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Dual batteries into a 92104 10ch receiver

Old 08-21-2014, 10:32 AM
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hogflyer
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Default Dual batteries into a 92104 10ch receiver

I was talking with a buddy who mentioned he was running dual LiFe batteries with his 92104 10-ch FHSS-3 receiver directly into the receiver ports 5 and 10 with no problem. This has me wondering as I was taught decades ago not to run dual batteries into the receiver, instead use a "Y" harness before the switch. Considering the technology in today's receivers and batteries is much different than in the mid 1970's could this not be the issue it once was?

What his thinking for this is that when using "Y" harness the batteries are in parallel, and by each plugging straight into the receiver the are in parallel also. His are run through separate switches and he claims this is safer than a "Y" into a single switch as there are two potential sources of failure - switch and "Y" harness, and either of them, being in series, will kill power to the receiver. By running direct to the receiver, if a switch fails there is still the other battery powering the system. This also allows him to test the receiver with each battery independently by just simply turning one off.

Now he has me thinking about doing this with my 1/4 scale Cherokee glider. I need two LiFe batteries for nose weight to counter the extra weight of the overkill 94851 servo's in the tail (rather us batteries than lead), and don't like the idea of using a "Y" harness or leaving one battery not connected. Putting a second switch on the other side of the fuselage for the second battery is no big issue. The more I think about it, it's just doing the parallel directly through the receiver instead of a "Y" harness.

So what is the general consensus? Good idea that will eliminate the potential point of failure of a "Y" harness, or bad idea that may damage a receiver?

Hogflyer
Old 08-22-2014, 10:39 AM
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Don't use a Y harness., and, with that radio, you should pretty much try not to have to use a Y harness for anything. 2 batteries to 2 switches to 2 ports. Provides higher current capacity to the Rx and switch redundancy. I had a jet once where I went to fly and neither switch would turn the jet on. So switches do fail. I also killed a couple pairs of Nicads years ago by using a Y harness on the batteries before the switch and they self discharged each other to nothing when I let the airplane sit for a couple months. You might as well also use 22ga wire for servo extensions and such as they will carry the current better.

PS: In the early 70's, I used to fly at the field north of the Beech plant when I was in the Air Force.
Old 08-22-2014, 11:32 AM
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From hangtimes.com Some of our most often asked questions deal with the practicality of using two packs. I find it incredible that there are still wives tales circulating about the practice of using packs in parallel… we’ll deal with the myth and reality, and have a look at WHY you might want to use a parallel / redundant system.
Q: What do I ‘get’ if I use two packs?
A: In essence.. it’s almost a ‘Free Lunch’.. by doubling up our receiver packs we get :
Reduced system voltage drop under a load due to an effective 50% reduction in system impedance. This means the system voltage will be higher under a given load with TWO packs instead of one. This is an exceptionally desireable advantage in an agressive aerobatic aircraft!
No single switch, pack or connector failure can kill the plane..
Flight time is increased by the additional capacity of the second pack
Q: Ok, that’s pretty kewl.. but my instructor sez I need diode protection or the packs will cross-talk or try to charge each other.
A: Sorry.. your instructor has been gaffed by Under-Informed Magazine Columists or the Battery Mis-Information Committee of your local fields Wives Tale Tag Team. There is NO need for diodes or a 'backup' circuit board (something else that adds weight, complexity and another possible failure point) and in fact there's no flight-safety significant energy transfer between packs at different charge levels.

You can test this yourself.. just plug a discharged pack and a fully charged pack together in a Y-Harness and check the two packs the next day.. You’ll find the system will pass a load test if checked through the ‘Y’, and you’ll also find that less than 20% of the charged packs energy has been ‘used’ by the discharged pack when you separately cycle-test the two of them. Next, consider that your average flight is 10-12 minutes, not 24 hours! The mythical "Energy Transfer Between Packs" scenario is simply NOT a flight safety issue and checking both packs before flight with a loaded ESV will certainly 'pick up' a weak pack before you fly it anyway!
Q: Can I run packs of different size in parallel? A: Yes.. if by size you mean ‘capacity’. Remember; battery packs are rated in three ways.. capacity, impedance and voltage. In a parallel system the number of cells in the 2 packs should be the same, and we recommend you use packs with similar impedance ratings but the capacity (milliamps) rating can be different.
Old 08-24-2014, 07:17 PM
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Originally Posted by Thud_Driver View Post
Don't use a Y harness., and, with that radio, you should pretty much try not to have to use a Y harness for anything. 2 batteries to 2 switches to 2 ports. Provides higher current capacity to the Rx and switch redundancy. I had a jet once where I went to fly and neither switch would turn the jet on. So switches do fail. I also killed a couple pairs of Nicads years ago by using a Y harness on the batteries before the switch and they self discharged each other to nothing when I let the airplane sit for a couple months. You might as well also use 22ga wire for servo extensions and such as they will carry the current better.

PS: In the early 70's, I used to fly at the field north of the Beech plant when I was in the Air Force.
Unfortunately Beech North is gone - went the way of a developer. It was moved once many years ago, then was going to be moved again but the new area didn't have enough room to be safe. Instead the developer worked with the City of Wichita to put a paved runway into Chapin Park (behind the drive-in at Hydraulic and MacArthur).

I know the "Y" harness is a failure looking for a place to happen. I guess some things just stick in the mind even though technology has changed like night and day since the 1970's when radios were wired differently and had the separate battery wire for a reason. I did install the second switch and flew all weekend with both packs into different ports. With 4200mA worth of battery I didn't even think about running them down. The radio worked like a champ even with the glider going out of sight for about 20-30 seconds when it hit a boomer at 1500'. And all the extensions are MPI 22ga extensions with heat shrink at all the junctions except were the ailerons/spoilers plug in from the wings to the fuselage.

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Old 08-24-2014, 07:19 PM
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Originally Posted by raptureboy View Post
From hangtimes.com Some of our most often asked questions deal with the practicality of using two packs. I find it incredible that there are still wives tales circulating about the practice of using packs in parallel… we’ll deal with the myth and reality, and have a look at WHY you might want to use a parallel / redundant system.
Q: What do I ‘get’ if I use two packs?
A: In essence.. it’s almost a ‘Free Lunch’.. by doubling up our receiver packs we get :
Reduced system voltage drop under a load due to an effective 50% reduction in system impedance. This means the system voltage will be higher under a given load with TWO packs instead of one. This is an exceptionally desireable advantage in an agressive aerobatic aircraft!
No single switch, pack or connector failure can kill the plane..
Flight time is increased by the additional capacity of the second pack
Q: Ok, that’s pretty kewl.. but my instructor sez I need diode protection or the packs will cross-talk or try to charge each other.
A: Sorry.. your instructor has been gaffed by Under-Informed Magazine Columists or the Battery Mis-Information Committee of your local fields Wives Tale Tag Team. There is NO need for diodes or a 'backup' circuit board (something else that adds weight, complexity and another possible failure point) and in fact there's no flight-safety significant energy transfer between packs at different charge levels.

You can test this yourself.. just plug a discharged pack and a fully charged pack together in a Y-Harness and check the two packs the next day.. You’ll find the system will pass a load test if checked through the ‘Y’, and you’ll also find that less than 20% of the charged packs energy has been ‘used’ by the discharged pack when you separately cycle-test the two of them. Next, consider that your average flight is 10-12 minutes, not 24 hours! The mythical "Energy Transfer Between Packs" scenario is simply NOT a flight safety issue and checking both packs before flight with a loaded ESV will certainly 'pick up' a weak pack before you fly it anyway!
Q: Can I run packs of different size in parallel? A: Yes.. if by size you mean ‘capacity’. Remember; battery packs are rated in three ways.. capacity, impedance and voltage. In a parallel system the number of cells in the 2 packs should be the same, and we recommend you use packs with similar impedance ratings but the capacity (milliamps) rating can be different.
That's some good information. Thanks for posting it.

Hogflyer

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