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Jun 19, 2001, 05:20 AM
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DavidWillis's Avatar

High current sub C question.


Hi all,
I've just been given a 17x3 buggy motor, which I'm reliably informed will work rather well as a replacement for the Speed 600 in my 1.8m Slope soarer. It has a 3:1 gearbox, and 12 x 8 prop. (both master airscrew)
Now, when I've run the motor up on a battery with the prop / gearbox the combo draws 30 -35 amps. This is the current I was told to expect.
When I run it through the speed control, after around 30 seconds the BEC cuts in. I've discovered that this is due to the cell voltage (7cells, by the way) dropping to below 6V which is the cutoff voltage set of the SC.
The cells I've used range from 2000mAH Ni-Cd to 3000mAH Ni-Mh. Both have approximately the same voltage / current profile, it would appear.
My question is, is this voltage drop typical of all sub C cells at this kind of current draw ?
Or, can I spend a lot of money and get better cells that hold up the voltage under higher current draws (I guess that this drop is due to a relatively high internal cell resistance).
Any comments / advice grweatly received !

Thanks
David
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Jun 19, 2001, 06:02 AM
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Are these cells Sanyo or Panasonic?

Are you sure of the 30 Amps you mentioned? Did you measure it? An average 17x3 should draw 20 - 25A with that setup and 7 cells - what motor is it?

How many Amps is the speed controller rated at? More than 35A?

What kind of connectors are you using between battery and ESC?

Last, if the problem persists, check if the battery is ok. You should measure the voltage of EACH cell during a moderate discharge. If someone goes toward zero before the other, that cell is dead.

[This message has been edited by Giacomo (edited 06-19-2001).]
Jun 19, 2001, 06:25 AM
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DavidWillis's Avatar
Cells are neither Sanyo or Pana, they are GP high drain series (very good value !).
They are OK @ 18-20 A current draw.
The motor is a Parma racing 17 turn tripple.

Yes, I'm sure of the 30-35 A. Measured it with a Fluke ammeter. Total voltage drop across cable (inc connectors) from battery to SC is 50mV. Drop from SC to Motor is 25 mV. Voltage drop across SC is 100mV. This is all at 35 A motor current draw. SC is rated at well above 35A - it will handle 480 A peak, and 80A continuous.
The cells are OK - they work fine in the Speed 600 setpu I have.

I'm starting to think that the cells are'nt happy at 35 A draw. Anybody use / recommend sub C cells for this kind of current draw ?

David
Jun 19, 2001, 06:40 AM
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Bill Glover's Avatar
I think Sanyos are good for 40A+.

See: http://www.overlander.co.uk/new/deta...erPacks&ID=316

[This message has been edited by Bill Glover (edited 06-19-2001).]
Jun 19, 2001, 06:45 AM
Registered User
I had four packs of GP SubC cells... went fine until 25A. Above that value the thin metal layers inside the cells overheat due to their excessive resistance, and the liquid inside the cell starts boiling, right in about 30 seconds, I experienced it too. They're good only for long duration motorgliders (they're 15% lighter than Sanyos too!). Get Sanyo or Pana, moreover their lower resistance will give you much better performance. Worth the money
Jun 19, 2001, 07:00 AM
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DavidWillis's Avatar
Thanks for all the info guys !
I've just ordered some sanyo cells from Overlander - I'll see how these fare.
David


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