EV Race Car

[andylaurence]

Interesting challenge. I don't know much about hillclimbing events, but sounds to me as if it's the perfect event to show off the strengths of an EV.

That's exactly what I thought! Short bursts of power.

Not sure you even need to match the 150 kW of the petrol competition because of the torque advantage of electric motors, but assuming you do, you should only need that maximum power for short bursts, so your motor can be rated for a much lower power output, say 30-50 kW. With only a six-minute sprint, the motor shouldn't have time to overheat if you run it at much higher power outputs for short periods.

The sprints are usually around a minute, although you do have to compete a few times at an event.

Until recently most EV speed and acceleration records were set with Lead Acid batteries. Even they are going to need a basic monitoring and protection system. Both NiMh and Li chemistries will die an early and possibly spectacular death without protection. You can't just simply scale up model car/aircraft practice.

OK. Could you explain why, please? I realise that I took a rather simplistic look at this and my understanding is in its infancy.

Accelleration is about torque NOT power, only the speed you can accelerate to is determined by the power. You may well NEVER use the peak power available from your batteries and/or controller. This would be very true of somewhere like Prescott.

Acceleration is directly related to torque at the wheels (assuming perfect traction). Power and torque are related too, only power factors in the speed too. If I have a motor producing 50lb-ft of torque, there won't be 50lb-ft of torque at the wheels (unless there's a 1:1 gear ratio). If I have a motor producing 50bhp, there will be 50bhp at the wheels too (assuming no transmission losses). It just makes for easier comparisons in my eyes. Of course, peak power is immaterial. What's important is the average power/torque in the rev range used. This is where electric motors win as they have a flat torque curve. Compare that with a bike engine!

Stick to a SAFE, affordable and reliable battery choice, at least to start with. A Hawker (Enersys) Genesis or Oddysey would be a good choice. As I recall the 26Ah size has the best peak power (measured at half the nominal voltage). http://www.enersys.com/

Thanks for that. I've had a look through the data on the Odyssey batteries. It seems I can get almost 150KW from a bank of 14 PC1500 batteries weighing almost 340KG. That'll give me a voltage of 168v which isn't ideal for most of the motors I've seen. Of course, I could run at half that voltage (84v), but I may struggle to find motors that produce 40KW @84v.

If I lose the weight of the engine/gearbox and the rear discs, then that'll help offset the weight disadvantage. There should be no issues with the run time of this setup as I should be able to run at full throttle for over an hour! Another solution is to run 48 PC625 batteries, which would save another 60KG but would cost a little more (just under £100 each). The PC1500 batteries are just under £220 each which brngs the batteries in at around £3000.

Cheers,
Andy

[EVguru]

Until recently most EV speed and acceleration records were set with Lead Acid batteries. Even they are going to need a basic monitoring and protection system. Both NiMh and Li chemistries will die an early and possibly spectacular death without protection. You can't just simply scale up model car/aircraft practice.

OK. Could you explain why, please? I realise that I took a rather simplistic look at this and my understanding is in its infancy.

Ok, take a typical 6 cell NiCad pack where the nominal terminal voltage is 7.2. The voltage remains quite stable untill one cell reaches its 'knee' and the voltage drops off fast. 1.2v is 16.6% of the total voltage and you're likely to notice such a large drop off. The top speed of your RC car is going to be 16.6% lower. Now imagine a ~144volt pack. One cell now represents less than 1% of the total which you're unlikely to notice. If you continue discharging you are going to reverse the cell and possibly cause it to explode.

Even at the 7.2 volt level, RC racers go to the trouble of matching cells to reduce the likelyhood of problems, but they still tend to burn up packs fast.

With my Scirocco running 192 volt, you could have half a dozen batteries down to less than 6 volt before performance dropped below normal road requirements. Flooring the throttle at this point would easily pull the weak batteries to substantial negative voltages. Do this at several hundred amps and you WILL blow a battery. Been there, done that. I've helped pick up bits of battery from the drag strip when the 'Manic Mazda' lit up the Arizona sky with a brief plasma display.

Ideally you need to monitor every cell and make sure that it doesn't drop below a set level. In reality this level of complexity often isn't justified and monitoring about every six cells will do since you can be reasonably sure that all the cells in a 12v lead acid are quite close in capacity and matching individual cells in sets of 6 isn't too onerous.

Acceleration is directly related to torque at the wheels (assuming perfect traction). Power and torque are related too, only power factors in the speed too. If I have a motor producing 50lb-ft of torque, there won't be 50lb-ft of torque at the wheels (unless there's a 1:1 gear ratio). If I have a motor producing 50bhp, there will be 50bhp at the wheels too (assuming no transmission losses). It just makes for easier comparisons in my eyes.

'Power sells engines, but torque wins races'.

Knowing the power at the wheels is meaningless in this application and getting the gearing right is essential in any case.

That'll give me a voltage of 168v which isn't ideal for most of the motors I've seen. Of course, I could run at half that voltage (84v), but I may struggle to find motors that produce 40KW @84v.

What motors have you been looking at?

A motor is simply an energy conversion device and the rating is only a single possible operating point. The 914 ran a motor with a 72volt nameplate rating on battery packs ranging from 120 to 288 volt. We self imposed a limit of 175volt accross the motor terminals when racing, but took it to over 200 volt in testing without problems.


If you're still talking about doing a Mini then I promise you that you won't be able to put down the torque possible from a 150Kw drive system.

[andylaurence]

Until recently most EV speed and acceleration records were set with Lead Acid batteries. Even they are going to need a basic monitoring and protection system. Both NiMh and Li chemistries will die an early and possibly spectacular death without protection. You can't just simply scale up model car/aircraft practice.

OK. Could you explain why, please? I realise that I took a rather simplistic look at this and my understanding is in its infancy.

Ok, take a typical 6 cell NiCad pack where the nominal terminal voltage is 7.2. The voltage remains quite stable untill one cell reaches its 'knee' and the voltage drops off fast. 1.2v is 16.6% of the total voltage and you're likely to notice such a large drop off. The top speed of your RC car is going to be 16.6% lower. Now imagine a ~144volt pack. One cell now represents less than 1% of the total which you're unlikely to notice. If you continue discharging you are going to reverse the cell and possibly cause it to explode.

Even at the 7.2 volt level, RC racers go to the trouble of matching cells to reduce the likelyhood of problems, but they still tend to burn up packs fast.

With my Scirocco running 192 volt, you could have half a dozen batteries down to less than 6 volt before performance dropped below normal road requirements. Flooring the throttle at this point would easily pull the weak batteries to substantial negative voltages. Do this at several hundred amps and you WILL blow a battery. Been there, done that. I've helped pick up bits of battery from the drag strip when the 'Manic Mazda' lit up the Arizona sky with a brief plasma display.

Ideally you need to monitor every cell and make sure that it doesn't drop below a set level. In reality this level of complexity often isn't justified and monitoring about every six cells will do since you can be reasonably sure that all the cells in a 12v lead acid are quite close in capacity and matching individual cells in sets of 6 isn't too onerous.

I see. Thanks for the explanation.

That'll give me a voltage of 168v which isn't ideal for most of the motors I've seen. Of course, I could run at half that voltage (84v), but I may struggle to find motors that produce 40KW @84v.

What motors have you been looking at?

A motor is simply an energy conversion device and the rating is only a single possible operating point. The 914 ran a motor with a 72volt nameplate rating on battery packs ranging from 120 to 288 volt. We self imposed a limit of 175volt accross the motor terminals when racing, but took it to over 200 volt in testing without problems.


If you're still talking about doing a Mini then I promise you that you won't be able to put down the torque possible from a 150Kw drive system.

I'd been looking at the rated voltage inputs. If they're as flexible as you suggest, then 168v should pose no problems. I guess I just need to start finalising specifications for components. As for putting down 150Kw, that's around 200bhp. There are FWD Minis with 200bhp. They seem to do OK. Ideally, I'd like to do something with four motors (one per wheel). Putting down around 40Kw per corner shouldn't be too tricky. I'm still not 100% sure on whether I'll use the Mini. If it's not the Mini, I'll look into a lighter vehicle, either a single seater or a light weight kit car. I really appreciate the input you've given. Lots of learning still to do...

Cheers,
Andy

[EVguru]

As for putting down 150Kw, that's around 200bhp. There are FWD Minis with 200bhp. They seem to do OK.

That's the problem with using power as your performance measure.

They most assuredly are NOT puting down the kind of torque that can be generated by at the wheels be a 200hp engine. They not only throttle the engine back, but use a torque limiting device, namely the clutch. They're also going to be a LOT lighter. You need to get away from thinking in ICE terms.

Whilst limiting the peak power on the 914 to 35Kw, we could still easily light up the 205 section Hoosier road legal race tyres when launching in third gear.

[ChopperMan]

Hi All,

I've been reading your posts with interest, being a motorsport fan that feels guilty buying an ICE sports car.

Have you seen what Westfield are developing?
http://www.westfield-sportscars.co.uk/N ... cle46.html

The world’s first one-make electric vehicle race series. It seems great minds think alike

[qdos]

Indeed great minds do think alike. All I can say is watch this space and you'll see some members of the BVS actually beat Westfield to it

[andylaurence]

As for putting down 150Kw, that's around 200bhp. There are FWD Minis with 200bhp. They seem to do OK.

That's the problem with using power as your performance measure.

They most assuredly are NOT puting down the kind of torque that can be generated by at the wheels be a 200hp engine. They not only throttle the engine back, but use a torque limiting device, namely the clutch. They're also going to be a LOT lighter. You need to get away from thinking in ICE terms.

Whilst limiting the peak power on the 914 to 35Kw, we could still easily light up the 205 section Hoosier road legal race tyres when launching in third gear.

I think I see where you're going here. Your approach is not one I'm familiar with. There will be a maximum torque at the wheels that is possible without wheelspin. My understanding is that this will be a similar value at any speed. Given that the gear ratio is fixed and the torque curve of the motor is relatively flat, then I need to rate my motor based on the maximum torque my tyres can deliver and the gear ratio. You're right. It is simpler! The main issue with this is how to calculate the torque it's possible to put through the tyres.

Cheers,
Andy