I have bought one year ago my phantom 2 vision+ and since a few days ago I have been piloting without any problems (around 50 hours flight)

One week ago I was flying the phantom for the first time after 4 months stop. As always I checked that the batteries where fully charged and made all the calibration tests before taking off.

I started flying just a few meters above the surrounding trees (20 metres) and was trying the new dynamic home feature by switching on and off the remote control to see the phantom behaviour.

After only 5 minutes unexpectedly I lost control of the phantom. The battery indicator on the phone passed in a few seconds from 70% to 1%, the light on the drone became red and the phantom started hovering down from its position without following my directions on the remote.

Unfortunately while hovering down the phantom touched some branches of a tree and went down very quickly from a height of approx. 15 metres.

After the crash I checked the phantom and it seemed ok. Nothing broken. I changed battery and tried to hover off again. The drone took off but was unable to lock its position in gps mode. It started moving randomly in every direction and at the same time rotating clockwise on itself.

I tried to IMU calibrate via software. The calibration seems ok but when I launch again the software its telling me that the drone needs to be calibrate with an error message.

Coming to the questions:

1)Any clues on what is broken and needs to be changed? Gps module?
2)How can I know if my battery is defective? As you can imagine knowing that your battery can fail in any moment is not quite reassuring.

There are two low battery levels. The first is a low battery warning normally at 20-25 % and the phantom will initiate RTH if set that way in the assistant software. Then if it hits critical battery level at 8% or below it will start to automatically land where ever it is and makes no difference what's under it.
You will probably want to check the problem battery in the assistant and see if the cells ar balanced or not. If no errors and it looks good there I would do some long low level close by flights.
I had one of my batteries that would drop from 65% to 8 % and my quad would auto land. Had to chunk that battery also.

I would remove the top shell and make sure the GPS plug is tight. Without GPS lock she will drift in any direction. You can inspect the GPS module while you have it open. Look for cracks in the board or the chip.  However if the GPS is completely off line the motors won't start. Good luck in trouble shooting.