Consistent performance over MS battery SOC? Possibly a performance boost

[tom66]

I'm aware that the S has a power limit at very low SOC around 30 miles or so.

Is this a technical limitation or a way of conserving battery lifespan?

The MS P85 is rated ~310kW with a 400VDC peak battery. Technical documents suggest a maximum inverter current of 1,000A, and some sources suggest up to 1,200A for the highest performance inverter. To get 310kW from 400VDC only requires around 750A, so where's the discrepancy?

As the battery SOC drops the available voltage must also drop, there is a suggestion of a terminal voltage of 4.17V/cell (96 series module pack) giving 400.3VDC max. But I haven't found anyone specify a minimum battery voltage. If 3.2V/cell is assumed (and this is just a guess), this gives a minimum maximum 310kW operational point of ~307V (giving a little over 1,000A at peak power.)

The current then hits about 1,000A. So I figure below about 307V, the power limiter kicks in, mainly because the inverter cannot possibly put out as much power (but also possibly to conserve battery life, both long term and short term.)

I realised this would mean surely the inverter is more efficient at peak SOC than at any point. This brings an interesting idea... On a future performance model (Roadster II) could a "boost" option be added for when the battery is at ~60% or greater SOC? On a Model S, this could provide as much as 90kW additional power at full SOC. Great to get even faster 0-60 and quarter miles.

There would be no significant additional heat produced in the inverter, or battery, compared to the low-SOC situation of maximum power. The only substantial additional power loss would be in the motor, but that's not difficult to improve.

I think Tesla wouldn't do this mainly because it would make people believe the car isn't as good over its full SOC range, which is wrong. But I could see it being an option for a future Roadster, maybe an additional 15~20% performance could really help.

 

[VolkerP]

You can see the pack voltage versus SoC in the supercharger threads. Charging near empty starts around 360V for the 85kWh pack. The 85kWh pack is believed to have 96s x 74p cell configuration. So the minimum battery voltage is around 3.6V.

The power limit at low SoC is attributed to voltage drop. Maintaining same power level would mean draw higher currents from the pack, which produces more heat inside the cells due to internal resistance.

The "boost" function is already there, in that a full pack delivers more power to the inverter and loses less power to internal resistance than a half empty pack. Check the Roadster quarter mile drag times - power delivery depends on SoC and pack temperature.

Tesla's battery management is mostly about avoiding high temps in the pack.

 

[GSP]

People that took their Roadsters to the drag strip got slightly quicker times with a full charge. Important if you need to arrive a the finish line a foot or so sooner than the other guy, but otherwise not noticeable.

GSP

 

[stopcrazypp]

As others point out, higher SOC does already provide a slight power boost. Plus you are neglecting to factor in voltage sag under load. If you look a battery discharge curve, you will notice vs float voltage or low draw (0.2C, ~17kW for the 85kWh pack) a high draw (2C, ~170kW for the 85kWh pack) results in a sag of about 0.4V per cell.
http://www.panasonic.com/industrial/includes/pdf/ACA4000CE254-NCR18650A.pdf

 

[sambennett]

Hi Guys - I am a newbie and looking forward to know everything I can to take care of my Tesla. Quick question to you, what is SOC? and why should I worry about it?

(this is my first electric car!)

-Sam

 

[Jason S]

State Of Charge -- the lower the voltage available (SOC) then lower the performance. The thread above is about how at some point the batter no longer has enough on demand power available to provide full acceleration to the car, which makes perfect sense when you think about it a bit.

 

[sambennett]

Thanks for breaking it down to me!

 

[qwk]

To add further to this, Tesla also underrated the power. My S85 hits 320+kW with a warm pack, and there are reports of P85 cars hitting 360kW.

 

[lloyds]

isn't it true that driving the car hard at a low SOC is bad for the battery pack as well?

 

[Johan]

Sorry to resurrect an old thread, but I'd like some good opinions on the following:

We know the only difference between a P85 and S85 is the inverter. I'ts rated at 1200A for the P and 900A for the S. At 100% SOC the P has more power than the S. At 0% both a P and an S will have 0 power. Imagine two peak power curves for a P model and an S model with power on the Y-axis and SOC on the X-axis. The P curve starts higher than the S curve and they both end at 0, so obviously the P curve will slant more downward, that's a given but what I'm wondering is:

- At what SOC (supposedly before 0% SOC) does the P curve trace the S curve (i.e. there is no longer a benefit to the P version of the car)?

- Can the bigger inverter on the P model keep it's curve flatter further out along the X-axis (i.e. as SOC is declining) due to the fact that the beefed up inverter can compensate for voltage of the pack dropping with lower SOC by pulling more amps from the battery? I guess another way of stating this question is: as SOC declines is there a point where the battery can still deliver >900A of current so that the P can better keep up peak power than the S version of the car, and if so at what SOC is the battery no longer able to deliver >900A?

 

[VolkerP]

I think the 900A vs 1200A rating is the motor current. If you look at the torque via RPM curves, the performance inverter produces higher torque until both hit the power limit curve. So I'd say the performance inverter is able to route more power at lower RPMs to the motor as long as the pack's power is not maxed out.