Green Power Challege: Battery tactics?

[Dynamic Position]

I spent an interesting day at the track side yesterday watching the Shell UK sponsored Greenpower Challenge where my son was taking part as part of his mechanics course which he has just started at school. The aim of the challenge is to see which team can travel furthest in 4 hours, include at least 4 pit-stops. They have to use identical 24 volt motors and 4 x 12 volt Lead acid batteries, but are free to design and build an appropriate car. It can only race if it passes the stewards safety test inspection.

An attached photographs shows a car in action. I wish this was available as a school subject when I was at school.

The school teams took to the task seriously, with team decisions being made and the driver of each car having a helmet radio link to the team, an experienced team also used hand-held signs at the track side.

During the race some teams had to deal with punctures, electrical faults and mechanical failures which cost them precious time on the track. One lost time as penalties for poor driving. All teams had to decide their battery change strategy.

The last half hour really showed where team tactics paid off. The lightest vehicle only stressed by the weight of its drivers! One team made regular swaps of the battery sets hoping to gain from the time it took to make the change.

In a race where speed is not the end all and be all and team effort in the design, build, driving and race tactics are everything the kids loved every minute even if they did not win. After all the newbies like my son would produce an even better car, race ready for next year. At least this year the team won a trophy for the best presented car which the school can display with pride.

The team analysis highlighted key areas for improvement, including battery change over times, and the requirements for a spare wheel rather than trying to mend puncture on a blown out tyre. More driving practice should have been undertaken, but the kids school teachers had the car racing it round the staff car park.

Now to return to the purpose of this post - battery tactics?

Would there be a technical advantage to design a heavier car having all the batteries on board saving time worrying about batteries or should the batteries be swapped out with its associated time penalty? Assume around 4 minutes to change the batteries. Remember the car that travels the furthest in 4 hours is the winner. Teams are not allowed to charge the batteries.

 

[flecc]

Sounds great!

There should be little disadvantage to carrying all the batteries if the tyres are run hard enough to keep rolling resistance low and corners allow high enough cornering speeds, since acceleration will not be a big issue then. Also, having the four batteries in the series parallel configuration for 24 volts would improve performance over two series pairs only.

An alternative approach if slow circuit corners mean more acceleration would be to develop high pressure sprung contact clamps for the batteries so there'd be no bolting time when changing batteries. If the pairs were trayed for quick swap and contact clamps used, there could be a big saving on that four minutes.
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[Dynamic Position]

Perhaps BEBA could have similar challenge in future but with bicycles instead?

This would raise the profile of the e-bike amongst the young.

Mechanics, a school subject so popular the kids will give their weekend to take part!

 

[Patrick]

Teams are not allowed to charge the batteries.

If battery capacity is an issues then one lesson they could learn from ebikes is that a driving style with minimal breaking improves efficiency. Accelerating drains the batteries and every time you break you waste expensively generated momentum.

If the cars tend to slow down during the course of the four hours then although avoiding the breaks would mean initially driving slower it might improve the average speed.

 

[Dynamic Position]

If battery capacity is an issues then one lesson they could learn from ebikes is that a driving style with minimal breaking improves efficiency. Accelerating drains the batteries and every time you break you waste expensively generated momentum.

If the cars tend to slow down during the course of the four hours then although avoiding the breaks would mean initially driving slower it might improve the average speed.

I think there is more likely to be a problem in that it is possible to run the car at the maximum speed available around the whole circuit, this presents problems because inefficiency's and losses through driving fast wastes the stored battery energy. It is too tempting for the driver to go as fast as possible, presenting the problem of running out of juice before the 4 hours is up.
I think it might be more efficient if the drivers are instructed to follow the fastest car on the track (if possible) to gain from effects of less air resistance? Like e-bikes, the lightest vehicle has the advantage .

 

[flecc]

Like e-bikes, the lightest vehicle has the advantage.

At a constant speed on the flat there is virtually no benefit from lighter weight with a given tyre size if the tyre pressures are correct for the weight.

There are only two possible losses. One from the increase in bearing friction/drag due to the increased weight, but this is negligible with a decent ball bearing, the second from the increased compression of the tread rubber. This should be offset by the performance gain of series/parallel batteries in a normal race condition and is minimal with thin treads anyway.

However, since the race is endurance based, running the batteries are two pairs will be better over time. Personally I think a mileage covered race is better as a test of the vehicle, reducing the driver skill component and giving more flexibility in battery combination.
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