EV Race Car

[floydster]

You can circumvent the requirement for pump fuel by adding a 50cc engine for recharging!

Cheers,
Andy

Nice one.

What classes did you race your last car in?

Floydster

[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

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

I realise this, but as they are the only figures that are usually quoted, how do I get around this?

Cheers,
Andy

[andylaurence]

You can circumvent the requirement for pump fuel by adding a 50cc engine for recharging!

Cheers,
Andy

Nice one.

What classes did you race your last car in?

Floydster

I've not competed in speed events before. I've competed in Solo and autotesting events before, and done some other track time.

Cheers,
Andy

[EVguru]

I've been building a Speed Hillclimb EV on and off for more years than I like to admit to.

My discussions in the past with the Bugatti Owners Club (Prescott Hillclimb) and the RACMSA were quite helpfull. Prescott will allow demonstration runs (as they have in the past) and the MSA suggested they would form an EV class as soon as there were vehicles to run. One possibilty would be a handicap class where a wide range of vehicle could compete.

Frank Wykes ran his Enfield (uprated to 120volt) at Prescott and his unofficial time was similar to that of a standard 1500 MG midget.

When I was involved with racing in the USA we ran a converted Porsche 914 and once shared track time with NASA (National AutoSports Association). The NASA cars were RX7s, 3 series Beemers (318, 325), and low end Porsche 911s. Many of these were running about 50hp of NOX to keep them smog legal. In the open practice session, our power limit was 36hp in order to last a 23 mile race, but with better suspension and tires and a good driver we could circulate as fast as any of the gas cars.

[andylaurence]

I've been building a Speed Hillclimb EV on and off for more years than I like to admit to.

My discussions in the past with the Bugatti Owners Club (Prescott Hillclimb) and the RACMSA were quite helpfull. Prescott will allow demonstration runs (as they have in the past) and the MSA suggested they would form an EV class as soon as there were vehicles to run. One possibilty would be a handicap class where a wide range of vehicle could compete.

Frank Wykes ran his Enfield (uprated to 120volt) at Prescott and his unofficial time was similar to that of a standard 1500 MG midget.

When I was involved with racing in the USA we ran a converted Porsche 914 and once shared track time with NASA (National AutoSports Association). The NASA cars were RX7s, 3 series Beemers (318, 325), and low end Porsche 911s. Many of these were running about 50hp of NOX to keep them smog legal. In the open practice session, our power limit was 36hp in order to last a 23 mile race, but with better suspension and tires and a good driver we could circulate as fast as any of the gas cars.

You've done more research with the authorities than I have. I've literally done a quick brainstorm and that's it. I'm off to the Bristol Motor Club meeting tonight, and will quiz people about it there. I know someone who will know the Blue Book backwards.

Cheers,
Andy

[andylaurence]

I'm still working on this, so a quick update: It seems I should be able to slot into a class at speed events. It's purely a case of which class they want to put me in. It's something nobody wants to decide on until the problem arises! I've been looking for suitable batteries, which is turning out to be a nightmare. Half of the battery suppliers list some specifications and half of them list prices. There's a crossover of about 5% where you get prices and specifications. None list everything I need! All the same, I've been trying to evaluate possibilities.

The limitation appears to be how quickly I can discharge the cells. I've seen a number of options:

1. Lead-acid: Cheap, plentiful and I don't need many. I can't find any details about how much current they can supply, so it's hard to work out exactly how many I'll need. I suspect they will supply around 3KW each (peak), which means I'll need between 30 and 50, depending on which direction I take. That's a scary weight of between 350KG and 1000KG, depending on size/quantity though. I realised that you can buy small (around 2Ah) units for not a lot of cash, and some of these will deliver up to 75A. That means that each one can deliver almost 1KW. I can buy 168 units for les than £2000 and get over 150KW, although not for long!

2. Lithium-Ion: I've yet to find a supplier that lists everything about these cells. It's hard enough to get prices! One thing I did discover was that the price of them is astronomical. I found units that will deliver almost 0.5KW for a shade over £30 each and weigh very little in comparison to lead acid.

3. NiMH: This one was a little off-the-wall. I can't find large NiMH cells, but 3.3Ah cells for model cars are two a penny. They discharge them as a matter of course in 5 minutes, which gives a sustained discharge rate of almost 40A. That's almost 50W each. I'll need 3000 of them, but they are cheap (just over a quid imported from the US). It's one hell of a soldering job though! At 55g each, the resulting battery pack will be under 200KG and power the car up the hill for 5 minutes using an average of around 200bhp. In short a 600KG kit car with the equivalent of 200bhp should be well within reach.

So what have I missed? The NiMH option looks a good option purely because it's simple (no BMS), the right weight and not expensive either. Next on the list once I've got my head round this is a motor/controller. I'm thinking about four smaller motors with direct drive to each wheel, regenerative braking and no mechanical brakes at the rear. This'll be far more fun than building a kit car!

Cheers,
Andy

[MalcolmB]

Hi Andy

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.

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.

You might be able to get a better idea of your energy requirements using this little calculator on the 4qd site: http://www.4qd.co.uk/faq/current.html
It only goes up to 48V, but if you enter your vehicle weight, average gradient and length of hillclimb etc, just divide the final current by three to give you the maximum current you would need from a 144 volt pack.

I can't really see you setting a blistering performance with lead acid, as you've already worked out for yourself. I'd go for lithium ion, and Thundersky seem to be the cheapest at the moment, but there are some quality control issues and you would need a BMS. More information here:
http://www.everspring.net/product-battery-pricing.htm
http://www.thunder-sky.com/order_en.asp
The cells offered by LifeBatt are very promising and there is a UK distributor http://www.lifebatt.co.uk/, but they are still relatively untested and expensive at the moment.

Malcolm

[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.

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.

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/

It's not how I'd do it (Gravitron prmoting their Karting comonents) but take a look at; http://www.compton.vispa.com/batterycharger.pdf

[MalcolmB]

Paul:

Off the top of your head what would you estimate as the average Wh/mile figure for an event of this type?

[EVguru]

Hard to say.

I based my design on Prescott.

Frank Wykes ran his Enfield (120v flooded lead acid) up the hill in about 55 seconds (as fast as a 1500 MG Midget on the same day). The 38 Ah Genesis batteries I have are rated at 400 amps for one minute.

You're probably not spending more than 50% time at full throttle INPUT, let alone full power.