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Old 08-22-2014, 11:32 AM
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raptureboy
 
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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.