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Old 08-24-2014, 07:19 PM
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hogflyer
 
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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