[Ovmsdev] Tesla Ideal vs Rated Range

[Tom Saxton]

The appeal of the ideal miles number is that it's a measurement of the
energy in the pack stated in terms of range under standardized
circumstances. For the Roadster, an ideal mile is 55 kWh divided by 244 EPA
miles = 225.4 Wh. (The 55 kWh value comes from data buried in a spreadsheet
that was posted on a Tesla Motors blog,
<http://www.teslamotors.com/blog/roadster-efficiency-and-range>) It's a
convenient unit of energy because it's stated in terms of range: if I drive
about 57 mph on level road in moderate weather, I'll get one actual mile of
range per ideal mile of energy.

Because it's an energy unit, the value after a full charge lets me see in an
intuitive way how my pack's energy capacity changes with time, miles, and
seasons. Because it's an absolute energy unit, it can be compared across
cars and drivers.

For the 85 kWh Model S, with a range of 300 ideal miles or 265 rated miles,
an ideal mile is 283.3 Wh and a rated mile is 320.7 Wh.

For a 2014 Leaf with an EPA rated range of 84 miles and a nominal usable
battery pack capacity of 22 kWh (281 - 6 = 275 Gids with a Gid equal to 80
Wh), a rated mile would be 261.9 Wh.

Once you have this number, it becomes very easy to estimate range. If I'm
driving on level freeway between 55 and 60 mph, I can get that range. If I
drive faster, I get less. At lower speeds I get more. Climbing a hill costs
7 ideal miles per 1000 feet and descending yields about 3.5 ideal miles per
1000 feet. In cold weather, around freezing, the range per ideal miles goes
down by 10%, plus another 10% if I use heat.

In practical terms, without needing to do any complicated math, if I have
twice as many ideal miles as my distance to home (or other charging), it
doesn't matter how I drive. If distance and ideal miles are close to each
other, I need to moderate speed accordingly. If I have fewer ideal miles
than miles remaining, I need to find a closer place to charge, or find a
shorter/slower route to my destination.

This ability to understand how much energy I have left and how conditions
affect my range is impossible with a guess-o-meter that is constantly
changing my estimated miles remaining based on how I've driven in the recent
past. With a number that ignores capacity variation, trying to build
intuition is hampered by not knowing whether my range is higher/lower
because of my driving or because of the battery pack's capacity (which
changes with season and cell balancing, etc.)

With this intuition I can not only estimate my remaining range based on how
I will be driving, far more important I know how to adjust my driving to get
the range I need.

It would be a great service to EV drivers if OVMS can provide this type of
energy-based battery instrumentation. For vehicles that don't provide a
charge estimate in energy units, it will be necessary to calculate an
estimate based on what the car does offer. This will be especially
challenging in cases where there's insufficient data to determine capacity
loss.

Having an estimated range based on recent driving can be useful when you're
cruising at constant speed in stable conditions, but that's a completely
different thing from knowing how much energy remains in your battery pack in
absolute energy units.

   Tom

From:  Nikki Gordon-Bloomfield <nikki at littlecollie.com>
Reply-To:  OVMS Developers <ovmsdev at lists.teslaclub.hk>
Date:  Wednesday, January 29, 2014 at 1:01 AM
To:  OVMS Developers <ovmsdev at lists.teslaclub.hk>
Cc:  OVMS Developers <ovmsdev at lists.teslaclub.hk>
Subject:  Re: [Ovmsdev] Tesla Ideal vs Rated Range

For what it's worth, I like setting it as an option.  As in " I know that I
can usually get 30 miles per charge". Since it will be different for
everyone that seems like a sensible option.

Sent from my iPhone

On 29 Jan 2014, at 07:17, Paul Churchley <paul at churchley.org> wrote:

> Thanks for that Tom,
> 
> I am wondering how best to apply that to the Ampera... indeed, to other cars?
> 
> I have made the assumption that the ideal range was the range likely
> achievable under ideal conditions. i.e. the max range you would expect to get.
> 
> Should we be changing the names, and indeed the purpose altogether, for these
> two values? Also, EPA does not really apply in other parts of the world where
> they might have their own range rating system.
> 
> I would prefer to see a more generic range estimation protocol that can be
> applied across all models and that is user adjustable perhaps through
> parameters?
> 
> Any thoughts?
> 
> On 29 January 2014 06:11, Tom Saxton <tom at idleloop.com> wrote:
>> Ideal miles is a terrible name because it implies something very different
>> from what it is.
>> 
>> Tesla was forced to come with a new name because the EPA changed how they
>> evaluate vehicles, giving less optimistic numbers for gas mileage as well as
>> electric efficiency and range. So Tesla called the new numbers the much more
>> descriptive "rated range miles", while still using the "ideal range miles" as
>> their estimate of what the range would be under the old EPA scheme.
>> 
>>    Tom
>> 
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