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

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This wont work. You can't expect redundancy by just using parallel wiring. If one battery malfunctions the other will try to charge it, possibly too fast which could cause a fire. The batteries need check valves so one can't charge the other and they stay independent. Schottky diodes are such a check valve.

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?

You could tap off each battery and the amp draw would be low. You just can't have redundancy for your motors without huge diodes, but you can use them for your other electronics.

I would replace that PM with an AttoPilot current sensor and a nice UBEC.

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 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...

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.

Attachments:

No, I use a Turnigy 4 port charger, and just do an independent, balance charge on them.

I'm a little concerned about your eCalc numbers.  The %throttle is fairly high, while your frame weight seem optimisticly low. 

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.

Yeah, I think the problem is 3S on 750kV is kinda low.  You need big props for that.  I usually try to get hover throttle around 40-60%.  Your 12" setup is close, you could consider 13 or 14.  I don't even think you need bigger ESC's.

I will yield to Lefebvre who is more knowledgable than myself. However I don't have to worry about how I mix, match, or charge my batteries. I can pair up a 1000mAh with a 5000mAh and leave one half charged. It doesn't matter, you can't screw it up and I find that comforting.
Remember I am a plane guy not a copter guy, so my needs are a bit different. Motor power redundancy is not as critical as in a copter, but APM/servo power is essential.

I do have to mention that I have winessed lipos self destruct in flight due to an internal short. I will admit that in this example redundancy wouldn't have helped since the battery melted through the bottom of the fuselage.
Sorry I fly planes mostly, and just play around with a little home built tricopter for fun.
I use the planes for serious mapping, but commercial ops are still illegal in the US, so it's all recreational at this point.

Do you run those Shottkys in your fixed wing acft; which is how you can do the above?

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