Technical, Techniques, Hints, and Tips > DC Motors (Brushed) and Speed Controllers

How much current?

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flag-d:
Fairmile type D MTB, 1/24 scale, 58" LOA, 15lbs displ running 4 x Graupner 500 E's, 4 shafts, 4 props, powered from 2 x 6V 7AH gel cells.  Just changed props to 45mm Rivabo 3 bladed brass which gives a better turn of speed than the 35mm's I was using.  With the 35's, the motors and ESC's remained cool at all times.  Now the motors run hot and the ESC's are warm-ish.

I have heard (urban myth?) that if you draw too much current from a gel cell, you can 'kill' it.  True?

Is there a way of knowing how much is safe to draw from any given gel cell battery?

I have to say that the increase in speed is not fabulous: could the props now be too big and therefore running slow?

Mike

Doc:
Mike,
Wish I could give you a definite answer about the current draws for gellcells, but I can't.  I'm pretty sure that there are 'formulas' out there, just haven't found enough need to find them yet.  A very, very rough rule of thumb is that if a gellcell doesn't last for more than 15 minutes, something "ain't right", at least for the boats I've built (limited).
From what you've said about the motors getting hot, I'd think that the props may be just a 'shade' too much load.  If the ESC is warmish too, then it's definnitely getting close to 'too much', the whole system I mean.  If you can 'adjust' the pitch on your new props, I think I'd make then a bit 'finer'.  I also wouldn't put much faith in my opinion...
 - 'Doc

PS - You always have the alternative of beating the @#$ thing till it obeys!

wombat:
You can probably kill a gel-cell from overcurrent, though it would have to be fairly massive. Looking on the Yuasa web-site they specify discharge characteristics for their cell up to 6CA (in your case 14*6 = 84A). The continuous current draw would have to be greater than that to kill the cells in the battery.

Tim (the Wombat)

Eddy Matthews:

--- Quote from: wombat on July 05, 2006, 08:46:37 am ---You can probably kill a gel-cell from overcurrent, though it would have to be fairly massive. Looking on the Yuasa web-site they specify discharge characteristics for their cell up to 6CA (in your case 14*6 = 84A). The continuous current draw would have to be greater than that to kill the cells in the battery.

Tim (the Wombat)

--- End quote ---

84 Amps! Most Nicads and Nimh cells would struggle to sustain that sort of current draw!

He also doesn't say if the 2 batteries are wired in series or in parallel - If they are in series (to give 12 volts) you can immediately halve that figure, and even then I doubt the batteries could survive for long at 42 Amps. A lot of these manufacturers give very misleading figures and often quote PEAK short term discharge as a continuous rating. Personally I think a gell cell will be on it's limit at anything much above 20 A discharge rate - They just aren't designed for high current applications.

Regards
Eddy

wombat:
Hi Eddy,

Looking at the Yuasa figures, I had calculated for a high discharge rated battery - they only spec down to 3CA for a normal battery. Saying that they do not specify a maximum current draw for the batteries.

The curves I looked at to get the figures were the rated discharge curves - terminal voltage vs time and current, so not a peak figure. I would agree that to run a battery of this size much above 20A on a regular basis is not the best of ideas, but they can sustain a much higher current discharge.  Short circuit current on the Yuasa batteries is 105A for 6V 7AH and 210A for 12V 7AH.


Tim (the Wombat)

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