View Full Version : Power Drain

08-13-2008, 02:39 PM
I need some advice on this subject,
In my R2 I have:
Intel D945GCLF motherboard (which may pull 20 - 30 watts)
2- 24v. 135w Scooter motors ( main foot motors)

I found a battery set that I can get inside R2:
2- 12v. 20Ah batteries ( much bigger than my 12Ah batts)

What I'm wondering is there a mathmatic way of quessing how long the batts. will run R2.
The feet will not be constantly in motion, But as needed.
Some of the other builders are running 12v. 18Ah batts and say they get plenty of run time.
But they are strictly R/C, No onboard comp.

These batts. are the biggest I can fit in R2 and was just curious.

08-13-2008, 02:59 PM
20Ah means that it delivers 20 Amps for 1hr before fully dead. You'll never get that much from a battery in reality. 75% is a fairly safe number for sealed lead-acid, but of course the motors will begin slowing down before that.

You're putting the batteries in series for the scooter motors. You can run them in parallel to the motherboard's power supply. That complicates the calculations a fair bit.

Anyway, let's do the numbers:

If the scooter motors are really drawing 135w (my thousand watt motors actually draw quite a bit more, the thousand watts is at no load). Let's figure they'll draw 250w each at some small load.

Let's figure your board will draw 25 watts and you're running it from 24v in series, the same as the motors (I'm assuming you got the wide-input power supply that we suggested).

With everything else, let's figure that you have 500w + 25 watts + 25 watts of "Everything Else"

These numbers are very "back of napkin" - you'd really need to measure the current draw to know for sure.

500+25+25 = 550watts @ 24v

Watts / Volts = Amps

550 / 24 ~= 23 Amps

75% * 20Ah = 15Ah useable

15 Amps / 20Amps/hour = 0.75 Hour = 45mins.

Again, this is a rough estimate without actual measurement and is intended to be relatively conservative.

08-13-2008, 03:08 PM
Man, 45 minutes?
Is it the comp that's eating all the power?
The motors are 135w, But will they pull that much if they're moving slowly?
The wide power input, I am going to order it and Thanks for that recommendation.
I was sure hopeing to get good time like the other builders.
Thanks Adren.

08-13-2008, 03:26 PM
Are you using exactly the same motors they are? Can you get actual current-drain specifications?

The computer is representing only 1/25th of the current draw in the above example.

As a motor spins slower it tends to draw more current until it's stalled at which point it will be a direct short-circuit.

The more load you put on a motor, the more current it draws.

My scooter motors are rated at no load, spinning free at max RPM, minimum torque. When you put a load on them they jump up very quickly in current draw. Mine run about 40A each at 36v (equivalent of 60A @ 24v) with a small load on them - but they're "thousand watt" motors.

You're just not going to know until you put a meter on 'em.

Another thing that's interesting - can you define "plenty of time" "like the other builders"? "plenty of time" is a little wishy-washy for me to figure out what they're ACTUALLY getting. I get "plenty of time" on one of those little nickle-sized RC cars. About 50 seconds. ;)

08-13-2008, 03:29 PM
They are runing 100w motors.
Let me see if I can get more info on them, Our group has done alot of research on these little guys.

08-13-2008, 04:05 PM
Plenty of use time, I guess is what I'm trying to say.
They run the R2's at large meetings (Like MegaCon) and drive them around alot.
One of our builders has only 2- 12v. 18Ah batts (in 24v.config) in his R2 and he said he's had no problem with
the batts going dead to early.

I have other items in R2 such as Leds, small motors But they will be powered by 4 other small
12v. 4.5Ah batts.

08-13-2008, 04:10 PM
So right off the bat you have atleast 270wts to their 200. That's not insubstantial...

Plenty of use time
drive them around alot.
dead to early.

f_sub-x_[("Plenty of use time" + "Drive them around alot") / "Dead too early"] = ROFL

I tried graphing that equation, but could find the "execute snicker" button on my calculator.

08-13-2008, 04:24 PM
You lost me on that one Adren.
As soon as I know more about the motors, I'll give you guys a yell.

08-13-2008, 04:32 PM
I'll try one more time, then let it go:

"plenty of time" is a little wishy-washy for me to figure out what they're ACTUALLY getting.

"Plenty of time", "drive them around a lot", "dead too early" are totally subjective. One person may think 45mins of continuous use is "plenty of time" where another person may think 45 years of continuous use is "dead too early" - or anything inbetween. If you're trying to figure out what kind of runtimes you're expecting, you need specific NUMBERS not bromidic and platitudinous words.

08-13-2008, 06:21 PM
One of our builders gets about 3 hours runtime off their battery setup.
But like one member mentioned, It's not all drive time.
You'll move R2 around and then you stand to take pictures.
If I could get 3 hours of R2 running like this, I'd be happy.
Would it be easy to figure out how long it would take just the comp to kill the 12v 20Ah battery?

08-13-2008, 08:44 PM
If you had 25% duty-cycle (ie. sitting around for 3/4 of the time), the 45mins I quoted would be multiplied several times - or closing on three hours.

Would it be easy to figure out how long it would take just the comp to kill the 12v 20Ah battery?

Yes, as above:

Watts / Volts = Amps
[(Capacity of Battery in Amp Hours / Current Drain in Amps) * ~75%] = Hours of runtime at 100% duty cycle

25 Watts / 12 Volts = ~2 Amps
20Ah capacity / 2 Amps drain = 10 Hours * 75% duty cycle of battery = 7.5 Hours