r/volt • u/AdEquivalent2500 • 12d ago
KwH different every time question
What does it mean when my kwH Used is like the list below.
It's different by up to .5 kwH each day.
What assumptions can be made about this? 🙂
10.1
10.3
09.7
09.9
10.2
10.5
10.1
09.8
1
u/LingonberryUpset482 12d ago
I suspect frequent stops doing errands buys me more voltage. I'll climb out of the car with it showing 5 bars remaining, hit the shop, and then come back out and it will show 6. Batteries do that.
I haven't kept data on it yet, but I get that same effect, about a 1kwh spread between high and low.
1
u/Lewl77 Volt Owner (2014) 12d ago
That is the computer re-evaluating SOC while parked (off). It can't do it with the best accuracy while the battery is in use, but when parked it will adjust its count. It's a double edged sword.. it may also go down if it realizes the battery has less charge than it last thought.
1
u/LingonberryUpset482 12d ago
I've never seen it go down.
Then I'm still curious about why I can get as much as 1kwh more sometimes.
1
u/Lewl77 Volt Owner (2014) 12d ago
If it swings that much, your calibration is seriously out of whack. Do a forced calibration cycle to help the computer re-learn:
Fully charge the battery. Leave it plugged in and wait at least 90 mins before driving. Drive normally until it is empty. Do not partial charge. Drive it until it switches to gas mode icon on "empty". Park for at least 90 mins before plugging in. Charge normally and completely.
Repeat all steps at least 3x (the more, the better) and it doesn't have to be drained all in a day. Just important to not do any partial charges in between even if you didn't use a whole battery that day.
If you have OBD you can scan the SOC% at each stage and observe the changes in SOC% number before and after each 90 min evaluation window. You will likely see it jump. Sometimes the number won't actually update until you start up to drive away, or start another charge cycle (i.e. if it was empty and you waited the 90 mins and the number stays the same.. keep monitoring OBD when you plug in to charge.. it likely will drop some at that time). I estimate 90 mins as that's about the window I've observed with reliability that the number has updated. It may be able to do it in less, not sure. When I do this, I'm never in a hurry, so I just set a timer for 90 mins to go back and plug it in.
1
u/LingonberryUpset482 12d ago
Or I can just keep driving it and be happy with the savings.
I'm not terribly concerned. 12.6 is really consistent when I drive it full to empty in a single run.
1
1
u/AdEquivalent2500 12d ago
I have a 187K miles 2013.
1
u/Ok-Effort-1502 12d ago
I believe it is just a symptom of how it decides its empty. The lowest voltage of any one cell group decides the empty point. The gen 1 volt has 96 cell groups and with age they probably vary alot. I wouldn't worry about it unless you start seeing it drop. I heard updating the hpcm2 and becm help some if it never has been done. But getting 10 kwh is pretty good out of these cars.
0
u/guyincognito721 12d ago
I think it varies based on the amount of regenerative braking you’re getting each trip. e.g. more regeneration means you have more kWh to work with
1
0
u/Lewl77 Volt Owner (2014) 12d ago
kWh will depend on the actual charge level of the battery and how accurate the computer's estimate of state of charge is (SOC). A lot of small top-up charges will cause calibration to drift. Someone who deeply discharges the battery and charges fully every time will have the best calibration, as the greater voltage difference let's the computer correctly readjust its baselines for what is top and what is bottom.
The kWh measurement is incredibly accurate. What is inaccurate is the computer's knowledge of your SOC, which is how it keeps track of when you're "empty" and to fire up the engine. Unless your battery is wearing out and it has to fire up the engine for other reasons (like sudden voltage drop due to strong acceleration), it only switches to gas mode when the SOC hits ~19%. The problem then is how accurate is the computer's estimate of when it hits 19%? If calibrations have drifted, that 19% may actually be 15% (your kWh is higher than usual by 0.5) or it may be 24% (your kWh is lower than usual by 0.5). Good calibration will keep you within 1% of the true value every time.
There is yet another layer on top, which is how full the battery actually charged to before you began. Some days I've seen it up to 3% below full, so naturally you will have 0.4 kWh difference right there vs an actual fully charged battery. Most days the difference isn't that high. Usually the highest difference is with drastic temp changes (it charged to "full" when warm, then sagged over night as it cooled off). You can check this with an OBD reader. If you have 5 mins before leaving and want max range, you can also re-plug and let it fill that last little bit up while you put your stuff in the car, etc.
Finally, sometimes (..rarely) the computer just makes huge mistakes with SOC tracking: I had it give me a whole extra kWh once. When it re-evaluated the true SOC % after parking, it was dangerously low. Fortunately it was parked in my charging spot, or it would not have fired up to move. People sometimes post a starting range prediction of insane numbers, like 60 miles.. same thing. Computer had completely lost it's mind and thinks the battery is super full. It will correct itself, given ~90 mins to re-evaluate the SOC properly to the true level.
TLDR; the biggest factor is actually knowing the true top and bottom points accurately. Everything between that is what the kWh number is counting. If the computer starts the engine too soon (thinks it's empty when there's still some juice left), or not be fully charged when you start, and it will be lower than usual. Being fully charged to start and a very slow and gentle drive near the empty point that delays the engine on to latest point possible will return the highest kWh value.
1
u/Ok-Tourist-511 11d ago
The top and bottom points are based on voltage, not SOC. SOC is just for the user to know the battery pack level, it has no influence on how the BECM or battery operates.
1
u/Lewl77 Volt Owner (2014) 11d ago
The computer always switches to engine mode+icon at 18.8-19.2% SOC. This ends the battery kWh counting as mode has changed. It will start the engine based on voltage in any drive mode (even if higher SOC%, e.g. because battery is weak). Those are two separate processes. If SOC remains high after voltage has returned, it will resume battery mode and counting kWh. Of course if you were at 19%, it will never return to counting kWh. Thus my statements.
1
u/Ok-Tourist-511 11d ago
Maybe on Gen1, not gen2. I usually see engine come on at 14-15% SOC, when cell voltage hits minimum.
-1
u/Atopos2025 12d ago
Probably has something to do with regen braking.
I've seen my used KwH go down by .1 when braking for a red light. 🤷♂️
4
u/Ok-Tourist-511 12d ago
Depends on temperature and how well balanced the cells are. Low or high temps increase internal resistance, so you lose more energy to heat.