Help choosing a RX battery pack

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Help choosing a RX battery pack

Postby Chskin » Sun May 05, 2013 4:12 pm

Hello

Just finished watching the video on RX batteries and I want some help choosing a RX battery pack for my plane. It is a 1/4 SE5a with a gas engine. There are 6 Hitec 645MG servos and a JR DMSS RX. The servos are rated for 4.8-6.0v. I do not want to use a BEC however I can't figure out how to get a LiFe or A123 battery into that voltage range. I am even considering using 2 batteries (1000mah each) as redundancy should one fail. What are your ideas?

Thank you

Craig
Canada
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Re: Help choosing a RX battery pack

Postby BillGriffiths100 » Sun May 05, 2013 6:05 pm

Something like these high capacity sub c cells is what you need to make up a pack of four
there are switching circuits that would allow you to use two packs one switching in if the other failed.
However if you make up two packs of five you can use diodes to prevent the packs fighting each other.
http://cellpacksolutions.com/Search_Dat ... tAodrAkAuA
If your servos were rated up to 7 volts you could use a pack like this:
http://www.hobbyking.com/hobbyking/stor ... _Pack.html
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Re: Help choosing a RX battery pack

Postby Chskin » Sun May 05, 2013 6:14 pm

Ok, here is a silly question. If I connected the batteries in parallel, why would they fight each other? I wanted 2000mah for this model. Using 2 1000mah batteries (one plugged into a spare RX port) would give me redundancy if one failed. Is that a valid idea or is the risk of battery failure so low as not to worry about it?
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Re: Help choosing a RX battery pack

Postby BillGriffiths100 » Sun May 05, 2013 9:18 pm

OK if one set of batteries falls low due to a cell failure the other paralleled set can drive current back through the other set discharging itself.
You wouldn't think that this would happen and normally it won't since the effect of back charging would only be to equalise the packs, but when a cell fails this will pull down the other pack.
To prevent this two diodes will prevent one pack from pulling down the other this applies to the ni type of cell lipo's are commonly paralleled in flight packs without diodes but it is possible that a similar thing could occur.
The reason for adding an extra cell to each pack is that the diode drop (0.7 volts typically) will drop a 4 cell pack to a nominal 4.6volts rather than 5.37.
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Re: Help choosing a RX battery pack

Postby Chskin » Sun May 05, 2013 11:10 pm

Ok thanks, some good information. Just two questions along that line. What type of diode would I use? Would one rated at 7v and 250 mh be sufficient? Secondly, could I use a BEC in place of a diode? Would it also stop reverse current?

Cheers
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Re: Help choosing a RX battery pack

Postby BillGriffiths100 » Mon May 06, 2013 8:37 am

No I wouldn't use those they sound like signal diodes, to be on the safe side something rated at at least four amps and more than 50 volts is your best bet something like a Silicon Rectifier 6A10 6A. Rated at 6 amps and 100 volts. Its not critical just some relatively beefy ones.
Two BECs would have slightly different output voltages if their outputs were put together leading them to constantly try to readjust their outputs so no it's not a good idea.
There are circuits that do the battery switching such as:
http://www.hobbyking.com/hobbyking/stor ... ystem.html
However all the extra circuitry involved brings in itself extra risk of failure.
Keeping it simple is a good idea.
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Re: Help choosing a RX battery pack

Postby Chskin » Tue May 07, 2013 4:31 pm

That's great. I will set it up with two LiFe 1000mah batteries each with a 6A diode in parallel. That should give me the redundancy I am looking for.

I appreciate your help.
Craig
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