Calculating Battery Pack for Electric Skateboard

[Zapheod]

So.. I am sick and tired of high gas prices and have decided to build an electric skateboard as a secondary method of transport. Main issue is that I have no engineering experience at all and am having problems finding formulas to calculate how long the battery will last.

Can anyone provide this formulas?

Sample data -

Motor - http://www.monsterscooterparts.com/36vo10mowsps.html"
Apparent draw of 35.6 amps

Now let's say I want to be able to run for an hour plus, what battery pack would I need?

Second issue is that obviously size does matter. Can anyone recommend a good website for battery packs?

Not sure what other information may be needed but please let me know if there is anything else that can be provided.

[link fixed]

 

[NoTime]

I couldn't get your link, some DNS issue.
The problem with computing something how long the battery will last is the load.
While the motor may draw 35.6 at it's max output, it is unlikely to do max load all the time.

Existing products, like this one, with 24v 4.5AH battery claim 5-7 miles on a charge.
That is roughly the energy content of a 24v power tool battery. You could carry more than one.
http://www.happyscooters.com/120W-24V-Electric-Skateboard-p-50-16fg232.html

 

[Zapheod]

http://www.monsterscooterparts.com/36vo10mowsps.html"

Corrected link.

The skateboard above is running a 120 Watt motor which only has about a 6 amp draw so it ends up with about 45 min to 1 hour ride time and frankly... slow.

If I am going to build one of these I want something better than that which is commericial available http://www.alteredelectricskateboards.com/m6_800.htm" even if it means running a large battery pack under the main board and extending the motor behind the board.
Frankly.. I am also a bit competitive. One of my friends built a gas powered skateboard with a top speed in the high 40s and I want something that is at least competitive.

so current issues - Formula to calculate duration at max power usage(probably battery amps/motor amp draw)*some effeciency factor(maybe .7 or.8) - Confirm?

and second issue - how to design a battery pack that can be bottom mounted to a skateboard (approx 5 inch total clearance with trucks and 9 inch knobby tires) that will allow for approximately 60 to 90 minute continuous usage with a 1000 watt or 800 watt motor.

 

[NoTime]

Well P=EI.
So your equation with the .7 or .8 efficiency factor is about right.

The 120 watt draw is 5A with the 24v 4.5 amp hour battery somewhat less than an hour run time.
That is slightly more than their claimed run time and it looks like it takes about the full 120 watts to move the board at 9 mph.

DC motor rpm is roughly constant with a particular voltage.
No idea how much power is need to move the skateboard at the speed you want.
Its going to be more, probably a lot more.

If you pick a max speed with gearing that uses less than 1000w max (it didn't say if this was continuous or an intermittent rating) then your batteries will last longer. If you pick a speed that uses more than the motor max continuous rating it will overheat and burn up if you run too long. Going up or down hills will make huge changes in power required.

No idea on the power pack.