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› Who remembers how to calculate current?
06-01-2004 06:14 AM  13 years agoPost 1
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rrElite Veteran

Galveston, Texas

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DO you take the discharge rate for example 10C times the capacity of 800 mah the battery? or what? I do know that it runs for about 10 min on my brushless setup.

If that is correct that would give me a discharge capacity rate of 8amps. Is this the correct way to figure out the amount of current a lipo can handle?

I'm just trying to figure out why my 830 mah TP 11.1v pack reaches temps of about 150F degrees.

I'll try to find it in one of the books, unless somebody remembers the formula right off hand.

Predator, Raptor 30, TRex CCPM 450TH, 3D Pro 400F

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06-01-2004 06:43 AM  13 years agoPost 2
Davo

rrVeteran

London, UK

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Yes, for your 10C rated 830mAh pack the max. allowable discharge current is 10 x 0.830 = 8.3A

If you can hook up an ammeter in series with the motor then you'll be able to see if you're drawing too much current.

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06-01-2004 06:47 AM  13 years agoPost 3
fritzthecat

rrKey Veteran

Virginia Beach, VA

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A 830 mah battery will provide 0.83 amps for one hour. Or 1.66 amps for 30 minutes. If it it is empty after 10 minutes (lets say 800mah used) the current draw would be 4.8 amps (0.8*6=4.8).
A Lipoly battery is designed for a certain current draw. This is expressed as 'C'. Capacity times C. I.e.(830*8=6.6)
According to TP, your 830mah battery has a rating of 8C so a current of up 6.5 amps is acceptable. This is the absolute max continuous and assumes good cooling air. Temperature in excess of 140F is a no no for Lipo batteries. I suspect your full throttle runs are less than 10 minutes.
The only way to really know how much current is sucked down is to use a meter. For less than 10 amps, any good multimeter will do. Radio Shack has them. For more than 10 amps, either the Astro Whatmeter or the Sears clamp on meter work.

Lipo FAQ:
http://www.rcgroups.com/forums/showthread.php?t=209187


Fritz


'Send Money, Women and Guns!'

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06-01-2004 12:56 PM  13 years agoPost 4
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rrElite Veteran

Galveston, Texas

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thanks guys,

My 830s rate at 7C so that is 5.81 amps max. My setup must be drawing more than that because my 830s reach temps of 150F. I may have to run the two two packs in parallel whereby doubleing the capacity and discharge rate to 14C = max amps of 11.62; Or change from 10 tooth to a 9T pinion. If I can live with the weigh if 108 gm pack, I think I prefer the former.

I have read that 140F temp warning from towerhobbies, but I did not know if TP packs could handle more temps than other packs.

I have a multimeter, do any of you by chance know what the amp meter symbol is on this multimeter below. I seem to have lost the instructions for it.

Predator, Raptor 30, TRex CCPM 450TH, 3D Pro 400F

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06-01-2004 02:51 PM  13 years agoPost 5
fritzthecat

rrKey Veteran

Virginia Beach, VA

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Sorry, no amp measurement on your meter. When I said 'good multimeter' I meant Fluke, Scope or Simpson for U$100. A great multimeter runs around U$300.
If you want to spend U$50 go to Sears and get their clamp on Ampmeter.
http://www.sears.com/sr/javasr/cata...nuModelNo=55801

Fritz


'Send Money, Women and Guns!'

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06-01-2004 03:19 PM  13 years agoPost 6
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rrElite Veteran

Galveston, Texas

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That sears $50 meter says that measures AC current up to 400amps, but what about dc current.

And is that the cheapest one?

Predator, Raptor 30, TRex CCPM 450TH, 3D Pro 400F

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06-02-2004 06:25 AM  13 years agoPost 7
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rrElite Veteran

Galveston, Texas

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fritzthecat, and the guys

Thanks alot,

I went to Sears and got one. My CP Hummer is drawing 10.3 amps. My 830 pack's max current is 5.8A. That's why my pack was 150F about to swell up and explode.

Predator, Raptor 30, TRex CCPM 450TH, 3D Pro 400F

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06-02-2004 06:53 AM  13 years agoPost 8
Rob_T

rrElite Veteran

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You can also use running time to estimate battery load

Min run time (minutes) = 60 / "C-rating"

Eg for a 7C battery if your run times are less than 60/7 (about 8 mins 36 seconds) then you are exceeding the battery's rating. This works for any capacity of battery.

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06-02-2004 07:05 AM  13 years agoPost 9
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rrElite Veteran

Galveston, Texas

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I agree, the problem is, its hard to estimate a good run time when the bird does not stay in the air long enough due to issues. I'm always setting the heli down for tweaking. So I decided to fork up the money.

Predator, Raptor 30, TRex CCPM 450TH, 3D Pro 400F

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