[JoeReal]

Has anyone noticed that after you drove your Volt for 25 miles or so, and you have remaining EV range of 30 miles, and then you plug it in to your home or a public charging station, and go back right away and check the EV Range, it drops down, like in my case, the remaining 30 mile EV range suddenly becomes 22 miles. It is erratic, sometimes it drops by just 1 mile, and other times as much as 8 miles. But it always drops. What is causing this? I drive in almost the same way and route most days. The sudden drop is one of the causes of why I am staying much longer at a charging station than my initial projection.

If I don't plug it in and continue my driving later, I hit that remaining EV range with great accuracy.

 

[Arcanox]

Remember that the range estimate is just that - an estimate. When you unplug it, it recalculates the range again based on the stored kWh and your efficiency history as well as a number of other factors. You're probably just noticing the estimate being off because it was recalculated from scratch right after the battery was plugged in (charging the battery causes a change in temperature and can change the balance of the cells slightly as well).

 

[emartin00]

It's been noted here before. My guess is that it spends the first few minutes balancing the cells and conditioning the battery in anticipation for charging. I don't bother to plug in if it's for less than an hour.

 

[JoeReal]

Remember that the range estimate is just that - an estimate. When you unplug it, it recalculates the range again based on the stored kWh and your efficiency history as well as a number of other factors. You're probably just noticing the estimate being off because it was recalculated from scratch right after the battery was plugged in (charging the battery causes a change in temperature and can change the balance of the cells slightly as well).

My range estimate though is accurate. If I don't plug it in and continue driving, I'll get or beat the EV range estimate displayed.

The drop, however, is real in the sense that if the EV range drop is bigger, I'll have to recharge longer. If the drop is smaller, then my recharging time is shorter, that is, after driven it for the same miles but with a higher EV range estimate displayed.

 

[Arcanox]

My range estimate though is accurate. If I don't plug it in and continue driving, I'll get or beat the EV range estimate displayed.

The drop, however, is real in the sense that if the EV range drop is bigger, I'll have to recharge longer. If the drop is smaller, then my recharging time is shorter, that is, after driven it for the same miles but with a higher EV range estimate displayed.

That doesn't make a whole lot of sense (that's quite a lot of energy to just suddenly disappear), but it could have something to do with what emartin00 mentioned, which is that the charging system probably spends some time at the very beginning of the charge cycle balancing the cells, which could result in a "lower state of charge" as observed by the battery controller. It could also be that some of your cells are misbehaving as has been observed by a number of users on the forum...I would keep an eye out for any DTCs or charging errors, as it may be a sign of a battery about to fail.

 

[Qinsp]

Guess:

It's probably got long term offsets and short term offsets for range estimation.

As you drive, you might notice the range estimate is moving. This would be short term adjustment for this driving session.
If you always drive the same way, you might notice a fixed number based on your long term driving style.

So... if you plug in, it resets the short term offsets to 0, and uses the long term that it has been working on for several driving sessions. So if you were hypermiling today, and covered 30 miles on 7 kWh, it might say you have 30 miles left out of your total of 14 kWh when you shut the car off, but before you plug in.

But if your normal long term style of driving is 52 miles for 14 kWh, when you plug it in with 7kWh remaining, it will say 26 miles instead based on the long term offset.

Clear as mud?

Easiest and most fun way to prove it is to drive like an animal until you deplete more than 1/2 the charge. Then when you plug in, it should get a range bump upwards.

 

[JoeReal]

That doesn't make a whole lot of sense (that's quite a lot of energy to just suddenly disappear), but it could have something to do with what emartin00 mentioned, which is that the charging system probably spends some time at the very beginning of the charge cycle balancing the cells, which could result in a "lower state of charge" as observed by the battery controller. It could also be that some of your cells are misbehaving as has been observed by a number of users on the forum...I would keep an eye out for any DTCs or charging errors, as it may be a sign of a battery about to fail.

Or the possibility that the portable OEM EVSE could be playing some tricks as well.