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Old Jul 30, 2013, 02:26 AM
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Belgium, Flemish Region, Geel
Joined Jun 2013
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Crash. Brown out, c-rating and ubec question

I just had a crash, presumably caused by a brownout to my flight controller ;all 4 motors stopped after one or two seconds of fulll throttle.

My battery is brand new, rcplus python 4S 4000mAH 25/50C. My quad has a calculated peak power draw of ~83A:
http://www.ecalc.ch/xcoptercalc.htm?...h=4.7&blades=2

I thought that would give me plenty of safety margin, as the battery per spec should be able to provide 100A continuously and 200A peak.

However I already noticed on the first flight that my lipo alarm went off briefly on full throttle, even though after landing the battery was still at 3.75v per cell. I should have paid more attention to this. On a next test flight, I did not get a lipo alarm beep, but at around the same voltage, all motors cut out.

Im powering my fight controller through one ESC (hobbyking afro).

My questions:

- did I underspec this battery, or is it just not living up to its rating? I also have a turnigy nanotech 4500 25/50C (not used yet) and I want to order some extra batteries soon, should I really get more than 100A cont', 200A burst discharge capacity for a <90A burst setup? Sure, more is better, but also heavier and more expensive. Do I need it, or is this rcplus battery just crap?

- Would or could a ubec have saved me? Those afro's have a small linear bec, but at 0.5A it should be plenty for only a FC, GPS and receiver. Is there any reason to suspect a separate ubec would have coped with this better? Even if the voltage sagged badly, a 4S battery could not have dropped below 10V (in fact, probably not below 13V since the lipo alarm did not go off), why then would a BEC struggle to provide 5V ?
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Last edited by Vertigo72; Jul 30, 2013 at 02:32 AM.
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Old Jul 30, 2013, 04:56 AM
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New Zealand, Canterbury, Christchurch
Joined Dec 2012
1,418 Posts
A switching BEC may have fared better, but even if that fixes the symptom, it won't fix the problem. Your ESC's have a linear BEC which likely becomes less efficient with higher voltage, so you're more likely to get irregular output when running 4S (assuming your ESC is specced for 4S max).

Try the nanotech pack, see what it's like, if you get he same thing then I'd start looking for wiring problems (intermittent short which would be much more likely on a metal or CF frame). If not, then it's that pack not handling the load and that it's rated poorly
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Old Jul 30, 2013, 05:22 AM
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Belgium, Flemish Region, Geel
Joined Jun 2013
832 Posts
Quote:
Originally Posted by WendoNZ View Post
Your ESC's have a linear BEC which likely becomes less efficient with higher voltage, so you're more likely to get irregular output when running 4S (assuming your ESC is specced for 4S max).
Then wouldnt you expect problems with a fully charged battery and low throttle/load (resulting in higher voltage), rather than what I witnessed, problem occurring with a half discharged battery under full load (resulting in voltage sagging)?

Im a bit stumped how even a bad battery with huge voltage sagging would cause this, but it hardly seems coincidence that it happened on a full throttle burst, so i cant really think of anything else?

btw, those afro esc's are indeed specced for 4S

Quote:
Try the nanotech pack, see what it's like, if you get he same thing then I'd start looking for wiring problems (intermittent short which would be much more likely on a metal or CF frame). If not, then it's that pack not handling the load and that it's rated poorly
Yeah Im going to try the nanotech once I repaired the crash damage. Ill tie it down while emptying the first charge, while giving ample throttle and full throttle bursts to "stress test" everything. THe frame is not conductive though, and while I cant rule out any wiring issues, it would surprise me a lot.
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Old Jul 30, 2013, 07:25 AM
Re-kitting planes since 1990
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Australia, WA, Beldon
Joined Jun 2013
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What connector are you using for the battery? What size wire goes off the battery connector?
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Old Jul 30, 2013, 09:05 AM
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Belgium, Flemish Region, Geel
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Originally Posted by nebbian View Post
What connector are you using for the battery? What size wire goes off the battery connector?
Deans connector with plenty thick wires. Id have to check to be sure, but IIRC AWG 12 to my power distribution board.

Anyway I just got back from the shop, and I was told it was most likely due to some chemical agent in the new battery, something to help store it for long duration (like when new). I was told the first few battery charges I shouldnt discharge it too far (which I didnt), and not pull max amps. The guy who works there claims to use the same battery in a 150A RC car with no problems, after a break in period.

Not sure if that is correct or just an excuse, but I will gently charge and discharge it a few times on my charger while I rebuild. Also picked up a ubec, just in case.
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