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Scratch building a battery for a 2C motor


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#1 Mike Tounian

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Posted 01 December 2010 - 02:40 AM

Hi folks, I am trying to figure out, as the topic says, how to scratch build a battery pack for my 2C motors, both constant and variable speed.

I have seen conflicting information about the voltage needed for these motors. For the constant speed motor, I have seen both 16 and 18 volts listed. For the variable speed motor, I have seen 22 and 24 volts. I have an engineer here at USC who will help me build a battery for it, but he needs some more information than I can provide.

-In terms of voltage, what is the correct voltage for these motors?

-Does this voltage need to be "procision", or exactly the correct number, or can say a 24 volt also power an 18 just fine?

-What is the wattage draw in amps for the motors?

-What is the current?

I literally know NOTHING of power, so I may not be asking these questions 100% correctly, but I imagine there are many on this forum that know exactly what I need and can help out. Thank you to everyone in advance for your responses.
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#2 Patrick Tong

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Posted 01 December 2010 - 03:52 AM

For the variable speed motor:
ProTek R/C 4S Li-Poly 15C Battery Pack (14.8V/2200mAh)

For the constant speed motor:
Thunder Power 2100mAh 3-Cell/3S 11.1V W 16 gauge wire


seach it at ebay. :rolleyes:
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#3 John Sprung

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Posted 01 December 2010 - 04:23 AM

We used to use motorcycle batteries, about 6-8 Amp-Hour ones. They're 12 Volt, so you use a pair in series. You can run the wild motor on 24 Volts if you're careful not to overspeed it. You're just at a lower setting on the rheostat than you'd have with 16.8 Volts for the same frame rate. Or, if you want slow, just use one of your pair of batteries, which gives you 12 Volts.

There are sealed lead-acid batteries for computer UPS's that would work fine. You can charge them with an ordinary car battery charger from AutoZone.




-- J.S.
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#4 Jean-Louis Seguin

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Posted 01 December 2010 - 12:41 PM

I just put some together some for a customer.

I used a 16.8V 5000mAh pack from Batteryspace with the appropriate charger.

http://www.batterysp...ghtscooter.aspx

http://www.batterysp...ugullisted.aspx

Motor specs are 16 Volts and they draw around 2.5 to 3.0 A when driving camera.

Too high a voltage will prematurely wear out the motor through excessive sparking.

Battery capacity can be higher than 5000mAh if you wish for more autonomy.

Cheers,
Jean-Louis
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#5 John Sprung

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Posted 01 December 2010 - 04:04 PM

Too high a voltage will prematurely wear out the motor through excessive sparking.


No, voltage across the motor is exactly the same for a given frame rate, no matter what the battery voltage. With a higher battery voltage, you're just dropping more across the rheostat. Sparking happens when the carbon brushes wear out and reduce their pressure on the commutator. Fresh brushes will prolong the life of the motor. Clean the commutator when you change brushes, and if it's out of round, have a motor shop turn it down for you.




-- J.S.
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#6 Jean-Louis Seguin

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Posted 01 December 2010 - 04:58 PM

No, voltage across the motor is exactly the same for a given frame rate, no matter what the battery voltage. With a higher battery voltage, you're just dropping more across the rheostat. Sparking happens when the carbon brushes wear out and reduce their pressure on the commutator. Fresh brushes will prolong the life of the motor. Clean the commutator when you change brushes, and if it's out of round, have a motor shop turn it down for you.

-- J.S.



I guess I was repeating erroneous information I heard from another source. It seemed logical to me but your explanation for the sparking seems more plausible as far as the variable speed motor is concerned. What about the governor-controlled motor like the constant speed 24fps model? I would think that operating with close to the prescribed voltage is more important in this case. The original poster seems to have both types. Better to build a battery that can be used interchangeably with both motors.

Cheers,
Jean-Louis

Edited by Jean-Louis Seguin, 01 December 2010 - 04:59 PM.

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#7 John Sprung

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Posted 01 December 2010 - 06:25 PM

With the constant speed motor, yes, you do want to stay with the nominal voltage. But that's not hard to do. All the old Arri stuff was designed for use with lead-acid batteries, which have a nominal voltage of 2 volts per cell, but fully charged will go a bit higher. Look up the "Nernst Equation" for the electrochemical details.




-- J.S.
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#8 Mike Tounian

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Posted 02 December 2010 - 12:54 PM

Gentleman, thank you all for your responses. We were able to figure out a perfect scratch build from your information.
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