Choosing battery help

choosing battery helpp

Greetings!
I am building an electric go-kart and would like advice/tips for selecting the battery and charger. I am considering LiFePO batteries from Sky Energy but we have a tight budget so I may opt to go to deep cycle lead acid if I can find the right deal.

The motor is the the brushless dc Mars0913 24-72 V with 12kw rms and 30kw peak.
http://www.kellycontroller.com/mars-0913etek-comparablepmsmbldc-motor-p-874.html
The controller is a Kelly controller, KHB72701,24-72V,700A,Opto BLDC Controller/With Regen
http://www.kellycontroller.com/khb7270124-72v700aopto-bldc-controllerwith-regen-p-838.html

I'm thinking I need a minimum of 8 batteries 2 packs of 4 batteries in series connected in parallel.

Can anyone offer any advice or wisdom of what time of battery configuration I'm going to need for decent performance? Anyone??

we also want to be able to achieve a top speed of at least 50 mphs.

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Thats dependent on the gearing. the sprockets on the go kart axle are ussually interchangeable. Calculate the max rpm (at the voltage you choose to use), then figure in the gear ratio's between the drive sprocket, final sprocket, and wheels. there are calculators online that could help you out

I realize the power output is reliant on the voltage but im just worried about the operating current of the motor. can u offer any advice about current? thanks

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Power is reliant on voltage and current (P=VI). In your situation, the most power you will be seeing is about 50kw. I figured this by using the controllers' max voltage and current which is 72v and 700A respectively. 50kw is pretty serious as far as power goes for a go kart. here's a pretty popular youtube video showing a 50kw electric go kart
http://www.youtube.com/watch?v=-xitMWUfeLk . I dont know what you want to hear about the current. Im not that sure about for how long the motor can sustain 700A (im assuming not very long). You probably will using be using alot less most of the time.

my suggestion remains the same. these are the batteries
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=14617
Each pack is 37v. 2 packs in series make 74v which is just what you need. the packs at 5Ah and can supply 45-90C which means 450A peak. so two in parallel and then two in series means a 74v 10Ah pack that can supply 900A bursts. for some reason hobbyking is offering me a one time discount right now while viewing the page. $152.56. strange. the normal price is $161.24 so x 4 = $645. only $610 in my cart currently. Email hobby king. maybe the can give you the offer that im currently seeing. Keep in mind this is only a 740w battery pack. this will on take you a few short miles depending on driving style.

BTW, what kind of project is this? Im guessing from your user name that a UCF student (central florida). Are there any EE's in the group?

EDIT: Didnt notice your motor is limiting you on power a bit. looks like it will be 30kw max for your build. oh well. my suggestions remain the same

Id forgo the super capacitor bank. the batteries themselves should be able to provide more then enough current when you want it. money would be better spend on more batteries, which would most likely be much cheaper as well as lighter. A reasonably sized "NOS" bank would most likely cost outside your budget.

Ill try explain the current a bit better. The C rating given the batteries is used to indicate a batteries continous current draw. this number is multiplied to the batteries current rating. In your case, the batteries current rating is 5Ah and the C rating is 45C continuous and 90C peak. this means that one pack should be able to put out 225A continuous and 450A peak. All those nano-tech batteries I was point you towards have a C rating of 45-90, but come in different current ratings, such as 2200mAh and such. you would take there current ratings (given in Ah) and multiply it types there C rating to get how many amps the batteries can provide.

now, this are specs just for the batteries. you need to look at the motor and controller spec sheets to see what the can do. from a quick glance, your motor can handle 350A continuous and 700A peak (1 minute). you batteries will be more then happy to supply that. next in line is your motor which you said can do 150A continuous and 550A peak. So it seems that your motor is limiting everything, then your controller, and lastly your batteries. 25kw is still very respectable for a go-kart. Im not sure if you can control the acceleration ramping, but if you could make it instant, you could stand still just burning tires.

Before you buy the batteries, please read about lipo and their safety. they are known to be a little more volatile then some other lithium types. cell monitors with a low voltage alarm is a must. just keep a close on the voltage when charging and you should be find