# Impact of battery voltage and current on Tesla motor performance



## brian_ (Feb 7, 2017)

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_ (Feb 7, 2017)

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 (Feb 4, 2019)

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 (Jul 30, 2017)

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 (Feb 4, 2019)

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 (Jul 30, 2017)

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 (Feb 4, 2019)

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_ (Feb 7, 2017)

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.


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## remy_martian (Feb 4, 2019)

Interesting. 

Now you have me wondering if there are any AC machines in the wild


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