[mzzyhmd]

Hi,
I’ve been making a mile long trip (both ways) recently to my kids daycare twice a day. And my consumption efficiency is extremely poor. Even though I’m only using a little bit of AC meaning never using it in full power and etc.

my question is when I do make a longer trip to work like 21 miles each way I only use 6 percent battery given only AC for first few mins then turned off. But these short trips to my baby’s daycare killed almost 6-7 percent in a day! Really? What gives? I’m still using B mode and when I got her back home yesterday I saw 68 percent this morning all on a sudden I’m seeing 63 percent? I thought for these short trips it will barely take a percentage off per day. Does it only become more efficient when the battery starts running at optimal temperature? I’m baffled and confused.


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[Bubbajet]

The initial energy cost of getting going (I think mostly due to cooling/heating) is substantial. It'll be worse if it's really hot (over about 95F I think?) and the car needs to cool the battery, and worse than that if it's cold (below 60F or so) and the car needs to heat the battery.

Your ICE car did this as well, you just didn't notice.

 

[mochalatte]

The reported SOC of an EV as much of a guess as the GOM range. Unlike ICE gas fuel gauges that directly measures the tank level with a float attached to a potentiometer, the SOC is indirectly estimated as part of the car's BMS system. You'll find scholarly papers and active research in the quest to estimate SOC for EV batteries. I'm sure this is a trade secret matter for VW and the other EV manufacturers trying to accurately estimate SOC.

And as a counter-point to your experience, I've seen my SOC actually increase from 70% to 71% on a 2.8 mile drive to an appointment, then only fall back to 70% on the return trip. When I plugged the car back in to the charger at home, the car didn't even charge. If you watch closely, I bet you'll eventually see a similar increase in SOC under the right conditions in your own driving.

 

[UWSP]

Hi,
I’ve been making a mile long trip (both ways) recently to my kids daycare twice a day. And my consumption efficiency is extremely poor. Even though I’m only using a little bit of AC meaning never using it in full power and etc.

my question is when I do make a longer trip to work like 21 miles each way I only use 6 percent battery given only AC for first few mins then turned off. But these short trips to my baby’s daycare killed almost 6-7 percent in a day! Really? What gives? I’m still using B mode and when I got her back home yesterday I saw 68 percent this morning all on a sudden I’m seeing 63 percent? I thought for these short trips it will barely take a percentage off per day. Does it only become more efficient when the battery starts running at optimal temperature? I’m baffled and confused.


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Agree, battery life has been very erratic with our vehicle as well ... both on short trips and longer trips. When weather gets cold it may be a big problem.

 

[Froggy]

Yes I am about to make a one mile trip , to be followed by a 50 mile one. Checking the efficiency should be interesting.

 

[xXChartmanXx]

I'm finding it really depends on how you drive it too. Just took a 56 mile drive (112 mile round trip). Had 55% battery at the start. Estimated 135 miles of range when I started. Almost all highway driving with light traffic averaging between 65 - 75 mph. Arrived with 81 miles range estimated at 28% of battery (So basically used 54 miles of range to go 56 miles). We charged up at a EA charger after dinner and headed back. Left at 80%, 221 miles estimated. No traffic and everyone around me was pushing me at 80 so we averaged 80-85 mph all the way home. When I got home it was at 131 miles of range left 54%. Did it really eat almost 90 miles of range to go the same distance do to the speed difference? Seems like a steep fall off once you hit 80 mph.

 

[Islander]

The reported SOC of an EV as much of a guess as the GOM range. Unlike ICE gas fuel gauges that directly measures the tank level with a float attached to a potentiometer, the SOC is indirectly estimated as part of the car's BMS system. You'll find scholarly papers and active research in the quest to estimate SOC for EV batteries. I'm sure this is a trade secret matter for VW and the other EV manufacturers trying to accurately estimate SOC.

Wow. I rely on battery meters every day and never gave a moment's thought to how they work or how accurate they are. Now at long last I want to know more. I read the paper you cited (well TBH, I skimmed half of it) and googled more papers, and they all seem highly technical and theoretical. Reading in-between the lines of what you wrote, it sounds like we don't know much about how any BEV estimates SOC. So ... we might figure that the displayed SOC might be off by as much as 3-4% under normal circumstances? And that the same factors that introduce large errors in the GOM (such as changes in driving speed, weather and elevation) could introduce additional errors in reported SOC? Are these same inaccuracies present at extremes of the SOC scale (determining whether the battery is at 100% or 0%)? Do you know of anything about this written for the poets among us?

 

[mlsstl]

Wind resistance increases rapidly with speed -- that's why airplanes need to be going a certain minimum speed in order to take off & fly. The air flowing over the wings is what creates lift. At higher speeds, air resistance has a squared relationship with speed.

 

[Jimspb]

I'm finding it really depends on how you drive it too. Just took a 56 mile drive (112 mile round trip). Had 55% battery at the start. Estimated 135 miles of range when I started. Almost all highway driving with light traffic averaging between 65 - 75 mph. Arrived with 81 miles range estimated at 28% of battery (So basically used 54 miles of range to go 56 miles). We charged up at a EA charger after dinner and headed back. Left at 80%, 221 miles estimated. No traffic and everyone around me was pushing me at 80 so we averaged 80-85 mph all the way home. When I got home it was at 131 miles of range left 54%. Did it really eat almost 90 miles of range to go the same distance do to the speed difference? Seems like a steep fall off once you hit 80 mph.

Speed is factored in as "wind" resistance. During a drive it is really the only variable that we control that has a significant impact on range. Since wind resistance increases as a squared function driving at 75 has more than twice the resistance as driving at 50. Sure there may be different drive system efficiencies at different speeds and wind resistance may not be perfectly linear but those are relatively minor effects. Go slower, go further.

 

[mochalatte]

Reading in-between the lines of what you wrote, it sounds like we don't know much about how any BEV estimates SOC.

I'm not in the industry or anything, but the very fact that it is hard to estimate SOC accurately together with the fact that it's so important probably makes SOC estimation techniques something that the manufactures see as a competitive advantage and will keep their cards close to the chest.

So ... we might figure that the displayed SOC might be off by as much as 3-4% under normal circumstances? And that the same factors that introduce large errors in the GOM (such as changes in driving speed, weather and elevation) could introduce additional errors in reported SOC?

The error bars on SOC would be smaller than the error bars on the GOM since SOC is only one of the components of calculating GOM range. The things you listed such a changes in speed, weather, and elevation are additional sources of error for calculating range on top of the SOC uncertainty. Temperature probably affects the accuracy of SOC, but it has an even larger effect on the GOM range estimation given the need for climate control in the car.

Do you know of anything about this written for the poets among us?

A bit more googling turned up this slightly easier survey:

The State of Charge Estimating Methods for Battery: A Review

An overview of new and current developments in state of charge (SOC) estimating methods for battery is given where the focus lies upon mathematical principles and practical implementations. As the battery SOC is an important parameter, which reflects the battery performance, so accurate...

www.hindawi.com

It might not be at the "for poets" level, but it gives more flavor on the challenge of estimating SOC. Maybe someone can find a good YouTube on the subject?

 

[xXChartmanXx]

Speed is factored in as "wind" resistance. During a drive it is really the only variable that we control that has a significant impact on range. Since wind resistance increases as a squared function driving at 75 has more than twice the resistance as driving at 50. Sure there may be different drive system efficiencies at different speeds and wind resistance may not be perfectly linear but those are relatively minor effects. Go slower, go further.

Good to know. So when I'm trying to get more miles out of the drive, slow down, steer into the wind (lol), and keep it steady. If I don't care about chewing Kwhr's crank the speed!

 

[Islander]

It might not be at the "for poets" level, but it gives more flavor on the challenge of estimating SOC.

LOL, not QUITE for poets! But very helpful. Thanks so much for this information.

 

[Nai3t]

Are these same inaccuracies present at extremes of the SOC scale (determining whether the battery is at 100% or 0%)?

Batteries have "tells" at the extremities that give a better indication of empty and full. But there's still a lot of math and coloumb counting going on because voltages aren't ready to measure actually when the battery is under load or hot .

But I'm sure as you've read by now, at least it's something compared to the extremely flat discharge curve for most of the middle range.

Also consider that it's not just a monolithic battery, it's a connection of cells all with slightly different discharge characteristics and can just be measured "as one."

 

[mzzyhmd]

So basically nobody noticed any significant decrease in SOC over a short distance using AC? If so maybe mine has a defect. Or AC consumes 10-20 percent battery than regular 5 percent as people claim. I’m seeing 1-2 percent decrease like even sitting in car using AC. This is sad that technology doesn’t allow to run a mandatory component like AC without compromising battery life significantly


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[Nai3t]

I wouldn't look at it that way. If you have a hot room (or cold, take your pick) you want to get it to a comfortable temperature quickly, more electricity will be needed to change the temperature, and once that's reached a set point, it takes less energy to maintain.

Likewise, if you were to start driving on a cold battery, the cell chemistry reacts less efficiently than if it were at temp. As the car is driven, the battery cones up to temp and becomes more efficient (although a temperature is reached where active cooling kicks on, another electrical draw).

I wouldn't look at it as "sad," it just is. But also, if you're watching SOC like a hawk, you're probably going to go crazy. The engineers can either unleash their algorithms and let them do their thing, or they can artificially "freeze" them if they want to project the illusion of stability and. (They're probably already doing a mix of this.)

If be more interested in how many miles are driven under wider range of SOC to evaluate the well being of a battery.

 

[SunWizard]

I’m seeing 1-2 percent decrease like even sitting in car using AC.

This is because the GOM makes a lower guess for your range as soon as you turn on the AC. Turn AC off, your GOM soon goes back up.

 

[mzzyhmd]

This is because the GOM makes a lower guess for your range as soon as you turn on the AC. Turn AC off, your GOM soon goes back up.

I meant decrease in SOC


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[ChristophW]

So basically nobody noticed any significant decrease in SOC over a short distance using AC? If so maybe mine has a defect. Or AC consumes 10-20 percent battery than regular 5 percent as people claim. I’m seeing 1-2 percent decrease like even sitting in car using AC. This is sad that technology doesn’t allow to run a mandatory component like AC witho

Naw, it may just be conditions of your short drive. I do a lot of similar short drives with very little impact on the state of charge. But around here it is very rarely above 95 F, at which point battery cooling would set in. Also, I park my car in the garage, not over hot pavement, so the battery is reasonably cool. At my destinations, I can often find a slightly shady parking spot too. That battery cooling onset is an expensive operation that will surely eat a few kWh, and if my circumstances were different I'd certainly see a significant impact on the state of charge. So my advice is to not worry too much for now.

All that said, my long-term efficiency is significantly lower than other people's at 3.1 mi/kWh which is very definitely due to the frequent short drives. And when we drive a little further it is at 75+, again hitting efficiency. I don't worry about this, because my electricity comes free of charge (pun intended) from solar panels.

 

[SunWizard]

I meant decrease in SOC

SOC is also just a guess, like the GOM, so can jump around due big A/C loads.

 

[mzzyhmd]

SOC is also just a guess, like the GOM, so can jump around due big A/C loads.

Running AC for 10 mins to cool the cabin can cause 2-3 percent battery loss I’m baffled


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