I am sailing in the Caribbean and bought a used Phantom 3 professional specifically for this trip.  I have flown it several times but today it told me that all 3 of my batteries were invalid.  I've owned this drone several months and this is the first time I have seen this message.

I just recently updated the DJI GO app, but the firmware was updated in the drone about a month ago with no issues.  

I did have an issue on the boat where the charger was not working and all batteries were dying.  However, I checked all the connections (it charges 3 batteries at once) and it started working again.  The other day I did not get the battery warning but was not able to fly as we were too close to the airport.

Any ideas?

UPDATE:  I cleaned the battery terminals in the drone with alcohol and cotton swabs and that solved the problem!