discharge rating need determination

[Chenige]

How do you determine the minimum discharge rating of a battery that you need in a setup?
Is it determined by the motor specs?

ADS300 Water-cooled Brushless Outrunner 3000kv 300w

Motor Size: 400 Class
Speed: 3000rpm/v
ESC Required: 30A
Voltage Range: 7.2v~11.1v (2s~3s Lipoly)
No Load Curr: 1.1A
Max Load: 24A

[GP73]

Let's say you want at least 300w for that motor:

300w = voltage x mah x discharge rate

Take those watts and divide by the voltage of your battery, let's say a 3S 2000mah (3S = 12.6v max).

300w / 12.6v = ~24A / 2000mah = 12 discharge rate

That's the theory because in reality any motor can and will generate more watts if you're running a little hotter. So in reality you should run the highest mah and discharge battery you can fit in your application.

[Chenige]

I understand the math but not sure I understand the calculation. In my case: 3s 5000 mah battery with above motor:

300w/(11.1v x 5000maH)=5.4 discharge rate.

So a 10C battery would work just fine? So if I buy the 20C batteries I will be safe?

So my run time would be 5000/ (5000x5.4/60) = ~ 11 minutes?

Heh, I am a mechanical engineer and just never understood how little creatures run thru those wires and make things work.

[785boats]

Chenige.
In reality, nobody (or very few people) runs a motor at its Max Efficiency Rating. By the time you have a prop on the boat (whatever type of hull it is) that gives you the top speed with the temps still in the acceptable range you will be drawing way more than the 300W or 24A given in the specs.
The suggested 30A ESC will be smoke. So your calculations have to start with higher numbers than the specs.
In my opinion you will be drawing 35A or so & would need a 60A ESC to be safe. This is just what I've found over the years.
Cheers.
Paul.

[Chenige]

????

Now I am getting very confused. I'm trying to keep the cost down because I need to buy 4 of each component because we are building 4 boats. Moving up to a 60A doubles the price of the ESC.

The motor spec says 30A ESC needed. The one I listed in a different thread for comment was 35A constant and 190A burst. http://www.hobbyking.com/hobbyking/s...idproduct=7377

Do I need to downgrade the motor or will this ESC be OK?

[siberianhusky]

Confused!
A 5000mah 20c battery is able to deliver 100amps, voltage doesn't matter here.
300w/12.6v = 23.8 amps What the motor will draw on a freshly charged pack, keep in mind as the voltage sags under load the amp draw goes up, at 11.1v your at 27 amps.
No experience with that esc but I'd choose the 60 the 30 it would be running to close to it's limit for my tastes, not enough headroom to allow for mistakes.

Volts x amps = watts. SO watts / volts will give you amperage.
The C rating x capacity has always given how much amperage you can draw. (20c x 5A = 100A)
So at 300 watts you are drawing 23 amps from a 5Ah battery that can deliver 100A. Remember this is theory, in practice nothing is perfect so the loads will likely be higher in an actual boat on the water.

[GP73]

As 785boats said better than me the watt generated and the amps drawn can be substantially higher than what's rated depending on the boat setup.

Tell us more about the size and type of boats you want to build and maybe we can give you a better suggestion.

But as a general rule: smaller prop = less amps, less wet area = less amps. I agree the 30A ESC wouldn't last long, unless the boat was a very light rigger or something like that.

Your math is correct: in *theory* you could get away with a 5000mah 6C rate battery, but then you have to make sure the burst rating can sustain the load too. In reality I would use at least a 5000mah 15C-25C burst.

Keep in mind that ratings are not necessarily accurate, that is true for motors, ESC and batteries. It's just a guideline.

Plus there is always waste in any electric circuit in the form of resistance. Check out http://scriptasylum.com/rc_speed/vdrop.html for a little demo of how much power can be wasted in heat.

????

Now I am getting very confused. I'm trying to keep the cost down because I need to buy 4 of each component because we are building 4 boats. Moving up to a 60A doubles the price of the ESC.

The motor spec says 30A ESC needed. The one I listed in a different thread for comment was 35A constant and 190A burst. http://www.hobbyking.com/hobbyking/s...idproduct=7377

Do I need to downgrade the motor or will this ESC be OK?

[Chenige]

Boats will be electrified from an old rc boat magazine.

20" lightweight wood mono
surface drive

I am building these with 10,11 and 12 year old kids to give them building experience and perhaps some gungho for rc boating and for me to get experience with LiPO before I convert some of my nitro hulls to electric (sport 40's).

Do not need to be ultra fast but more exciting than what I already built 10 years ago which used a 19T car motor and 7.2 v nicd with a small astroflight 214 35A max ESC.

thanks

[Chenige]

Thanks, I moved my questions to the fast electric boating questions section.