Power to FC for quick battery swap - using a suitable capacitor

I’ve been mulling over the options to keep the flight controller powered during a battery swap - using the “Rewind and Resume” capability.

reference: Mission Rewind on Resume — Mission Planner documentation

One of the options I’ve considered is using an appropriately sized electrolytic capacitor - with enough capacity to run the flight controller and it’s accessories (gps, telem radio, etc.) for a battery swap.

I was thinking of maybe installing such a capacitor to the Power-2 port on a carrier board.

I don’t know if this is possible - or what the possible pit-falls might be. One possibility might be the flight controller going haywire if the battery is isn’t swapped fast enough and the current from the capacitor begins to fade.

Before I begin experimenting, I thought I’d look to the community for comments. Thanks!

A hypothetical:
5V, 3F Supercapacitor
Supplying 500ma @ 5V

A perfect capacitor (no such thing) would do that for ~15 seconds if I got it right.
E=.5CV^2

A lipo of that size (~32x21x11mm) would be a better idea :slightly_smiling_face:

Time ago I tried that supplying the FC with a power bank through the USB connector during swap. It worked, although the remaining battery capacity was not initialized, and eventually was exhausted (I had to make it five times that of the battery for five batteries).
image

So the FC was missing recognizing the battery swap. This happened long time ago; I don’t know what happens now with recent FC firmware versions.

Mission Planner now has an option to reset the battery statistics. So as long as you’ve got a telemetry connection - you can set your failsafes properly for your battery size.

I haven’t actually tested this - but I’m confident it will work.

I’m just trying to figure out the best option for keeping the FC running. I’d like to also keep my other electronics running too - so I’m probably going to make some sort of connection to the Vcc downstream from the current sensor. That should keep everything alive. I may have to install a safety switch - because connecting even a small power source to the Vcc makes it possible for the motors to spin.

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Some quick numbers with the farad+diode+resistor solution: if you want to supply 0.5A during 30s with a voltage drop of 1V:
C=0.5*30/1F=15F.

You can find 15F/5.5V 3x2x1cm 50g (not too bad).

Charging: 5 times constant a flight time of 5’:
R=300/(5*15)=4ohm (say 5W), so the rush current would be 1.25A (not too good, but probably the C internal resistor would limit it more).

All above if I am not too wrong, and pending experimentation (it could work). A battery instead would be an ugly solution.

I would prefer the power bank. MP reset is a workaround (manual intervention). The FC alone should do the job.