In order to compute time remaining, past usage has to be projected into the future. And if heavy use of the battery begins after checking the "time remaining" estimate, then it will be woefully wrong. This is true for any battery. So they, like most other devices, will check the current "state of charge" (SoC) and give either the percentage the battery has been discharged, or the percentage of discharge remaining. A snapshot of what things are like at the moment instead of a guess about how things will be in the future.
But even this can be dicey, which makes a more complicated algorithm having the SoC as part of its calculation even more uncertain.
The Battery University starts their article on battery SoC thusly:
Measuring state-of-charge by voltage is simple, but it can be inaccurate because cell materials and temperature affect the voltage. The most blatant error of the voltage-based SoC occurs when disturbing a battery with a charge or discharge. The resulting agitation distorts the voltage and it no longer represents a correct SoC reference. To get accurate readings, the battery needs to rest in the open circuit state for at least four hours; battery manufacturers recommend 24 hours for lead acid. This makes the voltage-based SoC method impractical for a battery in active duty.
But they have to guess at the SoC in active duty so every device manufacturer does their best using various methods and they can get close enough to satisfy most users.
This can lead to some difficulty for BEV car makers as they really can't avoid the "time remaining" problem and they must provide a "miles remaining" estimate. This is how one Tesla driver experienced the problem:
Took my first road trip using a Supercharger this weekend. From Charlottesville to Greensboro NC for the ACC Tournament. Did a full range charge (254miles) and headed down on Thursday, a pretty cold day (upper 30s) and headed straight to the Burlington NC supercharger, about 199 miles. I didn't drive fast, did some hypermiling (putting in N down long hills), and got there with about 20 miles to spare. So got about 219 miles on the 254 mile estimate. Charged there and headed to Greensboro to watch my Hoos roll through the ACC tournament. On Sunday after the tournament, we headed from Greenbsoro to Burlington, plugged in and went and had something to eat. Temps were dropping and slight drizzle falling. Got charged up to 252 miles, and headed out on the 199 mile trip. Did the same thing, drove slowly (55-60), hypermiling when I could, had to have some defrosting on because everything fogged up without it. Wipers running. I noticed we were losing miles fast --after 80 miles of driving, we had used about 120 miles of range, so we were down to 132 left with 119 to go. I kept doing my best but eventually we were in the negative -- when we had about 75 miles to go, we were down to 70 miles of charge. Weather was getting nasty, snow on the roads, cold (32) and moderately windy. We pulled up Chargepoint and there was nothing anywhere near us. I calculated what we were losing in my head and figured we could make it to the Hyatt Place in Charlottesville that has chargers with a few miles to spare (about 20 miles from our destination). Fortunately we did make it there with 5 miles of charge left, and 20 miles to home. We plugged in, went to a nearby restaurant and had a couple drinks, and came back to 40 miles of charge and headed home. I was pretty disappointed though that 250 miles of charge would only get me about 180 miles. And this was not driving fast. Yes it was cold (32) and snowy, but that's a lot of mileage loss... I still love my Tesla.
It's a good thing there was a charge station close enough.
Like I said, this applies to any battery. Here is a SoC table for lead acid batteries from Energy Matters:
DEEP CYCLE BATTERY STATE of CHARGE & VOLTAGE State of Charge Sealed or Flooded Lead Acid Gel battery AGM battery 100% 12.70+ 12.85+ 12.80+ 75% 12.40 12.65 12.60 50% 12.20 12.35 12.30 25% 12.00 12.00 12.00 0% 11.80 11.80 11.80
And one should note that reading these values has to be done under certain conditions to get the best estimate.
There are some new methods that are being tested to see if can get more accurate in our estimates, but since this kind of information always involves predicting the future, it will never be perfect.
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