Reduntant Receiver Batteries
11-08-2013, 04:04 PM
#4
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I would say use two battery packs and two switch harnesses. Place a diode in the postivie wires of the leads that connect the switches to the receiver on both switch harnesses. Then, if one pack or switch harness fails as a short circuit the other pack and switch harness will not be affected. In order to compensate for the 0.7 volt drop of the diode it will be necessary to have at least a 5-cell pack. The diode current rating should be high enough to handle your servo load including possibly stalled servos.
I'm assuming you have a glow/gas engine and NiXX batteries. If you are flying electric then ignore me.
I'm assuming you have a glow/gas engine and NiXX batteries. If you are flying electric then ignore me.
Last edited by JPMacG; 11-08-2013 at 04:08 PM.
11-10-2013, 06:06 AM
#6
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The chance of a pack "shorting" are on about the same level as having a California Condor swoop down and grab your plane. Never, ever, in my 40 years of electronics repair have I encountered a "shorted" cell.
11-10-2013, 07:00 AM
#7
My Feedback: (2)
The OP was asking about a product advertised for the specific purpose of protecting one pack from a failure of the other pack. The diodes do exactly that at about 1/10 the cost of the Battery Backer.
And yes, cells rarely fail as hard shorts, but what about switch harnesses. They never develop shorts? It is not unreasonable to imagine a cable getting pinched so that the insulation is compromised and then developing a hard short. I have seen a Futaba J connector that was installed with the insulation peeled back.
The OP is concerned about unlikely failures or he would no be considering the Battery Backer.
By the way, I don't use redundant packs and I have never had a battery pack or a switch harness failure.
And yes, cells rarely fail as hard shorts, but what about switch harnesses. They never develop shorts? It is not unreasonable to imagine a cable getting pinched so that the insulation is compromised and then developing a hard short. I have seen a Futaba J connector that was installed with the insulation peeled back.
The OP is concerned about unlikely failures or he would no be considering the Battery Backer.
By the way, I don't use redundant packs and I have never had a battery pack or a switch harness failure.
Last edited by JPMacG; 11-10-2013 at 07:22 AM.
11-11-2013, 03:59 AM
#8
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It is very common for NiCad batteries to short out. When this happens, it is a single cell in the pack, and the voltage will drop for the pack by approximately 1.2 volts.
11-11-2013, 06:29 PM
#10
You can learn a lot here: http://www.hangtimes.com/parallel_packs.html
Many pseudo battery "experts" put forth the argument that plugging two battery packs into the same receiver with out blocking diodes is NOT a good thing, claiming that his creates a host of problems and the two packs will end up fighting each other or "cross charging".
These concerns show a lack in the understanding of the charge and discharge potentials involved in Ni-Cd cells. One pack cannot charge the another (equal number of cells) as the discharge voltage of a pack can never be as high as the voltage required to charge the other pack. For the doubters here is an experiment: completely discharged one pack to 4.0 volts and then connected to a fully charged pack having an equal number of cells. There will be less than a 10% transfer of charge in a 24 hour period. Since shorts rarely occur in fully charged packs the risk of one pack "dumping" into one with a shorted cell are insignificant. A simple ESE preflight test would detect a pack with a shorted cell.
Many pseudo battery "experts" put forth the argument that plugging two battery packs into the same receiver with out blocking diodes is NOT a good thing, claiming that his creates a host of problems and the two packs will end up fighting each other or "cross charging".
These concerns show a lack in the understanding of the charge and discharge potentials involved in Ni-Cd cells. One pack cannot charge the another (equal number of cells) as the discharge voltage of a pack can never be as high as the voltage required to charge the other pack. For the doubters here is an experiment: completely discharged one pack to 4.0 volts and then connected to a fully charged pack having an equal number of cells. There will be less than a 10% transfer of charge in a 24 hour period. Since shorts rarely occur in fully charged packs the risk of one pack "dumping" into one with a shorted cell are insignificant. A simple ESE preflight test would detect a pack with a shorted cell.
11-11-2013, 09:31 PM
#11
My Feedback: (2)
I would extend Red's argument to its logical conclusion. Dual packs are not necessary.
Oh, but you want to use dual packs because you feel they give you better reliability? Then take that concern to its logical conclusion and use diodes!
Oh, but you want to use dual packs because you feel they give you better reliability? Then take that concern to its logical conclusion and use diodes!
Last edited by JPMacG; 11-11-2013 at 09:35 PM.
