So after much reading and research into the whole "backup" BEC saga. I think I've found a solution!
My first idea was just to run a 5.5v backup lipo in parallel with the 5v UBEC output.
This idea has been vitoed by most. Too many unknowns apparently.
The other was to parallel all the BECs up, but this would cause harmonics due to all the square wave interaction.
So I figured we cant mix power supplies. Other than using a microproccessor controlled mosfet to switch
power supplies when and if the bec dies.
We use a relay :) Bear with me.
The main power supply powers the relay. With the relay closed (NO) power is routed via the bec to the apm.
If the bec fails, the power to the relay fails and is switched to the auxillary power supply (battery or another bec)
This completely seperates the two power sources.
Will the relay be able to switch fast enough to the auxillary power source without the APM losing power?
If not, im sure a power capacitor will keep the APM alive long enough for the bi-pass surgery to take effect?
Above is a recording of the ciruit just in case my explaining skills are dodgy :)
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I dont know the electronic specifics, but I have an RVOSD gen4 that uses either 1 or two batteries (via jumper setting).
The selection is to either use one battery to power the rvosd board, or both the primary and backup batteries; auto selected via the higher detected voltage on the batteries.
So for example, if both batteries primary and backup are 3s same exact voltage it will use both batteries equally. But while motor is spinning (motor will draw from the main only) the osd servos and its connected systems (ie gps) will draw from the backup battery; until it gets to be a lower voltage. Or for example when motor is shutdown and voltage from main battery becomes more than backup battery, it will again draw power from the main.
A typical seniero where this is helpful,. is a lost plane.
Lets say we start with a low battery condition; youve mistakenly run too low on power and plane is too far away to make the return trip home.
So starting with power to motor is killed when the BEC tries to keep the servos alive. (this description is for planes not quads)
The Autopilot keeps running on backup battery untill reaching the voltage of the main, then it will use both batteries until they both eventually die.
The advantage is, this leaves telemetry (if intact and able to transmit) running so are able to track down the grounded plane from gps corrdinates obtained by the groundstation
(ie people running to the scene, but operator patiently google earthing the last known gps and trajectories) and then hopefully from the recovery team can get a better place to be looking.
This would be a handy feature, with apm(x)
thereby balancing its usage accross both batteries. but motor can only draw from main.
I believe rvosd is a member here, who could explain better the technical details.
but its a nice way of handling dual batteries...
I have done a board for two LTC4415s and an OKI-78SR-5. This provides one battery input (7-36V) and one BEC input with a single BEC output. The board is 1x1" and contains surface mount parts. The LTC4415s are around $4 q1 and the OKI regulator is about $3.50. Both are available from Digikey.
Zip File
The SCH and PCB files are PCB Artist files, PCB Artist can be downloaded for free.
I agree that using Schottky diodes is much simpler, but my concern is the reduced torque for servos (if you have a tri or other configuration that has servos in the propulsion chain). For suitably rated diodes it looks like 0.36V diode drop is about as low as you are going to get (.475V for non-surface mount parts). If this is ok, use a pair of diodes, one anode to each power source and the cathodes tied together for output. 1N5820 is suitable.
Using a 4 cell nicd/nimh buffer pack in parallel with the regulator is not uncommon in the rc helicopter world.
If the main UBEC is being fed off a Lipo, the switching voltage that the relay is seeing depends on the load resistance. As soon the voltage drops below the threshold, the relay opens the circuit, causing the load resistance to increase to inf, making the Lipo voltage recover, which re-opens the relay. So, it may result in a looping behaviour that is wearing out the mechanical relay.
Why not get rid of the relay, just short circuit the 3 wires, and put a simple diode between Sw2 and the point where used to be the relay. The diode will prevent any current from being drawn from the backup system while the main BEC voltage is above the backup source (5.5V) minus the diode drop-out (0.7V), setting the threshold at 4.8V, plenty for the APM to work.
If there is fear that the backup battery will divert some current back to the main UBEC, an alternative circuit based on a diode-or can be implemented.
@Dean: Please send me that board layout :) Thanks
@Robert: I like the idea of an open source UAV UPS. It can be used for all models.
The issue with the LTC4415 is the 6v abs max on the inputs and outputs. You are driving inductive loads (servos) with minimal clamping and filtering. While I have not measured the BEC bus voltages I would strongly suspect that there are spikes well over 6v on those lines. The 18V part (LTC4358) gives you some margin.
While they are surface mount, they are not leadless or fine pitch so they are pretty easy to assemble onto a board.
Also, I never got why anyone uses linear regulators for the BEC, OKI/Murata make a 7805 pin for pin replacement that is a high efficiency switching regulator (OKI-78SR-5/1.5-W3). It is has a higher dropout than an LDO (approx 2v), for for 3 cell and higher packs you get 90%ish efficiency instead of under 50% for a linear regulator (not to mention that linear regulators fail often and in bad ways). The OKI regulator is cheap, very light and doesn't generate any heat to speak of.
If anyone wants a board layout for the LTC4415 or LTC4358 (not recommended) respond here and I will provide one.
As already mentioned, use diodes. It is the simple and proven solution. Most electronics does not require exactly 5V. So either use 5V UBEC's with low drop diodes, or for higher amp rated diodes at lower costs use 6V UBEC's and normal 0.7v diodes.
Just OR your different supplies using high current Schottky diode/s.
Only drops 100mV or so, old PC supplies are full of them.
(The stripey ends go together, to the + line)
I've been doing it for years, no problems, yet :-)
You don't want to see my diagrams. ;)
I was just thinking, this is a great idea, why don't we make a thread somewhere in the forums and open source develop a backup system board. I can envisage dual battery input (don't have to use both) through a pair of LTC4358 or similar. Then that feeds twin linear voltage regulators going to an LTC4415.
Then we have redundant voltage regulators and the option for redundant batteries.
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