Steven, would you please measure if you have also voltage on the +/- pins of the input rail while backup battery is connected?
It is ok to have voltage from the backup battery showing up in the APM, the real issue is if it is drawing current. Since it drained your battery it is pretty likely that it is drawing current somehow.
Jake, the RX is connected all the the time to the APM, of course. And I think it is drawing current. On the pins I connected the RX is 0,9V, so it is powered (with 0,9V) all the time and this is may draining my battery. What do you think?
Even with RX disconnected, the battery is empty in one day.
I think the APM itself is probably drawing the current. You have to remember that with the vbat still connected to the APM you have to deal with all the circuitry connected to that pin unless you put in a diode on that side as well.
I put ferrit coils to the Lipo + line, the UBEC (powering the in rail) and the one ESC ( powering the out rail). No change, no GPS-Lock.
I have installed a plug to separate the battery from APM. But when disconnecting the plug, the saved almanac information is lost. So on the fly field i must first connect USB to get a lock, than plug the backup battery in and than connect the lipo. It is a workaround for now, but not the solution.
Please excuse my ignorance, perhaps I am not seeing the problems you are but wouldn't a simpler / safer solution, what that doesn't include cutting traces and messing with the APM include just powering the APM separately? I use a 68 gram 1200mah 2s battery, and an LM7805 liner regulator / or 5v ubec. Now I can fly all day, swap batteries, and not lose the almanac from resetting the GPS. On a similar note, if my ESC(s) or attached servos are introducing noise into my setup, then I would benefit from powering the APM and the copter separately. I just wanted to throw that possibility out there and get some feed back. What do you think?
I guess if you want to deal with the time, weight, cost, and maintenance of dealing with another battery that would work fine.
A single 50 cent watch battery should be able to power the backup on the GPS for a year. We're just trying to fix the APMs design flaw so that the GPS operates like it should.
What is the PIN2 (ENABLE)?
A switch on the GPS?
If so, if turned off, is it possible to connect another external port on the GPS?
Yes. You just ground this enable pin and it will keep the onboard GPS off. Then you can just connect another serial GPS to the same port, which is pinned out on the main board.