What is the correct way to connect the AttoPilot 180A ground wire without causing a ground loop when using two separately fed 6Ss into a PDB while only sensing one of the packs (I only want to monitor 1 pack)? Pixhawk is powered via the top power port using the original Pixhawk PM via a separate 3S (I’m not running the Pixhawk from the PDB).
I have hit a massive dead end after trawling the forums for hours, help greatly appreciated.
The "Correct" way? That would be the way it works best!
Ok, So, I have such a setup, with parallel battery packs, each battery connected separately to the PDB. One Battery is monitored with the Attopilot, In this setup we accept the fact that our battery readings are representational of one battery only, and we hope the other, non-monitored battery is acting exactly like the monitored battery. So for me, this has been working for me but is not a real good solution as the non monitored battery could have a bad cell and be draining current from the good battery, this would make your reported current to high, and your voltage drop faster than it should, poor readings may affect how the pixhawk reacts, and provide strange looking data in the log files.
In your case the pixhawk power supply is not monitored but on an independent battery, so you will not know when the FC will cut out. I use a BEC off the flight batteries. You do have a BEC or regulator, don't you?
With that out in the open this is done by installing the Attopilot on the leads for one battery as per the Wiki. Calibrate the voltage with actual measurements. To calibrate the current reading you will need to get an independent battery monitor that will connect in series during an actual flight, measure the actual max current for a single battery, multiply by two, assuming the same current is provided by each battery, then proportionately calculate a value for Amps_per_volt parameter so the reading sent to the FC will be close to your calculated current.
As I've stated this results in fuzzy results as we are assuming that each battery will provide the same current and voltage, and that's just not true. Differences in the health of each cell cause the entire pack to react in an individual way. Very small differences in cell voltages of each pack have good results, however, one week cell will bring down the other battery pack. I spend way to much time balancing cells with matched sets of batteries, and have some good results, but on my next build I will look avoid this setup as better setup is out their.
Thank you for your reply. I am aware that I am only monitoring one pack on the system. I have chosen this option as it is "safer" than linking packs together (series or parallel) before they meet the PDB when using an Attopilot sensing board, if the AttoPilot fails I still have one working pack.
I understand your frustration with dodgy cells, if you want to preserve your batteries and balance charge them one cell at a time (or top up a low cell) i recommend charging via the balance leads one cell at a time. This avoids charging and discharging good cells during a normal "balanced charge). Make up your own adaptor using pin headers (positive the smae on balance plug and directly next to is cell one... work your way down to the desired cell and charge at the lowest rate).
3DR (Pixhawk) has not come up with a "safe" way to monitor voltage and current for 6S+ users, forcing them to use third party products to support higher voltage setups. This means the power feed of the main pack (+ve & -ve) has to be cut and the +ve power passed through the Attopilot shunt resistors on the Attopilot (in my case 2 resistors in parallel as I am using the 180A). If the Attopilot board fails the unit comes down. 3DR need to build a power module capable of 12S with monitoring of 300A MAX (my X8 pulls 264A MAX).
My main concern is grounding the Attopilot not Attopilot Mission planner configuration. As my Pixhawk is powered from the top power port via the OEM PM (separate 3S) and not via power from anything connected to the PDB (Speed controller BEC or BEC from PDB) I am concerned that there is no "REAL" ground for the Attopilot. Speed controllers are purely sending signal into channel 1-8 (my 2 x 6S packs are dedicated to motors). My pixhawk also has redundancy from the servo rail with a zener diode on the rails via a separate (5.3V BEC powered via 2S). Receiver also powered from seprate 2S, no BEC (got to love the optima 9).
I have decided to ditch the Attopilot 180A option for current and voltage monitoring and use a system that requires no cutting of power wires and convoluted setup and inaccurate monitoring for a "SAFE & ACCURATE" voltage and current monitoring system.
My Optima 9 supports 35V MAX directly from PDB via the SPC connection to the RX, drawing hardly any A (now there is a "common" ground from on my system, receiver to PDB and from my receiver to Pixhawk via the PPM encoder providing power on the rails via channel 1).
HTS-SS Blue - Sensor Station (data port connection to RX, does not require additional power/BEC)
HTS-VOLT Voltage Sensor
HTS-C200 200-Amp Current Sensor
I can monitor V and A draw without any broken link in my system directly on my TX in real time as the V sensor accommodates 10AWG.
My question now is can I have 2 x 5V going into the Pixhawk via the rails (one in addition to main Pixhawk main power coming via top port using OEM PM powered by a separate 3S? power on rails via PPM encoder 5V and now want to add a BEC powered by 2S on rails for backup?
My understanding of the Pixhawk is you can provide 5v power to the main top port AND to the servo rail. In the event of failure of the main power port, the pixhawk will take its power from the servo rail.
So you are looking to have redundant power supplies just for the servo rail, from separate power sources? My gut reaction would be to tie the grounds from the two BEC's, however, I'm not an expert and I think your correct to be concerned. If you where getting the power from the ESC's it would be incorrect to tie together multiple ESC outputs. I think separate power source for electronics, motors, and perhaps servos, with separate grounds should be as involved as this gets. Redundant power just for the servo rail is overkill, simple is good.
It sounds like you have lots of different batteries, all used at the same time. It must be some work to plug everything in for each flight. I use three flight batteries in parallel with a parallel balance connector, and that seems like a lot of plugs.
As far as balancing cells over the three lipo packs, my efforts never end. Your idea of individual cells makes sense, but only for small adjustments. If I have a low cell, say 3.3v and I connect this cell to a fully charged cell at 4.2v then don't I run to possibility of pulling to much current from the good cell? I have actually tried to resurrect a bad cell at 2v by connecting to a good cell with very limited results, I was able to bring the voltage back up for a few flights but the end result was that by connecting the damaged cell to a good cell I just caused damage to the good cell