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Does it cost more to charge the last 20% of the battery?

heysteveh

Member
Apr 3, 2016
178
155
Peoria, AZ
I have a Model 3 and charge at home. I know that when charging, as the battery gets closer to 100% it takes longer to charge, e.g. going from 80% charge to 100% takes a lot longer then, say, from 40% to 60%. My question is does this mean that it will also take more electricity to go from 80% to 100% charge, thus costing more money per Kw hr to charge?
 

SSonnentag

Rocket Scientist
Apr 11, 2017
1,711
2,166
Arizona
Short answer, no.

I suspect that since the charge rate slows at the top end, and less heat is produced, that the efficiency may actually go up a smidge. But for all practical purposes, unless you're charged by the minute, it won't cost you more.
 

TT97

Active Member
Aug 6, 2017
2,168
2,896
Los Angeles
I have a Model 3 and charge at home. I know that when charging, as the battery gets closer to 100% it takes longer to charge, e.g. going from 80% charge to 100% takes a lot longer then, say, from 40% to 60%. My question is does this mean that it will also take more electricity to go from 80% to 100% charge, thus costing more money per Kw hr to charge?

As the car reaches 100%, it will reduce the amps charging increasing the time it takes to charge. The car uses the same amount of kWh to go from 40% to 60% as it does to go from 80% to 100%.

There is some efficiency loss in charging that would be greater with the slower charge but we are talking pennies difference (possibly even less).
 
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heysteveh

Member
Apr 3, 2016
178
155
Peoria, AZ
Short answer, no.

I suspect that since the charge rate slows at the top end, and less heat is produced, that the efficiency may actually go up a smidge. But for all practical purposes, unless you're charged by the minute, it won't cost you more.

As the car reaches 100%, it will reduce the amps charging increasing the time it takes to charge. The car uses the same amount of kWh to go from 40% to 60% as it does to go from 80% to 100%.

There is some efficiency loss in charging that would be greater with the slower charge but we are talking pennies difference (possibly even less).

Thanks!! That answers my question.
 

darth_vad3r

Well-Known Sith
May 6, 2019
1,574
1,119
Canada
If you are charging at 11 kW you might notice a taper around 95%.
If you are charging at 6 kW it would be even later, like 97-ish?

Charging from 75-95% should be equal in time to charging from 50-70, 60-80, etc.

If you never charge past 95% you will not see a taper in the charging rate at home charging power levels. Supercharging is a different story.
 

darth_vad3r

Well-Known Sith
May 6, 2019
1,574
1,119
Canada
Short answer, no.

I suspect that since the charge rate slows at the top end, and less heat is produced, that the efficiency may actually go up a smidge. But for all practical purposes, unless you're charged by the minute, it won't cost you more.

I'd say short answer "no, below 95%, yes a bit above 95%."

And strictly speaking the DC charge-only efficiency may go up, but not the "charge efficiency" overall when you include all the AC power coming out of the wall ... because ~250 W goes to powering "the car" just for being "on", and if you let that continue for an extra hour unneeded, that's an extra 0.25 kWh on your power bill.

5% of a 75 kWh battery is 3.75 kWh ... 0.25 kWh is 6.67% of that ... so I think it's hard/impossible to make up that ~7% loss with any increase in DC charging efficiency since the DC charging efficiency could already be around ~95%.
 

davewill

Active Member
Feb 5, 2014
1,821
1,964
San Diego, CA, US
Short answer, no.

I suspect that since the charge rate slows at the top end, and less heat is produced, that the efficiency may actually go up a smidge. But for all practical purposes, unless you're charged by the minute, it won't cost you more.
Actually it goes down a smidge. As the amperage goes down, the fixed overhead of the charging system (cooling mostly) remains the same, so that the efficiency declines. But it's a pretty small consideration as the taper doesn't happen until the very end.
 

heysteveh

Member
Apr 3, 2016
178
155
Peoria, AZ
If you are charging at 11 kW you might notice a taper around 95%.
If you are charging at 6 kW it would be even later, like 97-ish?

Charging from 75-95% should be equal in time to charging from 50-70, 60-80, etc.

If you never charge past 95% you will not see a taper in the charging rate at home charging power levels. Supercharging is a different story.

To be more specific I have a model 3SR plus and I am using the mobile connector that came with the car plugged into 14-50 NEMA outlet on a 50 amp circuit. So my car usually charges @ 32 amps. Hopefully I got all the terminology right . Thanks for the helpful reply!
 

darth_vad3r

Well-Known Sith
May 6, 2019
1,574
1,119
Canada
To be more specific I have a model 3SR plus and I am using the mobile connector that came with the car plugged into 14-50 NEMA outlet on a 50 amp circuit. So my car usually charges @ 32 amps. Hopefully I got all the terminology right . Thanks for the helpful reply!

Ya, so you are in the ~6 kW camp ... you definitely won't see a taper at 95% ... it might be 97 or 98% until you see any taper.

240 V x 32 A = 7.68 kW to be precise, but you probably have some voltage drop, and anyways it's close enough ... if you look at the supercharger profiles posted in the threads here (by @Zoomit) you can follow the taper down and try to eyeball where 7-6 kW is ... it's well past the 95% point. Well, maybe not "well" past ... but it's definitely after 95, not before.
 

darth_vad3r

Well-Known Sith
May 6, 2019
1,574
1,119
Canada
it only costs you more if you are supercharging at a supercharger that is on a per minute based pay method.

If you are charging at home and spend more time to add the same amount of kWh to the battery, it costs you more than if you spend less time to add the same amount of kWh to the battery (because the car stays 'on' the whole time you are charging, and the power to do that (~0.25 kW) comes from the wall too).
 

SigNC

Active Member
Aug 23, 2017
1,465
1,275
NC
If you are charging at home and spend more time to add the same amount of kWh to the battery, it costs you more than if you spend less time to add the same amount of kWh to the battery (because the car stays 'on' the whole time you are charging, and the power to do that (~0.25 kW) comes from the wall too).

Sure, the care can charge pretty much at the max home charging rate up till 99% or close to it. I charge at 240/40amp and i don't recall a taper earlier than 99. (granted I don't charge that high much) So only that last little bit is it slowing down much and if they are charging at a slower rate it would be exactly the same.
 

darth_vad3r

Well-Known Sith
May 6, 2019
1,574
1,119
Canada
Ya, so you are in the ~6 kW camp ... you definitely won't see a taper at 95% ... it might be 97 or 98% until you see any taper.

240 V x 32 A = 7.68 kW to be precise, but you probably have some voltage drop, and anyways it's close enough ... if you look at the supercharger profiles posted in the threads here (by @Zoomit) you can follow the taper down and try to eyeball where 7-6 kW is ... it's well past the 95% point. Well, maybe not "well" past ... but it's definitely after 95, not before.

re: SR+ supercharging profile charts by @Zoomit , this was the first one I found here from May (not sure if there are any newer ones?): 150kW Supercharging for Model 3
20190511-3sr-v2-chrg-png.406671


The orange dotted line looks like it gets to about 7 kW only after 96%.
10-11 kW looks closer to 94%.

These may have been non-ideal conditions, and the grey line is higher because it's a more optimistic projection. Charging slowly at home on L2 is a lot more ideal than Supercharging at crazy high power, so I'd expect home L2 charging to be closer to whatever the actual ideal supercharger profile is at the high end above 94%. Unless of course other factors like very cold or hot ambient temperatures come into play.
 

darth_vad3r

Well-Known Sith
May 6, 2019
1,574
1,119
Canada
Sure, the care can charge pretty much at the max home charging rate up till 99% or close to it. I charge at 240/40amp and i don't recall a taper earlier than 99. (granted I don't charge that high much) So only that last little bit is it slowing down much and if they are charging at a slower rate it would be exactly the same.

Which trim? LR I suppose if 40A. At nearly 10kW the charts above seem like you should see a taper after 96% (ideal profile) on the SR+ anyways. I think the LR plus is similar though, due to everything wrt limits usually being SoC%-related. Could be different though.
 

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