FFE 101: A positive "status number" does not indicate regen

I originally posted this in another topic here, but though it might be useful to elevate to its own discussion:

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A positive status does NOT indicate regen. This is a very common misconception.

Now, regen can certainly contribute to having a positive status, but remember that the "status number" in the FFE display is simply the difference between what the original range estimate was (when you turned the car on) and what the current range estimate is (now that the car has been driving along for a while). That's it. It's just a subtraction, and it is constantly varying as you drive, depending on how you drive.

Your driving style and the current driving conditions are the primary inputs that affect the status, not regeneration. A big status number doesn't necessarily indicate that a lot of regen has occurred.

Consider this: If you were to drive perfectly steady on level ground, but very slowly (which uses much less energy per mile than driving fast) your status will start to go up, and possibly positive. Conversely, if you were to start driving very fast, or drive up hill (which uses much more energy per mile than driving slowly) your status will start to go down, and possibly negative.

On my mountain trip documented here, I covered so many miles (doing down hill) near the end of my trip where a very small amount of net energy was being used per mile (because so much of my energy usage was being offset by regen) that the algorithm correctly started estimating that I could continue for another 155 miles under those conditions. I say "correct" because the algorithm is "stupidly" logical. If there were another 155 miles of the same downhill driving, I'm sure that I could have, in fact, traveled that far.

In my case, yes, regen was an important factor in the crazy big range estimates I got, but only in that the plentiful downhill regen significantly changed the net energy used per mile, causing the range estimate to change drastically.

So, the "guess-o-meter" behavior can be confusing, misleading, mysterious, etc.. But, once you know how it works (it's just math) it is a little easier to cope with. Obviously, the "real" number to go by is the percentage of charge. That, combined with driver experience (i.e., "getting to know" what the FFE can do with that charge).

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Btw, you can also see the actual miles regenerated in the "trip summary" displayed on the left-hand dash area whenever you turn the car off. It will often be a surprisingly small number (and usually have very little to do with the status).

Another way to think of the status number...

As I described above, the status is simply a difference. It is the difference between two "parallel" estimates... what I think of as the "fixed budget" and the "running budget".

When you start the car, "some algorithm" (let's not care what the algorithm is right now) computes a range estimate based on the battery SOC. This first estimate is called the "fixed budget", and appears in the FFE display (labeled "Budget"). It is conceptually a fixed estimate (computed only once) but it does decrease in real miles traveled as you drive, like a reverse odometer.

The second range estimate is actually constantly re-computed. Again, "some algorithm" uses the current battery SOC and rate of consumption (Wh/mi) to compute this range as you drive. This second estimate (let's call it the "running budget") is displayed on the battery graphic, and you can also see its value reflected in the form of the "status number".

status number = running budget - fixed budget

If you think about how these numbers will all interact as you drive, you realize that the status number basically shows how much "debt" or "credit" you currently have against the original fixed budget. Drive "spending" lots more energy than was budgeted for, and you will see a deficit (decreasing and/or negative status). Drive "saving" (or sometimes "making", in the form of regen) lots of energy, and you will see a credit (increasing and/or positive status).

Note that regen is not required to have a positive status, nor vice versa, through regen can certainly help increase status, sometimes significantly. But, there is no direct correlation between the amount of regen and the status number.

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The status number approach may also make more sense when you engage the navigation system. When engaged, the status number becomes the "surplus number". It's called the surplus because it represents how much "extra" range the car can travel beyond the programmed destination. The whole system basically works the same way, but with slightly different variables.

In navigation mode, the "running budget" is computed just the same, but the "fixed budget" is not based on the battery SOC. Instead, it it simply the distance to your destination. And, obviously, the distance to your destination will decrease in real miles traveled as you drive (just like the fixed budget decreases).

Note that it is entirely possible to enter a destination that will instantly give you a large negative surplus. For example, to some far point outside your current "running budget". Or, you can enter a destination that will instantly give a large positive surplus. For example, some point just a couple miles away.

(And again, note that a large positive surplus number might have nothing to do with regen.)

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Finally, once you understand what the status and surplus numbers mean, you can understand the blue "cup" at the bottom of the energy meter. The cup is designed to help you "stay within budget". If you drive such that the energy meter stays "within the cup", your running budget will always closely match the fixed budget. In other words, staying within the cup means that you should be able to travel the budgeted distance (when not using the navi) or travel the distance to your destination (when using the navi).

WattsUp said:

Another way to think of the status number...


Finally, once you understand what the status and surplus numbers mean, you can understand the blue "cup" at the bottom of the energy meter. The cup is designed to help you "stay within budget". If you drive such that the energy meter stays "within the cup", your running budget will always closely match the fixed budget. In other words, staying within the cup means that you should be able to travel the budgeted distance (when not using the navi) or travel the distance to your destination (when using the navi).

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Thanks for going to the trouble to explain this stuff so clearly.
I’d add a small detail to your last paragraph. If you drive so the white horizontal line in the budget cup is right at the top of the budget cup blue lines, your running budget will track very close to the fixed budget. But if you’re able to drive so the white horizontal line stays near the bottom of the cup then you’ll generate a positive status number and a favorable variance between the two budgets.

TedMH said:

Thanks for going to the trouble to explain this stuff so clearly.
I’d add a small detail to your last paragraph. If you drive so the white horizontal line in the budget cup is right at the top of the budget cup blue lines, your running budget will track very close to the fixed budget. But if you’re able to drive so the white horizontal line stays near the bottom of the cup then you’ll generate a positive status number and a favorable variance between the two budgets.

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Thanks, and yes you are correct!

Keep your cup full, and you'll never be thirsty. Keep your cup a little empty, and you'll have something extra to drink later.