly credible and repeatable but I think you point is that they may not reflect a specific use case. This is true.
I have found the "rated range" to be useful as long as I remember to take into account terrain and weather. The nav system calculates something closer to a real world range since it does take terrain and driving history (and perhaps other factors) into account. I have found the nav system to be very accurate and I rely on it on all my trips. If anything, I have found it to be conservative in estimating range.
If I am concerned about range, I use the trip energy graph since it updates in real time based on actual energy usage. If I find that I the energy at the end of the trip is declining and will be too low, all I need to do is slow down a bit to see the graph recover.

 

[rhumbliner]

If I am concerned about range, I use the trip energy graph since it updates in real time based on actual energy usage. If I find that I the energy at the end of the trip is declining and will be too low, all I need to do is slow down a bit to see the graph recover.

Absolutely. After spending all summer traveling around the US -- mostly off of the SuperCharger network -- I found the trip energy graph to be the most useful tool for avoiding tow trucks. As others have suggested, when charging I routinely enter my next destination into the nav system and use the energy graph to determine how much of a charge I need to continue. It would be a shame if the SoC vs RM debate obscured the truly useful tool that is the energy graph.

 

[bxr140]

I know it puts a bee in a lot of people's bonnets, but its got to be rated miles***: The average human being would prefer to work in miles. Its simply more intuitive with the current state of battery capacity and charge speeds...and we want average human beings to buy more EVs. I know its a bit offensive, but % is a holdover from the ICE gas gauge. We can move beyond...

The current day flaw with RM is that it can be inaccurate. Mine is actually very spot on (as is my trip planner), but I'm also at like 284 lifetime wh/mi, so I understand that those with less efficient tendencies find the RM frustratingly optimistic. What would be nice is if there was some level of learning for the RM calculation, or at least a user-settable multiplier. Bonus points if for long distance travel it could take trip planner data and have a dynamic value based on your actual next leg.

***Note that I'd really prefer it just be an option for the user to display either, or both. I don't need the interface to be as customization as Android, but it would be great to have a little more control than our current iOS-like options...

 

[Gwgan]

Maybe if I could see the remaining miles and the battery percent together at the same time I could learn to trust/adjust to the RM figure but I’ll stick with percent for now since I must make a choice. Call me old fahsioned. Around here we mostly speak of time, not distance, when thinking about trips (2.5 hours to Portland, 4.5 hours to Boston, etc) with a nod to weather (Winter) and traffic (Wiscasset) so if I learn that it takes me 45 min to drive to work and that takes 10% of the battery then a 90 min trip ought to take about 20%.

 

[bxr140]

I always select "web page views available" instead of "percent battery available" on my iPhone.

That's a very false equivalent. Over a charge cycle people use their devices in many different ways, each for an infinitely variable amount of time.

People use their cars for one thing--miles.

You don't yell "babe, grab another dozen eggs--we're down to 16.6%" when your SO is running out to the grocery store, you say "we have two left".

 

[mblakele]

All the eggs in a carton are about the same size. All miles in an EV... aren't. The first mile in my commute sometimes registers as high as 1500 Wh. By the end of the trip my average is closer to 330 Wh/mi. The whole trip uses about 15% of the battery, or about 50 RM, but only 40 actual miles. Knowing that I'll use 15% is good enough for planning purposes, and for some reason I feel less range anxiety with the percentage than I did with RM.

 

[Gwgan]

I got the idea of using percentage while reading these forums in the years waiting for the X, the early S drivers had this all sorted out already.

 

[mknox]

I just can't think in terms of percentages--not useful.

Not trying to be snarky, but how did you ever get by with gas cars? Their fuel gauges are calibrated in percentage (or, more accurately in 1/4 or 1/8th of full, but still based on tank capacity and not how far the tank will take you).

 

[mknox]

I would prefer both percentage and miles. Why not give us what we want and make it selectable?

Tesla does this quite well.You set the "fuel gauge" in percent and use the Trip Computer to show you your expected range based on actual driving style and conditions. Much like an ICE with a fuel gauge and a trip computer that calculates miles to empty based on your driving.