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Author Topic: Calculating Resistors and Battery packs Help?  (Read 1523 times)


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Calculating Resistors and Battery packs Help?
« on: November 22, 2010, 12:15:59 PM »

   I need some drastic help working out which resistor I need. Right I have a speed 400 motor which needs 7.4V and it says that at max efficiency it drains 3.3A. I am either going to use a 9.6V or an 8.4V battery pack to power my main motor. And please do check my calculations and tell me off for any silly mistakes.

So starting off on the 9.6V pack    9.6V-7.2V=2.4V    using 3.3A


Using R1=V/I

R1=0.727272727        Therefore R1=0.73Ω

And using the 8.4V pack   8.4V-7.2V=1.2V

R1=0.36363636           Therefore R1=0.36 Ω

Am I getting this very wrong, or is this correct, in which case I canít find a 0.73Ω or a 0.36Ω resistor!! In which case I am guessing that I would have to use a 1Ω resistor. Which would be quite high if I where to sue the 8.4V pack?

The reason for the Resistors is for my Bow thruster as it runs on a different voltage to my main Motor. Or is it just far far easier to use a 7.4V power pack and also run my lighting off of that?

Whilst on this subject of power packs. I know ití possible to wire up two packs in parallel, but do I need a balancer for this, or is it just fine to hook them up in parallel??

Many thanks :-)


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Re: Calculating Resistors and Battery packs Help?
« Reply #1 on: November 22, 2010, 12:34:57 PM »

It is generally not a good idea to fit resistors into a motor supply line. The main reason being that they take an awful lot of power away in waste electricity.

I would tend to use a 7.4 volt battery as this will match the motor and if you use a speed controller the voltage will be, on average, less anyway. You might get away with direct 8.4 volts on the supply, briefly.

Regarding fitting batteries in parallel then a lot has been said on here against this idea. Perhaps other members might enlarge on this technical stuff.

Hope this helps

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Re: Calculating Resistors and Battery packs Help?
« Reply #2 on: November 22, 2010, 12:50:33 PM »

Hi, tjones27

Er, you don't need a resistor :}
Your arithmetic is impeccable, but not applied in a good cause

first, the S400 7.2V is not  a voltage, it is the size of wire it is wound with, and hint about a voltage to start at!

You can run this motor at any voltage from 1.5 to 20 so long as you keep the current below 10A, and there is no way that a bow thruster is going to exceed this
and it says that at max efficiency it drains 3.3A (I run a S400 7.2 on 18V in a ducted fan aircraft - it has lasted about 10 flights so far)

If you are running the bow thruster with a speed control - presumably reversible, then staying away from full throttle is (effectively) the same as reducing the voltage applied.

So do, by all means run it off the main battery
If you need tactile reassurance take the boat out from the dock, do a minute of bow-thrust pirouettes and feel the motor :}  It should feel hot but not scary hot
You could easily (if needed) wind an aluminium or copper cooling coil round the motor and cool it that way - but it shouldn't be necessary.

btw - resistors of the size you calculated do very commonly exist  - they are called lengths of resistance wire salvaged out of dead hair-dryers (I have 2 daughters) and they are also presented to look like car light bulbs!  12V, 12W is one ohm!



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Re: Calculating Resistors and Battery packs Help?
« Reply #3 on: November 22, 2010, 01:05:34 PM »

If your motor says 7.4 volts and you have to run it at that voltage why not just use a pack with 6 cells and not suffer energy losses through a resistor?
Your motor would probably handle the higher voltage anyway. Assuming that you are going to have some kind of speed controller that can handle the slightly higher voltage - would that control the speed and eliminate the need for resistors ?
With electrical power a little forward planning would eliminate the need for extra efficiency losing additions

As regarding connecting two NiCad /Nimh batteries in parallel, on this type of type I would put a diode in the positive line on each battery connector on the 'boat-side' of the connector - in that way one pack could not try to charge the other. It's not best practice though, I'd be inclined to go for higher capacity batteries in the first place.

Also if you can't find an exact value of resistor you can use higher values in parallel that would give you the odd values you require - but I'm not suggesting the formula because I've forgotten it!


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Re: Calculating Resistors and Battery packs Help?
« Reply #4 on: November 22, 2010, 04:05:08 PM »

As stated above, use the same pack and power down with a suitable ESC (I use M-Troniks units).

Connection of two packs in parallel can lead to one pack discharging the other - this happens if the two packs are not exactly of the same charged capacity and voltage - a suitable diode pack will ensure this doesn't happen (see Action Electronics - they have a small relatively inexpensive unit). However, if they are both equal, they can be connected in parallel without a diode - I do this on one of my boats. As someone else suggested, you could always revert to a higher-capacity (single) pack.

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