electric go kart conversion. battery question!?

dwaring28720

hi no idea if ive posted this in the right section, but i apologise now if it is, ive decided to do some tinkering with my retired kart basically im pulling out the honda gx160 engine from my Honda pro kart and want to fit it with a "36v 800watt escooter motor"

http://www.ebay.co.uk/itm/36-VOLT-80...item2a163820ed <-- sorry if i am not meant to link things like this it was just for explanation purposes.

and basically my question is, can i run this motor at full using 3x car 12v battery, ie

http://www.ebay.co.uk/itm/12V-CAR-BA...item19be9d6312

as i may have got this calculation wrong but according to what i read you take the CCA "Cold Cranking Amps" and multiply this by 12.6 to get the watts, so the battery in question produces 400 CCA x 12.6 gives me 5040 watts and with 3 of them running in series to produce the 36v ... am i getting this right before i go out and buy bits that aint gonna work


thanks in advance

The Chase

I'm not too sure why you are using cold cranking amps here. If you are asking 'how long will these batteries power the kart for' then you should be looking for amp-hours. I'm also not too sure where you have this constant of 12.6 but the Watts provided to your circuit should follow basic Ohm's law (I=V/R)

if we rearrange this to;

P=I*V (power = volts multiplied by amps);

And because we know V=36v and P=800W then we can rearrange further to get the amps required from the batteries;

I=P/V (I = 800/36)

Amps drawn from the battery when the motor is at full 800W load will be 22.22 amps.

I know the battery in my car is rated at 40 amp-hours, multiply this by three to represent your system in series and we can supply 120 amps for an hour.

120/22.22 = 5.4 hours

So yes, three car batteries in a series circuit would power an 800W motor for 5.4 hours before they needed recharging.

Hope this helps.

dwaring28720

thanks for your answer very clear and helpful, you even answered my next question of how long the batteries would last after a full charge, i came across the CCA figure when browsing elsewhere for a quick answer

http://wiki.answers.com/Q/How_many_w...attery_produce

thanks very much for your help

The Chase

Glad to be of help, good luck with your project.

potatoecannon

I believe you may have confused series/parallel there: if the voltages add AND the current capacities add, then total energy availe would have gone up by 9 for 3 batteries...

In series, each battery will still only supply 40 amp-hours, and the voltages stack.
If you ran the batteries in parallel, you'd only have 12 volts available, but would then have the 3*40 = 120 amp-hours.

Each battery is capable of supplying 12V * 40 amp-hours = 480 Watt-hours of energy. 480*3 = 1440 Watt-hours total.

You will get approximately 40 amp-hours / 22.22 amps = 1.8 hours run time @ full load (Same as calculating 1440/800).
Note, this does not take into account starting current (locked rotor current), which will be several times higher than the 22 amps. (Hint: don't buy controller components rated to 25 amps)

CCA (Cold Cranking Amps) is a rating of what the battery can do for a very short time in cold weather (I think it's 1 minute) - you do not want to design to this, it's hard on the battery, and would yield a very short run. The 3*12.6volts*400CCA=15,120 Watts (20 hp) calculation is the peak power that you can draw from this setup.

Again, 400 amps may be somewhat...difficult/expensive to design for (in regards to your controller). Find the curves for the motor you want to use to get the max current it will draw.

As a tip, look into PWM (pulse-width-modulation) circuits for controlling the power you feed the motor; any other method will reduce your runtime (less efficient-creates more heat).

potatoecannon

One more note (if you pick this project back up again): Batteries do not like to be run from full charge to full discharge - you will kill them rather quickly (unless you shell out for "deep-cycle" batteries). Something to keep in mind.