Wondering if i might ask those of you with far superior electrical knowledge than me (thats most of you lol) a question...
I'm building a hex for aerial photography and have been planning a dual battery in parallel for redundancy.
We're using the APM2.6 and would like to use it's PM so i can monitor the voltage through the GCS.
Trouble is, that PM is limited to 70 amps (from memory) and the ESCs will draw up to 120.
So i've thought that instead of making a 3 plug parallel harness i could make up a 4 plug and run 2 systems. One side for the motors (direct wired to the parallel harness) and the other side for accessories like gimbal et al (wired from the APM PM which runs back to the parallel harness).
My question is in a couple parts:
-am i introducing a short in the system?
-will i get accurate voltage readings?
- is there some quantum effect that i'm missing (lol- but i do think of electricity like water and am aware that it not always the case))
- am i just crazy?
- your thoughts?
Sorry for the noob question; i just don't want to wire this up and watch my first effort at this smoke up a couple grand and weeks of my spare time.
I've attached my planning schematic (i stand ready to be flamed for my horrid lack of electrical drawing skills!). You can see the wiring harness i plan above the batteries. The APM PM is on the Right (top) system, and the motors/ESC are on the left (bottom). The two power distribution boards before the gimbal et al and the ESCs are those cheap ask ones from HK which are supposedly good for 160 amps each.
Thanks for any and all advice,
Windjimes
Replies
Yeah. the use of diodes for this is arcane; even multiplexing 5V power inputs as in the PixHawk. Note that if the diodes are properly rated, they won't "burn". Simply put, simple use of a few of today's high power, low cost MOSFETs that were not available five years ago will do the trick! Look at how today's ESCs are now using only a few MOSFETs instead of a bank of 12 or more. However, I have run two 4S 80C(120C burst) batteries in parallel and NEVER had an issue. However, these were hard-case (racing car certified), so they had extra protection from crash damage.
...old posts live on and still have relevance!
Marco Riccardo Napoleone said:
Schottky diodes has a dropout voltage of (in the best case) of 0,3V
With 120 Amps the power loss is P=V*I = 0,3*120=36W...
A lot of power will be disspated in heat and the diode could burn very soon. We developed an ideal diode controller with a very low voltage dropout. It remains cold even if you have a 100A flow and all the energy is provided to the UAV circuit without losses.
iskess said:
Here a post thya will answer your question
The frame is that wooden SpiderHex from BlueSkyRC. I've lengthened the CF arms to take the 12" props and the gimbal is a tiny CF brushless one for a GoPro.
I didn't plan to have the gimbal on with the 10" props but looking at it now as you said its not good. I swear it looked fine back when i made the order!
Just compared the frame by hand with a 2L soda bottle about half full and the frame is well under that (its rigged sans motors esc and battery)... but thats about as inaccurate as it gets. I will go get a hook scale so i can know for sure.
Do i solve this by going to deeper 12" props and/or 14" with a larger esc? That would bring it to 56% and 36% respectively with plenty of payload. What do folk aim for thr% @hover?
Thanks for your help and the tips on charging... good to hear from someone who does it already.
Thanks R_Lefebvre... when you charge do you parallel charge both the matching batteries? Or is it ok to charge them via the same charger just not neccessarily wired together? I did buy both my matching batteries at the same time same manufacturer and i have this Turnigy 800x (400 x2) charger here
I attached my ecalcs at various configs: i have 10" cheap props for practice plus 11"s and 12"s in CF to carry a small gimbal et al.
How careful will i have to be on those 10" props at +70% thr @ hover?
As to my original post i was adding 6 x 20amp ESCs and getting 120... but it doesn't seem like i will get anywhere near that. Is that right? The motors i have are the AX-2810Q from HK. On the website they claim 444 watts but Ecalc (with my best-guess custom figures) doesn't come close or to 30amps for that matter.
eCalc hex10 9ah.pdf
eCalc hex11 no gimbal.pdf
eCalc hex12.pdf
I disagree with iskess.
All of my large machines use batteries in parallel for redundancy. I don't use any diodes.
While it is true, you *could* have a situation where one battery suddenly gets a short, which in turn grabs a whole bunch of power from the second battery, causing a fire, this type of thing spontaneously occurring is extremely rare. In fact, I've never actually heard of a battery suddenly shorting out in-flight. I've only seen it after a heavy crash.
A more likely scenario is where one of the batteries is merely "weak". It could have reduced capacity, which could lead to a crash due to reduced flight range/duration. Or it could have a voltage that sags too much under load. Running parallel batteries helps with both these problems, with absolutely no risk of a fire or other problem. The stronger battery simply holds up the voltage of the weak one. When the power system demands power, more of it comes from the strong battery than the weak one. You still get reduced range, but the situation is much more manageable.
The only thing is, you do have to take some care with this setup. I purchase batteries in pairs. Two identical batteries, hopefully from the same batch, with very similar characteristics. I always fly them as a pair, and charge them as a pair. You must also ALWAYS make sure both are fully charged before plugging them in.
So using this method, you get protection against 90-99% of common failures, with only a very small risk of a very rare problem. And let's face it, if you're in the air when a battery spontaneously shorts out, and that was your only battery, that aircraft is coming down anyway so...
iskess... thanks very much for your help and my apologies for such a late reply. The Mrs has been on a business trip leaving me as Daddy Day Care for a few days.
I've taken your advice and ordered the 180a attopilot from sparkfun, hopefully it will arrive before week's end. I already have a 5amp UBEC which i will use with it to power the board.
Do you work commercially with your multi? Have read up on it and would be interested to offer it down here. Obviously a learning curve but what system would you recommend? Is it ok to use regular DSLRs and setup yourself or is it necessary to go with one of the dedicated offerings to provide a decent product?
I would replace that PM with an AttoPilot current sensor and a nice UBEC.
Doh, i should have thought of that. So no redundancy as it looks like diodes of the required amperage will be too heavy.
So minus the redundancy, if i stick with the dual battery (for flight times) and run two systems to avoid going over the 70 amp limit for the APM's PM will the system work?
Also, just looking for lighter weight options... could i use a fuse in the wiring harness that would blow and isolated a malfunctioning battery? Hmmm... but then i'd need a fuse on both batteries and if one malfunctioned it might blow both fuses leaving me SOL. Would that be right or might it be wired so that situation would be avoided?
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