EV Race Car

[andylaurence]

I've been mulling over the idea of a competition BEV. It's not the first time I've thought about it, but with my track car now up for sale, I have a spare space in the garage to actually carry out this task. The first part of a project is to scope out what it needs to do. The car will need to run for 6 minutes at full throttle, and will compete against cars with up to 200bhp weighing under 500kg. The base car I'd like to use is a Mini as they're cheap, plentiful, lightweight, small, and there's an aftermarket parts scene at least as good as any other model.

If I were able to keep to 500kg, then a 150Kw motor would see me competitive. I'd need 15Kwh of energy for this, which is 1250Ah @12v in battery terms. So 13 100Ah batteries would do the business, which gives a running voltage of 156v. Not ideal given the ratings of the motors and controllers I've seen out there. The figure that is more worrying though is the current required to run a 150Kw motor @ 156v. Batteries won't normally do 1000 amps as I understand it! As such, I'll need more batteries. If I use two banks of 12 batteries, I can run at 144v, which will require each battery to push out 12v @500A. That's within specs, from what I've seen.

That's my thought process about the EV side of things so far. Who can tell me how many mistakes I've already made? Do 150Kw motors exist that run @144v? Are suitable controllers available? Will I be able to do this without a budget running into tens of thousands? Next thoughts will revolve around transmission, but I need to read the technical regs first to check that this will actually fit into the right class!

Cheers,
Andy

[qdos]

I think you can make it a bit easier by using more than one motor

[ChrisB]

.......... I'd need 15Kwh of energy for this, which is 1250Ah @12v in battery terms. So 13 100Ah batteries would do the business, which gives a running voltage of 156v.........

Hi Andy

I've spotted a bit of a problem striaght away, 13 x 100ah 12volt batts would give you 156v but only @ 100ah , due to the fact you've got them in series, this way the voltage goes up but Ah stays the same , put them in parrallel and then you would get 1300ah but only at 12volts

Thats problem one, there may be more , but I think the 500kg might be another I'm afraid .

I'm not well up on EV racing but there are peeps on here that have done it.

ChrisB

[andylaurence]

I think you can make it a bit easier by using more than one motor

I'd been pondering this one. I've seen multi-engined vehicles driving single wheels, and they allegedly work quite well. It's essentially like a 50% torque split viscous differential. Traction is also very important, especially off the line. A 4WD vehicle would have a distinct advantage. Perhaps four 50Kw motors driving each of the wheels woulod be a good bet.

Cheers,
Andy

[andylaurence]

.......... I'd need 15Kwh of energy for this, which is 1250Ah @12v in battery terms. So 13 100Ah batteries would do the business, which gives a running voltage of 156v.........

Hi Andy

I've spotted a bit of a problem striaght away, 13 x 100ah 12volt batts would give you 156v but only @ 100ah , due to the fact you've got them in series, this way the voltage goes up but Ah stays the same , put them in parrallel and then you would get 1300ah but only at 12volts

Thats problem one, there may be more , but I think the 500kg might be another I'm afraid .

I'm not well up on EV racing but there are peeps on here that have done it.

ChrisB

156v @100Ah is 15.6KWh, which is enough energy for the application I have in mind. I was converting the required energy into the number of lead-acid batteries I'd need as a real world example. I wouldn't want to use 12v for the car. Imagine the current!

As for the weight, I realise this is going to be an impossible target. 24 batteries alone will weigh almost 500KG! I'm clearly going to need more power to overcome the weight disadvantage. Of course, this means I need more batteries to supply this power. Catch 22? Perhaps. I just hope there's a break even point!

Thinking more sensibly, I've based the energy requirement on full throttle for the entire 6 minutes run time. Realistically, that won't happen, but perhaps I should include contingency. Still, it's likely that I could run 200KW with that amount of energy available. Time to do more sums, and more investigation, I think.

Cheers,
Andy

[floydster]

Hi Andy,

What class are you building the car to compete in? I've often thought that an EV would be ideal for speed events but I've never been sure about what class you could enter them in as most classes for speed events stated that pump fuel must be used.

Floydster

[andylaurence]

Hi Andy,

What class are you building the car to compete in? I've often thought that an EV would be ideal for speed events but I've never been sure about what class you could enter them in as most classes for speed events stated that pump fuel must be used.

Floydster

That's exactly what I'm thinking of. I don't have a copy of the blue book, so I'm only doing preliminary estimates of whether it's feasible before I look up the regs. I was thinking the under 1400cc sports libre class. You can circumvent the requirement for pump fuel by adding a 50cc engine for recharging!

Cheers,
Andy

[ChrisB]

.........156v @100Ah is 15.6KWh, which is enough energy for the application I have in mind. I was converting the required energy into the number of lead-acid batteries I'd need as a real world example. I wouldn't want to use 12v for the car. Imagine the current!

Agreed, but watch out , not sure if its a typo but you've said 156v@100Ah is going to give you 15.6KWh, you cant really say that as it all depends on the batteries you use and the Wh they will give you. I'm almost 100% sure you cant just calculate KWh from voltage and Ah

ChrisB

[andylaurence]

.........156v @100Ah is 15.6KWh, which is enough energy for the application I have in mind. I was converting the required energy into the number of lead-acid batteries I'd need as a real world example. I wouldn't want to use 12v for the car. Imagine the current!

Agreed, but watch out , not sure if its a typo but you've said 156v@100Ah is going to give you 15.6KWh, you cant really say that as it all depends on the batteries you use and the Wh they will give you. I'm almost 100% sure you cant just calculate KWh from voltage and Ah

ChrisB

I'd have thought that those were the only things you can calculate KWh from. The batteries are rated at 100Ah and 12v. 156v is 13 x 12v batteries, still at 100A. 15.6KWh is 156v @100A for one hour. 156v x 100A = 15600VA = 15.6KVA =~ 15.6KW. The batteries can supply this current for 1 hour, hence they collectively have a capacity of 15.6KWh (ignoring the difference between KVA and KW). If I have this wrong, I'd really like to develop my understanding and learn why.

Cheers,
Andy

[ChrisB]

.........156v @100Ah is 15.6KWh, which is enough energy for the application I have in mind. I was converting the required energy into the number of lead-acid batteries I'd need as a real world example. I wouldn't want to use 12v for the car. Imagine the current!

Agreed, but watch out , not sure if its a typo but you've said 156v@100Ah is going to give you 15.6KWh, you cant really say that as it all depends on the batteries you use and the Wh they will give you. I'm almost 100% sure you cant just calculate KWh from voltage and Ah

ChrisB

I'd have thought that those were the only things you can calculate KWh from. The batteries are rated at 100Ah and 12v. 156v is 13 x 12v batteries, still at 100A. 15.6KWh is 156v @100A for one hour. 156v x 100A = 15600VA = 15.6KVA =~ 15.6KW. The batteries can supply this current for 1 hour, hence they collectively have a capacity of 15.6KWh (ignoring the difference between KVA and KW). If I have this wrong, I'd really like to develop my understanding and learn why.

Cheers,
Andy

Ahhhh common error, it does really depend on what batteries you are going to use, there are few batteries that will supply the 1hr rating known as C1 some of the modern Li-ions are pretty good at this though.

Battery ratings are a big bone of contention, and are easily missunderstood.
Lets take say a Yuasa 12v 17Ah sealed LA, if you look at the figures it may appear to be able to supply 17A for an hour when actually its 17Ah @ C20 which works out that it will actually supply 0.85A for 20hrs but that is its guaranteed output.
Yes it will supply more and will supply several hundered amps for a few mins but your unlikely to get 17amps from it for 1 hr or if you do you will be damaging the batteries long term life and you wont get its its full life cycle from it.

Different batteries have different spec, and we havent even touched on the purket effect

Hope that helps a little ??

ChrisB