Amp Draw Calc for LARGE X8 (Pre-Flight) Need an Attopilot?

Hi,

I am currently trying to determine the anticipated amperage draw for a large X8 platform that our team is using to carry a ~10lb sensor for use in the field of snow science and hydrology. It has (fortunately) come to my attention that we need to reduce the 22.2V coming from our batteries to under 18V when supplying power to the PixHawk using the standard power module, which came with the PixHawk flight controller. It seems that we have two options:

1) New power module with increased power handling capacity 

http://www.ebay.com/itm/like/251927302504?lpid=82&chn=ps

2) AttoPilot - I'm still unclear as to exactly what this would do although many other forum posts on this topic have addressed this to some degree. 

http://autonomousavionics.com/products/attopilot-180-amp-voltage-an...

My question is twofold - 

1) Which option, if any is the best route to take. What specifications need to be met so I don't max out either option?

2) How to calculate the theoretical amperage draw from an X8 (sets of two coaxial motors). I have not found much material on how to calculate theoretical values for this type of configuration since the motors are 'working in tandem'? Does this matter? Help here! 

- T Motor U10 KV100 

http://www.rctigermotor.com/html/2014/Efficiency-Type_0314/198.html

- 29x9.5 CF Props 

http://www.rctigermotor.com/html/2013/prop_0805/13.html

- 22.2V 6s 25C 22,000 mAh batteries (option to run either two batteries, or four, in parallel depending on payload/configuration): 

http://www.getfpv.com/batteries/flight-packs/tattu-22000mah-6s-25c-...

- Gryphon Dynamics X8 (V2) Frame XQ1400 VZX = 1400mm 

http://gryphondynamics.co.kr/product/x8-airframe-kit-new-version

- PixHawk Flight Controller

- FPV = Separate system

- Around 50lbs Takeoff Weight

http://www.getfpv.com/batteries/flight-packs/tattu-22000mah-6s-25c-...

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Do not run all your power through a single resistor on large aircraft.  Period.

You don't need amperage data that badly.  You barely need it at all.  If you really must, get an Amploc current sensor, which slips over a wire, with zero chance of failure.

And then I recommend to use a proper UBEC instead of a no-name voltage regulator from Ebay.

Thank you for your response Rob.

To clarify, my main goal is to ensure that I will not burn out my PixHawk with 22.2V coming into the power module. I'm not 100% clear as to what the capacity of the stock voltage regulator is and how this will translate powering the PixHawk. The basis of my concern comes from these two articles:

http://diydrones.com/profiles/blogs/powering-your-apm-drone-or-how-....

http://copter.ardupilot.com/wiki/common-optional-hardware/common-po...

http://copter.ardupilot.com/wiki/common-optional-hardware/common-po...

I suggest just using something like these to power the Pixhawk.

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=...

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=...

The second one has bonus points for a failsafe power switch, which allows you to boot up the Pixhawk after connecting the power leads and letting is stop wobbling on the landing gear as most large multirotors do.  Power up, pull the pin, good to go.  This is what I'm doing on my Octo.

I'm running two 6s22ah packs in my X8 and U8pro 170kv motors.  As Rob pointed out, running all your power through one sensor is a bad idea.  Mine uses an attopilot 180 fed though only one of the two packs.  So if the Attopilot fails, I still have the other pack that's wired direct to the DB.  As for the power to the Pixhawk I run one ubec wired with attopilot into the power port.  Another two ubecs to the servo rails with a switching circuit that only feeds one at a time in case one were to fail.  Also I add a reverse zener diode across the +- rails in case of power spikes.  Only needed if your powering noisy stuff like digital servos.  One thought about your setup.  6s with the 100kv U10 may not fly so nice.  Even my 170kv has been a tough go for stability.  I'm switching up to 7208 or 7215 135kv KDE direct motors on 8s packs.  It was a pain to realize I had to ditch all my ubecs for HV version lol!

Hi Shaun;

I'm helping Robert with his X8 unit and we're struggling with power/motor/screw type, and noticed you mentioned using a KDE motor with an 8s pack. Did you do this since you posted this reply? Thx.


Shaun Bell said:

I'm running two 6s22ah packs in my X8 and U8pro 170kv motors.  As Rob pointed out, running all your power through one sensor is a bad idea.  Mine uses an attopilot 180 fed though only one of the two packs.  So if the Attopilot fails, I still have the other pack that's wired direct to the DB.  As for the power to the Pixhawk I run one ubec wired with attopilot into the power port.  Another two ubecs to the servo rails with a switching circuit that only feeds one at a time in case one were to fail.  Also I add a reverse zener diode across the +- rails in case of power spikes.  Only needed if your powering noisy stuff like digital servos.  One thought about your setup.  6s with the 100kv U10 may not fly so nice.  Even my 170kv has been a tough go for stability.  I'm switching up to 7208 or 7215 135kv KDE direct motors on 8s packs.  It was a pain to realize I had to ditch all my ubecs for HV version lol!

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