Impact of battery voltage and current on Tesla motor performance

3835 Views 9 Replies 3 Participants Last post by remy_martian, Oct 10, 2021

catphish

Discussion Starter · Oct 8, 2021

I am looking to properly understand the impact of battery specification, both voltage, and peak current, on the performance of a Tesla small drive unit, and the vehicle it is installed in. I discussed this briefly, but want to go into some more detail. While writing this post, I think I've answered pretty much all my own questions, but I'm going to post it anyway for information / discussion / corrections.

My basic understanding, assuming throttle is at 100% is as follows:

100% of battery voltage is always used, only current varies (directly proportional to power)
The inverter attempts to provide the motor with 600A regardless of RPM for 100% torque
The voltage required across the motor to achieve this 600A is (somewhat) proportional to the rotational speed
The power drawn from the battery is (basically) the same as the power consumed by the motor
At low RPM, the motor can have its 600A (and constant torque) at low motor voltage, and hence low battery current.
Eventually a point will be reached where 600A is drawn and motor voltage is equal to battery voltage, so peak 600A is drawn from the battery.
As RPM rises further, the motor voltage required to achieve 600A exceeds battery voltage. The inverter continues to apply full battery voltage to the motor, but current (and torque) falls as RPM rises. Power remains constant.
Eventually the reducing torque and increasing air resistance are equal, hence top speed.

I'm fairly confident that this is how an induction motor works, but I have two big questions:

* With the Tesla Small Drive Unit, at what RPM will I run out of voltage for a 200V, 300V 400V battery? ie if I have a 200V or a 300V battery, at what RPM will i reach peak power and start to experience reduced torque? (this question is potentially answered later in the post)

* If I use a battery pack that cannot actually supply 600A peak, is the correct solution to reduce the maximum output power of the inverter accordingly, so for a 300V 400A battery pack, limit the inverter power to 120kW? This would seemingly have exactly the same affect as a lower voltage battery, the only difference being that the inverter is artificially limiting battery voltage.

It seems to me that if one can't fit a fully sized 96S 600A battery pack, one would get exactly the same reduction in performance, at all speed/rpm, using a 400A 96S pack, or using a 600A 64S pack. Both would deliver full torque up to some RPM limit, and then constant power (146kW) beyond that. Am I missing any benefits of one option over the other, or am I completely misunderstanding anything?

I found a graph that implies that constant power (reducing torque) begins at 40mph in the Model S (pretty much regardless of which model). I suppose we can assume this is correct for Tesla's wheel size and standard gearing at 96S. Will this reduce somewhat linearly with reduced power, approx 30mph if one were to reduce voltage 25% (ie to 72S)?

The final question I'd have to answer would be how the car actually feels overtaking at 30mph vs 50mph vs 70mph, but I fear no amount of maths will be able to do that, as aerodynamics will have considerable impact.

Thanks for reading my ramblings. Please tell me if I'm missing anything important.

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brian_

· #2 · Oct 9, 2021

catphish said:
I'm fairly confident that this is how an induction motor works, but I have two big questions:

Yes, and it's how a synchronous permanent magnet AC motor works, as well.

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brian_

· #3 · Oct 9, 2021

catphish said:
I found a graph that implies that constant power (reducing torque) begins at 40mph in the Model S (pretty much regardless of which model). I suppose we can assume this is correct for Tesla's wheel size and standard gearing at 96S. Will this reduce somewhat linearly with reduced power, approx 30mph if one were to reduce voltage 25% (ie to 72S)?

Roughly, yes. Look at the peak torque curves for the BorgWarner HVH motors for an example of how available voltage affects the "knee point" where torque starts dropping off because the rated current can no longer be sustained. Although the HVH comes in both PM and induction versions, the data is for the PM; that doesn't matter much for this purpose.

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remy_martian

· #4 · Oct 10, 2021 (Edited)

Which HVH has an induction rotor?

They're all PM afaik. All that changes is stack length and whether it's parallel or series wound. Diameter is also changed between families of machines.

I mean, there's no reason you couldn't put an induction rotor in, but I've never heard of it.

As far as fitting a full sized pack, you can use 8 modules to make the 400V using some deft cutting wheel skills with your angle grinder to make 'em 12s(x/2)p, so you CAN fit that into almost any car.

Problem is, everybody wants 0-60 in 2 sec and 150mph out of their subcompact. The other problem is those modules are going extinct.

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catphish

Discussion Starter · #5 · Oct 10, 2021

remy_martian said:
Problem is, everybody wants 0-60 in 2 sec and 150mph out of their subcompact.

Personally I am hoping I can see a 0-60 time below 4 seconds, and enough continuous power to drive at motorway speeds (70mpg). I am trying to get a feel for whether I'm being realistic, and how much power will actually be required for this. Would be good to find some threads with anecdotal information about what people have achieved in small cars.

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remy_martian

· #6 · Oct 10, 2021 (Edited)

You never mentioned the car or its expected curb weight, which I'll guess is 4000lb

The SDU is only rated at 35kW continuous, so not sure where your 600A numbers are coming from. That's barely enough to cruise at highway speeds.

At 350V it will be using ~100A.

0-60 times should be roughly that of a Chevy Bolt EV (6.3sec) from the LDU if you give it full rated power of 200-ish kW for no more than 12 sec.

You could just buy a Bolt - it costs less than a build would, especially if your time can be put elsewhere.

This is where identifying the car matters, and, yes, you can do "maths" to figure out performance. How much battery can you stuff, what's the weight, Cd, etc? Is it worth doing?

As basic transportation, thinking you can do it cheaper than buying an EV is a false economy - those days are vanishing quickly. By the time you're done with your math, an EV in the US will cost about $18k out the door. That's what a Model S pack costs at the salvage yard.

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catphish

Discussion Starter · #7 · Oct 10, 2021

remy_martian said:
You never mentioned the car or its expected curb weight, which I'll guess is 4000lb

Apologies, I forgot to reference my vehicle / project in this post. The vehicle is a Vauxhall VX220, about 2100lbs with its original ICE. I believe batteries are going to add 400-500lbs to that number. It has an ICE power output of 107kW.

remy_martian said:
The SDU is only rated at 35kW continuous, so not sure where your 600A numbers are coming from. That's barely enough to cruise at highway speeds.

I'm aiming to match the specs of the SDU which I believe are 35kW continuous, 90kW for 12 minutes (or 75kW for 15 minutes), and 220kW (that's where 600A comes from) peak (10-12s). I am therefore hoping to (at least approximately) build a battery that will meet these requirements.

You're right that 35kW is barely enough to cruise at decent speeds, but your comment has lead me to do some obvious research. A mk1 Fiat punto 1.1 (similar weight), achieves a top speed of 93mph with a power output of 40kW, so I think it's reasonable to assume that the Tesla SDU could propel this vehicle at highway speeds without going too far above the 30kW continuous output.

I'm fairly confident that if I could actually get get 600A @ 360A (216kW) out of a battery, this vehicle will accelerate, at least to 60mph, at a pretty decent pace. I believe the ideal would be a 96S 100AH battery. This should roughly be a match, but that may not be possible, so I'm thinking about compromise. I suppose anything between 110kW and 220kW would be acceptable to at least match the original performance.

The big problem I anticipate is going to be finding space for battery modules, and knowing what the safe output current of a battery is going to be for continuous (that's fairly easy, 1C), 15 minutes, and 10s peak. This is particularly challenging when buying scrap.

Thanks for your input, and giving me some thinking points. Any other thoughts are always welcome, particularly if I'm being stupid

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remy_martian

· #8 · Oct 10, 2021

Provoked thought and analysis. You've nicely worked through it.

Usual safe output, max continuous, from a liquid cooled Lithium cell is 3C, with short bursts as much as 9C-10C.

Ignoring voltage sag, the issue for a battery quickly becomes run time, not available current, imo. Which is the happy accident Mother Nature gives an EV as far as instantaneous performance from a Lithium pack.

Then reality sets in....weight limits/distribution (in a sports car handling trumps 0-60 imo....focusing a sports car on the drag strip is blasphemous) and where to stuff the cells pops into primary consideration. Note that weight distribution is a 3D thing in a sports car.

Your project, but having the equivalent of a morbidly obese mistress in the passenger seat all the time kinda defeats the point of a sports car, especially when your pack can't be thin and at the chassis bottom.

Keep the weight down and the vehicle dynamics will be rewarding. 20% increase in curb weight could make it handle and ride like a truck.

You CAN trade top speed for 0-60 with gearing, though that's not easy with the SDU.

Wish we had the VX in the US...it's cute.

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brian_

· #9 · Oct 10, 2021

remy_martian said:
Which HVH has an induction rotor?

They're all PM afaik. All that changes is stack length and whether it's parallel or series wound. Diameter is also changed between families of machines.

I mean, there's no reason you couldn't put an induction rotor in, but I've never heard of it.

You have to find the right (or maybe just old enough) web page, brochure, or catalog. The induction version uses the same stator as the PM version - it's just an alternate rotor. In a similar situation, the popular Siemens induction motor comes in PM, too.

I checked some bookmarked web pages and documents, and among the ones that still work I didn't see a reference to the induction version, but the attached older documents all list it. The induction option was clearly not popular, and appears to have been discontinued back in the Remy era.

The point is just that some of these basic motor characteristics are comparable regardless of PM or induction operation.