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Limits of Model S charging ?

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Kevin Harney

Active Member
Apr 30, 2008
2,052
3
Herndon, VA
#1
We started with Superchargers with 10 chargers in them and giving 90 kWh. then we went up to 12 chargers that are giving 135 kWh. Why not 15-20 chargers yielding over 200 kWh ? How fast can the car take a charge before it exceeds the recommended c rate ? and why not just go straight to that level instead of increasing incrementally ?

At the max c rate how many miles/hour are gained ?

Please excuse me if the above terminology is incorrect but I think you get the idea :cool:
 
Last edited:
#2
Kevin Harney said:
We started with Superchargers with 10 chargers in them and giving 90 kWh. then we went up to 12 chargers that are giving 135 kWh. Why not 15-20 chargers yielding over 200 kWh ? How fast can the car take a charge before it exceeds the recommended c rate ? and why not just go straight to that level instead of increasing incrementally ?

At the max c rate how many miles/hour are gained ?

Please excuse me if the above terminology is incorrect but I think you get the idea :cool:
Click to expand...

Tesla runs its batteries up to 4C. Using that as a theoretical limit, they would be able to fill the battery in ( 60 / 4 ) x {usable fraction} minutes, at a rate of 240kW for the 60kWh battery and 340kW for the 85kWh battery.

Tesla's research target is 5 minute charging, but I don't know that that's charging to full. 5 minutes charging would require a C rate of {usable fraction target} x 12, or {usable fraction target} x 720 kW/1020kW for 60kWh/85kWh batteries.
 
#3
ItsNotAboutTheMoney said:
Tesla runs its batteries up to 4C. Using that as a theoretical limit, they would be able to fill the battery in ( 60 / 4 ) x {usable fraction} minutes, at a rate of 240kW for the 60kWh battery and 340kW for the 85kWh battery.
Click to expand...

So theoretically the car could charge in 15 minutes (less time for taper and heating and such) so maybe 20-30 per fill is the max rate. Why not triple the capacity of a Supercharger ? especially seeing the possibility of a 100+ kWh battery in the Model X ? and knowing that each supercharger cabinet could actually be charging 2 cars at once ?
 
#4
ItsNotAboutTheMoney said:
Tesla runs its batteries up to 4C. Using that as a theoretical limit, they would be able to fill the battery in ( 60 / 4 ) x {usable fraction} minutes, at a rate of 240kW for the 60kWh battery and 340kW for the 85kWh battery.

Tesla's research target is 5 minute charging, but I don't know that that's charging to full. 5 minutes charging would require a C rate of {usable fraction target} x 12, or {usable fraction target} x 720 kW/1020kW for 60kWh/85kWh batteries.
Click to expand...

But max discharge and max charge rates are different. I really don't think it can handle 4C charge; 135 kW is the max, for now, for Model S and I'm pretty sure Musk mentioned this himself. But I'm sure that could go up to 150 or something with new batteries in 2 yrs. maybe. I always think that R/C hobby batteries (li-po) are already capable of 10C+ charge rates. That's a full charge in like 10 min. or less. I think Tesla should build some batteries capable of that, just for demonstration - or as an option. That would come at the cost of decreased capacity however, and of course lack of any 1MW chargers. :)
 
#5
Even if the engineering problems were solved, Tesla has said before that they're concentrating on expanding the network as fast as possible. I'm sure they'd rather spend their money putting in more SCs at an adequate charging rate than fewer sites at maximum charging rate.
 
#7
Beckler said:
But max discharge and max charge rates are different. I really don't think it can handle 4C charge; 135 kW is the max, for now, for Model S and I'm pretty sure Musk mentioned this himself. But I'm sure that could go up to 150 or something with new batteries in 2 yrs. maybe. I always think that R/C hobby batteries (li-po) are already capable of 10C+ charge rates. That's a full charge in like 10 min. or less. I think Tesla should build some batteries capable of that, just for demonstration - or as an option. That would come at the cost of decreased capacity however, and of course lack of any 1MW chargers. :)
Click to expand...

Even if we use a 3C rate that is still a 250 kWh charger which is double. Charging two cars at a time is quadruple. Anyone know the actual recommended C rate for charging these batteries ?
 
#8
Cables and inlets is a hughe issue if "just" trying to scale from 135 kW to 270 kW. Heat also will be a big problem.

Also as many have pointed out typical LiIon chemistries have a max C rate when discharging that is some 3-4 times bigger than maximum charging C rate.
 
#9
But don't forget that a charger stack is shared between two cars so increasing the capacity of the stack also means increased charging rate for the "second car". I'm not sure it would make that much difference in total charge time, though.

I agree with the sentiment - "put in more SCs, not beef up what we already have". At least until the system is more fully built out.
 
#10
PhilBa said:
But don't forget that a charger stack is shared between two cars so increasing the capacity of the stack also means increased charging rate for the "second car". I'm not sure it would make that much difference in total charge time, though.

I agree with the sentiment - "put in more SCs, not beef up what we already have". At least until the system is more fully built out.
Click to expand...

I am totally with that thought. more chargers is way more important right now. Just wondering the limits possible. Perhaps an easy increase would be one cabinet per charger instead of one per two chargers. ie 8 stalls = 8 cabinets instead of 4.
 
#11
I think Telsa will use two ways of minimizing the "inconvenience" of waiting at a Supercharger site. The first is to build out until there are Superchargers every 40-50 miles on all major roadways. If you go above 150kw you increase exponentially the cost of electrical equipment, wire, and the labor for installation. Second is to get the battery pack up to at least 400 miles. Then on a road trip you would stop once, and it would be a meal break where you were going to spend some time anyway. Problem solved.
 
#12
S-dog said:
I think Telsa will use two ways of minimizing the "inconvenience" of waiting at a Supercharger site. The first is to build out until there are Superchargers every 40-50 miles on all major roadways. If you go above 150kw you increase exponentially the cost of electrical equipment, wire, and the labor for installation. Second is to get the battery pack up to at least 400 miles. Then on a road trip you would stop once, and it would be a meal break where you were going to spend some time anyway. Problem solved.
Click to expand...

Sort of - A larger pack would take longer to fill at the same rate. This is also my question as to the limits. If we can get a 150 - 250 kWh battery eventually it needs to fill faster for the stop time to be reasonable. Interesting trade off.
 
#13
S-dog said:
I think Telsa will use two ways of minimizing the "inconvenience" of waiting at a Supercharger site. The first is to build out until there are Superchargers every 40-50 miles on all major roadways. If you go above 150kw you increase exponentially the cost of electrical equipment, wire, and the labor for installation. Second is to get the battery pack up to at least 400 miles. Then on a road trip you would stop once, and it would be a meal break where you were going to spend some time anyway. Problem solved.
Click to expand...

I think that the current wiring from the Supercharger Cabinets to the pedestals, the connector from the pedestal to the car, and wiring inside the car may all be limited to 333 Amps by wire sizes, etc.

Look at the CHARGING POST CIRCUIT SCHEDULE on page E-1 of Madison, WI Supercharger Application. In Madison, they used 250 mcm copper wire from the Supercharger Cabinets to the Pedestals (Charging Posts). 333 Amps is already pushing the limits for that size wire.

360 Volts is the approximate pack Voltage in an 85 at low State of charge when the most power can be accepted. 333 Amps * 360 Volts is 120 kW; for a 60, that scales to 105 kW. The real, peak charging rate to a car in today's Superchargers appears to be the wires and connectors used. As S-dog points out, scaling up from these levels, the costs rise rapidly.

For today's Superchargers, max charging power to a single car appears to be limited to 120 kW for an 85 and 105 kW for a 60 by the wiring used, independent of the batteries.
 
#14
(LMB spouse)

OP, also note that there is no "hard and fast" max C rate. It varies with temp, state-of-charge, and desired battery lifetime. Most hobby and power tool applications are not worried about maximizing number of charge cycles while minimizing battery capacity degradation. Tesla is. Their approach may be conservative, but I suspect they will not increase charging current or decrease charging taper unless they have a lot of data showing it won't hurt battery life. Within reason, battery life is the most important factor given current Li-ion technology.
 
#15
Kevin Harney said:
Sort of - A larger pack would take longer to fill at the same rate. This is also my question as to the limits. If we can get a 150 - 250 kWh battery eventually it needs to fill faster for the stop time to be reasonable. Interesting trade off.
Click to expand...

However it also means that the charge-current taper wouldn't kick in as soon... so with a theoretical 200kWh pack, you could be looking at the charger operating at max power for longer, hence more range in the same time (after a certain point).
 
#16
scaesare said:
However it also means that the charge-current taper wouldn't kick in as soon... so with a theoretical 200kWh pack, you could be looking at the charger operating at max power for longer, hence more range in the same time (after a certain point).
Click to expand...

Fair enough. Good point. So the current battery can fill to 60-70% in 30 minutes right ? How long to fill a 200kWh pack to 60-70% or what ever the taper point is ...
 
#18
Very roughly speaking, the taper slope starts to really become noticeable about 30 mins in on a 120kW charge so say after 60kW or so delivered or about 70-75% capacity.

On a 200kWh battery, you don't hit 75% until 150kWh delivered, so that means the same supercharger would be operating at maximum power for 80 minutes before taper... so while the 85kWh pack might just have finished up at max after that time, you'd have nearly twice that amount for the larger pack.
 
#19
Kevin Harney said:
Fair enough. Good point. So the current battery can fill to 60-70% in 30 minutes right ? How long to fill a 200kWh pack to 60-70% or what ever the taper point is ...
Click to expand...

scaesare said:
Very roughly speaking, the taper slope starts to really become noticeable about 30 mins in on a 120kW charge so say after 60kW or so delivered or about 70-75% capacity.

On a 200kWh battery, you don't hit 75% until 150kWh delivered, so that means the same supercharger would be operating at maximum power for 80 minutes before taper... so while the 85kWh pack might just have finished up at max after that time, you'd have nearly twice that amount for the larger pack.
Click to expand...

That's what I was looking for. Thanks. Does that also mean that if you had an 85 kWh battery and a 200 kWh battery and you stopped at the same supercharger for 30 minutes that you would get the same miles for both cars ?
 
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