I’m having a look at fitting one of my racing cars with the rear electric motor from a Tesla.

The motor & controller aren’t too difficult to sort out but I just want to see if I have the right guess for the size of the battery needed. I’d like to use a 300 kW motor and I typically use full throttle about 80% of the time on the track, with the rest under hard braking & occasionally part-throttle after the corner apex. It’s for sprint races so under ten minutes racing maximum in one race. With the much more powerful motor I’d expect to be at full throttle for maybe 75% of the time as what weren’t corners before would be, due to the higher speeds.

So 300 kW x 0.75 × (10/60 minutes) = 37.5 kW/h

That seem about right?

Um …. 37.5 kW/h plus a bit more for the part-throttle times, plus a bit more for idling around, etc.

Spiny Norman said:

Um …. 37.5 kW/h plus a bit more for the part-throttle times, plus a bit more for idling around, etc.

Don’t forget the inverter and motor efficiencies.

The inverter is going to be 95 – 96%. The motor somewhere between 80 – 90%. Tesla claims the higher figures but I’d be inclined to use about 85%. Taking those two into account you’re up around 46.5 kWh (please get rid of the divide sign in your figures – shudder). Add on the bit more and you’re up around the 50 kWh.

sibeen said:

Spiny Norman said:

Um …. 37.5 kW/h plus a bit more for the part-throttle times, plus a bit more for idling around, etc.

Don’t forget the inverter and motor efficiencies.

The inverter is going to be 95 – 96%. The motor somewhere between 80 – 90%. Tesla claims the higher figures but I’d be inclined to use about 85%. Taking those two into account you’re up around 46.5 kWh (please get rid of the divide sign in your figures – shudder). Add on the bit more and you’re up around the 50 kWh.

Okay thanks for that. I’m not sure I’d be able to fit a battery that size into the car ….. I might have to go back to designing the three litre V8 I was looking at instead.

Spiny Norman said:

sibeen said:

Spiny Norman said:

Um …. 37.5 kW/h plus a bit more for the part-throttle times, plus a bit more for idling around, etc.

Don’t forget the inverter and motor efficiencies.

The inverter is going to be 95 – 96%. The motor somewhere between 80 – 90%. Tesla claims the higher figures but I’d be inclined to use about 85%. Taking those two into account you’re up around 46.5 kWh (please get rid of the divide sign in your figures – shudder). Add on the bit more and you’re up around the 50 kWh.

Okay thanks for that. I’m not sure I’d be able to fit a battery that size into the car ….. I might have to go back to designing the three litre V8 I was looking at instead.

A rough estimate shows a ~230 kg battery pack.

Spiny Norman said:

Spiny Norman said:

sibeen said:

Don’t forget the inverter and motor efficiencies.

The inverter is going to be 95 – 96%. The motor somewhere between 80 – 90%. Tesla claims the higher figures but I’d be inclined to use about 85%. Taking those two into account you’re up around 46.5 kWh (please get rid of the divide sign in your figures – shudder). Add on the bit more and you’re up around the 50 kWh.

Okay thanks for that. I’m not sure I’d be able to fit a battery that size into the car ….. I might have to go back to designing the three litre V8 I was looking at instead.

A rough estimate shows a ~230 kg battery pack.

I haven’t looked at the weights.

A suitable 18650 cell is about 46 grams, I’d need about 4,500 of them.

sounds like a fun project, i’ve watched lot of electric cars racing V8s etc on the tube, they do well

transition said:

sounds like a fun project, i’ve watched lot of electric cars racing V8s etc on the tube, they do well

They sure are, but the problem that I face is the same as every other performance EV car – Needing quite a large battery pack to get the power & duration needed. It’s difficult to justify compared to the V8 I’m designing.

Spiny Norman said:

transition said:

sounds like a fun project, i’ve watched lot of electric cars racing V8s etc on the tube, they do well

They sure are, but the problem that I face is the same as every other performance EV car – Needing quite a large battery pack to get the power & duration needed. It’s difficult to justify compared to the V8 I’m designing.

Make the races shorter :)

sibeen said:

Spiny Norman said:

transition said:

sounds like a fun project, i’ve watched lot of electric cars racing V8s etc on the tube, they do well

They sure are, but the problem that I face is the same as every other performance EV car – Needing quite a large battery pack to get the power & duration needed. It’s difficult to justify compared to the V8 I’m designing.

Make the races shorter :)

Well I do try to drive faster than anyone else. :)

Spiny Norman said:

transition said:

sounds like a fun project, i’ve watched lot of electric cars racing V8s etc on the tube, they do well

They sure are, but the problem that I face is the same as every other performance EV car – Needing quite a large battery pack to get the power & duration needed. It’s difficult to justify compared to the V8 I’m designing.

guessing you can flog the batteries fairly well, with some risks, and more rapid decline in output (AH capacity) with cycles

of course there are derating factors, depends how much want to conform to them (manufacturers recomendations), not unlike high performance piston engine really, try and test, see what fails

How much could you recover in regenerative braking?

Have you considered using a hybrid?
Small ICE + small electric?

*kWh

dv said:

*kWh

I already asked him to get rid of the divide sign :)

I found a place in Australia that’ll sell me a 370 kW Tesla motor, but the battery pack is going to be far too heavy.
I’ll have to go back to designing the three litre V8 I guess – At least it’ll sound much better. :)
Like this

What was the weight?

Spiny Norman said:

I found a place in Australia that’ll sell me a 370 kW Tesla motor, but the battery pack is going to be far too heavy.
I’ll have to go back to designing the three litre V8 I guess – At least it’ll sound much better. :)
Like this

Bill, I may have led you astray after thinking about this a bit more. I was assuming that you need the full 300 kW output from the motor, or 80%, but surely you need to know your maximum power required at the wheels?

dv said:

What was the weight?

They reckoned the battery pack would be about 400 kg. That’s close to the weight of the entire car with the conventional engine.

sibeen said:

but surely you need to know your maximum power required at the wheels?

Yes, it’s more. Always more.

Spiny Norman said:

sibeen said:

but surely you need to know your maximum power required at the wheels?

Yes, it’s more. Always more.

Ah, OK then, my original figures stand, and of course with a 375 kW motor they just got worse.

sibeen said:

Spiny Norman said:

sibeen said:

but surely you need to know your maximum power required at the wheels?

Yes, it’s more. Always more.

Ah, OK then, my original figures stand, and of course with a 375 kW motor they just got worse.

Yep. :(

Use a small battery pack to supercharge air into the engine and draw waste gases out.

Some battery technology from the Formula E racing series.

A Formula E battery is a collection of over 5000 cells! Inside the carbon casing made by McLaren Applied Technologies are separate modules, each housing a huge number of small AA sized cells made by Murata (Sony). By connecting the cells inside each module in series and parallel, then interconnecting the modules, the entire battery can produce 800v and a storage capacity of 54kwh.

This is twice the performance of the older Gen1 battery, the increase in energy storage allows the cars to race for the full +45 minute race without recharging or car swaps. The downside being the unit’s weight, there are 250kg of cells alone, plus the associated internal structure, connections, cooling and casing bring the battery up to over 400kg.

More

My car wouldn’t run anywhere near as long as they do, but have more power so I think I’d still end up with a ~250 kg pack. I’ll keep thinking about it anyway.