As the n00b (that’s me) continues along the journey to the Mexican border, I’ve had a few more thoughts.  I think these will mostly apply to driving the Roadster, though the concepts should apply to other EV’s.  In the Roadster, the most readily available measure of the strength of charge (SOC) on the battery is called Ideal Miles.  On a standard full charge, I get 181 Ideal Miles, and this will change (go down) over time as we put miles on the Roadster.  In about 4k miles that we’ve driven so far, with another 1-2k to come in the next week or so, that hasn’t decreased yet, so the decreasing capacity of the battery isn’t a speedy affect.

Anyway, Ideal Miles change when reported in Standard mode and Range mode.  In Range mode, two things happen.  The first is that the ~10% buffer not reported in Standard mode is added in.  So just changing to Range mode seemingly changes the car from 181 miles to 206 or so (no actual change in battery SOC has occurred though).  The second thing that happens is that the last 10% of the battery becomes accessible to charge.  So I can nudge the full charge up to about 233 miles, though that is only a real increase of about 24 miles.  There are some other changes around A/C and heat in range mode, and I believe accelerations are “limited” (not necessarily obvious from the outside :D).  A power limited Roadster is still a Roadster that doesn’t need to spend very much time between 0 and 40.

That’s all by way of information and explanation.  If you’re driving road trip style as I now think of it (50-60 to keep efficiency up, and charging time down), then absent other factors, you’ll be getting about 1 real mile covered for 1 ideal mile.  After watching the nav miles (real miles to destination), Ideal Miles (SOC of the battery), and Estimated Miles (a calculation of how far you can go, if the last 30 miles are representative of all of the remaining miles), my new preferred number to keep an eye on is (Ideal Miles – Nav Miles).  This number should always be positive (a negative number is bad as it means you will be stopping short;  in Range Mode, it’s even likely).  In standard mode, you could aim for that to zero out as you arrive at the destination / charging point, as standard mode has about 20 miles in reserve.  In Range Mode, I now target about 50 miles remaining, mostly to be comfortable (I’m on vacation – not a mission to prove how macho I am).

Aggressively, I target 30 miles in Range Mode on that number (Ideal – Nav) when I arrive.  The two times I’ve cut it that close, I actually arrived with 25 miles.  Soon after, the Roadster reported that the battery charge was too weak to be estimated, and so it just gave me a “–” instead of a number.  In theory it’s about 20 miles, and in practice the Roadster claims it could stop any moment now.  It is a very persuasive and effective message to transform the driver into a charging station-seeking missile  🙂

Anyway, Ideal Miles – Nav Miles is pretty steady even over hours of driving.  It does vary some, and to get an idea of whether it is going down or up, now you look at the Estimated Miles compared to Ideal Miles.  When Estimated < Ideal, then Ideal – Nav will be decreasing.  When Estimated > Ideal (not common, though going downhill can get it up pretty good), then Ideal – Nav is increasing (you are getting energy back that will be in the battery when you arrive at your destination).  So that’s sort of been my ah-hah of the morning – I’ve been relying more and more on Ideal miles anyway.  Now I feel like I have a good way to relate Ideal and Estimated miles, in a way that is tangible at the end of the drive.  Put another way, I don’t trust Estimated Miles much at all – I’ve seen it swing too radically (-48 to +78 during the Grants Pass to Orland run – others have seen Estimated go to 999;  hah!).

Ultimately, Ideal Miles remaining when you arrive is what you’ll use to shorten your charging time, or do your puttering about while you are at the destination, and so forth.

None of this is stuff that I’ve ever thought about in the 3-4 months we’ve had the Roadster until now.  In town and doing errands, I might go really crazy and drive 100 miles in a day.  I can drive that 100 miles as fast I can get away with, and never come close to the battery getting low.  And in 1-8 hours in the garage overnight, the car charges back to full while it’s not in use.