Limits of Model S charging ?

[kennybobby]

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 ?

Panasonic data sheet for the NCR 18650B 3400mAh cells indicated 0.5C, although their generic data indicated 0.3 to 0.7C for the cylindrical cells depending upon model.

So for an 85kWh pack with 96 CELLS (modules) of 74 cells in parallel that would be ~126 Amps. Not sure how many cells in parallel in a 60 kWh pack...

 

[Kevin Harney]

Panasonic data sheet for the NCR 18650B 3400mAh cells indicated 0.5C, although their generic data indicated 0.3 to 0.7C for the cylindrical cells depending upon model.

So for an 85kWh pack with 96 CELLS (modules) of 74 cells in parallel that would be ~126 Amps. Not sure how many cells in parallel in a 60 kWh pack...

English please !! :biggrin: Does that mean they have a charging rate of 2C ? Which would be max of 170 kWh charger ? How does that relate - mathematically- to the 360 volts above and the 333 amps above

 

[kennybobby]

English please !! :biggrin: Does that mean they have a charging rate of 2C ? Which would be max of 170 kWh charger ? How does that relate - mathematically- to the 360 volts above and the 333 amps above

Charging rate 0.5C is 0.5 x C = one-half of the C. C = 3.4 Amp-Hrs (3400 mAH), therefore charge rate is 1.7 Amps per cell times 74 cells in parallel = 126 Amps. The 85 kWh pack has 96 of these parallel modules in series to give a maximum voltage of 96 x 4.15 V per cell = 398.4 Volts when full.

The 360 volts was for a depleted pack needing to be charged (@3.75 V per cell). The 333 Amps was based upon the max current that could be carried by the gauge of wire--it is only how much the wires could handle, not related to how much the cells can handle. The wires in the charger obviously need to be oversized for the task.

i'm thinking that a 60 kWh pack still has 74 cells in parallel so it's charge current would be the same as the 85 pack. Difference being in max voltage: 84s vs 96s.

These are assuming that the pack Depth of Discharge is only using 85% of the total available--not fully charging to the top and not totally discharging all the way to the bottom. The cells could go to 4.2 V, but Tesla limits to 4.15 at the top. There is also a margin at the bottom--the pack is not totally drained when you hit 0 miles, hopefully the controller is set up to not let you damage the pack by going too low. Panasonic uses 2.5 V as the low voltage cutoff in their datasheets.

 

[Kevin Harney]

Charging rate 0.5C is 0.5 x C = one-half of the C. C = 3.4 Amp-Hrs (3400 mAH), therefore charge rate is 1.7 Amps per cell times 74 cells in parallel = 126 Amps. The 85 kWh pack has 96 of these parallel modules in series to give a maximum voltage of 96 x 4.15 V per cell = 398.4 Volts when full.

The 360 volts was for a depleted pack needing to be charged (@3.75 V per cell). The 333 Amps was based upon the max current that could be carried by the gauge of wire--it is only how much the wires could handle, not related to how much the cells can handle. The wires in the charger obviously need to be oversized for the task.

i'm thinking that a 60 kWh pack only has 52 cells in parallel so it's charge current would be 52 x 0.5 x C (3.4AH) = 88 Amps.

These are assuming that the pack Depth of Discharge is only using 85% of the total available--not fully charging to the top and not totally discharging all the way to the bottom. The cells could go to 4.2 V, but Tesla limits to 4.15 at the top. There is also a margin at the bottom--the pack is not totally drained when you hit 0 miles, hopefully the controller is set up to not let you damage the pack by going too low. Panasonic uses 2.5 V as the low voltage cutoff in their datasheets.

Well done. That helps but what I am still missing is link between volts, amps and kWh of charging, and kWh of battery.

 

[scaesare]

Well done. That helps but what I am still missing is link between volts, amps and kWh of charging, and kWh of battery.

Power (watts or W) = current (Amps or A) X Voltage (V).

You can't arbitrarily charge at a voltage above what the pack can take. The pack in the S is wired in an arrangement with a lower-end voltage of ~300v maximum top end voltage of ~400V (as kennybobby points out above).

Divide the charge power delivered by the voltage it is operating at to get the current that will be delivered: 120kW / 360 V = 333A

Or... take the voltage and current the charger is operating at (visible on the center console screen when supercharging)and determine power being delivered: 400V x 250A = 100kW

Thus you can think of the pack as one large 350V (nominal) battery that can deliver ~250A for an hour.. for roughly 85kWh of capacity.

Thus any "C" comparisons are against that total capacity. Dumping 85kW in to it from a supercharger would be a 1C charge rate. A 1.5C rate would equate to delivering 127.5kW to it, etc...

 

[Kevin Harney]

Power (watts or W) = current (Amps or A) X Voltage (V).

You can't arbitrarily charge at a voltage above what the pack can take. The pack in the S is wired in an arrangement with a lower-end voltage of ~300v maximum top end voltage of ~400V (as kennybobby points out above).

Divide the charge power delivered by the voltage it is operating at to get the current that will be delivered: 120kW / 360 V = 333A

Or... take the voltage and current the charger is operating at (visible on the center console screen when supercharging)and determine power being delivered: 400V x 250A = 100kW

Thus you can think of the pack as one large 350V (nominal) battery that can deliver ~250A for an hour.. for roughly 85kWh of capacity.

Thus any "C" comparisons are against that total capacity. Dumping 85kW in to it from a supercharger would be a 1C charge rate. A 1.5C rate would equate to delivering 127.5kW to it, etc...

So given that I am getting that an 85kWh battery would charge in 42 minutes from a 120kWh charger (less losses for heat and taper). But if the battery should not be charged at greater than 0.5C how come we are not limited to a 42.5 kWh charger ?

 

[widodh]

Some interesting information here: [POLL] Would you prefer a bigger battery or less SuperCharger tapering?

 

[scaesare]

So given that I am getting that an 85kWh battery would charge in 42 minutes from a 120kWh charger (less losses for heat and taper). But if the battery should not be charged at greater than 0.5C how come we are not limited to a 42.5 kWh charger ?

0.5C is just an example given above. It's not a rule for all cell chemistries, and there are other factors (such as active cooling, max charge point, etc..), so Tesla is comfortable charging at slight more than 1.5C for some portion of the charge curve.

 

[David99]

Elon said at a meeting in Europe that the charge rate of the Superchargers is actually conservative to make sure the battery lasts longer. It could be charged faster but they don't want to sacrifice battery life. The limitations are in many parts. The wiring on the Supercharger and car. But also cooling the battery while charging. The car's AC runs at max a few minutes into the charging session to cool the battery. If you just double the rate you get 4 times the ohmic losses creating 4 times as much heat.

I remember a Tesla guy say batteries could theoretically be charged within minutes if properly cooled. The losses would be huge and require very powerful cooling. It all comes at a cost. Putting such a powerful cooling system into the battery adds significant weight and size.

 

[kennybobby]

The 0.5C charging is the basic standard recommended by Panasonic, but is not a maximum or limit on the charge rate capability of the cells. Faster charging is possible as long as the cell temperature limits are not exceeded. The standard charging protocol runs at Constant Current (CC) at 0.5C until the cell voltage hits 4.2, then holds Constant Voltage (CV) at 4.2 and continues charging until the current tapers down to 0.1C. This will provide fully charged cells over a certain number of charge/discharge cycles and last a given length of time.

Another protocol was developed (Tesla's) to charge at a faster rate for a shorter time, reduce the max voltage (4.15), start the taper sooner and end charging sooner. This can reach an effectively fully charged cell state in a shorter time, and it may be that it doesn't reduce the overall lifetime of the battery either. They offer a long battery warranty period so they must be confident that their protocol does no harm.

 

[Cottonwood]

With existing car and Supercharger design, the maximum charging limits are more determined by maximum current than by the batteries.

Maximum charging power reported on 85's is about 120 kW or 120kW/85kWh=1.41/hr or 1.41C for the 85; for a 60, that is 105kW/60kWh=1.75/hr or 1.75C for the 60. That is a pretty big difference for the same basic cells.

Looking into more detail, the 85 battery pack is put together as a serial connection of 96 sets of 74 cells in parallel (16 modules of 6s x 74p). The 85 battery has an approximate, very low state of charge Voltage of 360 Volts, and 360 Volts * 333 Amps is 120 kW.

The 60 battery pack is a serial connection of 84 sets of 60 cells in parallel (14 modules of 6s x 60p). The 60 battery has an approximate, very low state of charge Voltage of 315 Volts, and 315 Volts * 333 Amps is 105 kW.

It really looks like the design limit of the wiring and connectors is 333 Amps. For an economic, cost-effective design, this design limit probably applies to the wiring and connectors from the Supercharger Cabinet to the Battery.

The existing chargers have a max output Voltage of about 400 Volts, so increasing battery Voltage would mean a lot of Supercharger retrofits. If the battery capacity were increased so that the battery could accept the 333 Amps for longer and to a higher Voltage, the charge rate could increase slightly before the taper started.

This appears to be strong evidence that the existing Superchargers and/or the existing cars are limited to a maximum charging current of 333 Amps by wire and connector design. Further, the existing chargers appear to be limited to a maximum battery Voltage of 400 Volts. Even if the maximum current could be used at the maximum Voltage (very difficult) that creates a limit of 133 kW. With the existing Model S, that is about 443 rated miles per hour, only slightly more than the existing 120 kW, 400 rated miles per hour limit.

 

[Matias]

Another protocol was developed (Tesla's) to charge at a faster rate for a shorter time, reduce the max voltage (4.15), start the taper sooner and end charging sooner. This can reach an effectively fully charged cell state in a shorter time, and it may be that it doesn't reduce the overall lifetime of the battery either. They offer a long battery warranty period so they must be confident that their protocol does no harm.

Capacity degradation is not covered in warranty.

 

[apacheguy]

@Cottonwood - How can the 70D pull 371 amps if the limit is 333 A? See #56

Supercharging the 70D - Page 6

 

[techmaven]

Since the wiring in the Superchargers are capable of so much more current, I wonder what are the costs of moving to 150 kW. I am looking forward to seeing if there are any Supercharging improvements to the 90 kWh packs.

 

[apacheguy]

Wonder what the current carrying limit is of the on board wiring that creates a direct connection from the charge port to the pack? Any ideas?

 

[Cottonwood]

@Cottonwood - How can the 70D pull 371 amps if the limit is 333 A? See #56

Supercharging the 70D - Page 6

Well, the 135 kW Superchargers are labeled as 330 Amps out, but folks have measured 371 Amps on a 70D. If an 85 could get that current, with the initial charge Voltage of 360 Volts, that would be 371 Amps x 360 Volts or 133.6 kW, pretty close to 135 kW...hmmm...