Ok, I ALMOST had a really bad thing happen (Pretty sure it would have been bad) earlier this week. I always fly with 2 batteries on the copter. When I charge them I always SYNC charge them to ensure they are both at the same voltage and I only charge/fly them in the same pairs. The other day I charged 3 sets of batteries for a shoot. During the shoot I flew the first 2 sets. I didn't need the third set. After I returned home I went to go store the batteries. The first two sets stored correctly. When I went to plugin the 3rd, unused set, to my surprise one battery was fully charged, the other was at 20%... Im really glad I didn't plug these into my copter... my guess is it would have resulted in a fire, or something bad happening of some kind. From here on out I will be checking each battery using a voltage checker before placing them on the copter. Just thought I would share this little experience. Josh
Whew. Actually, what some of us think (and you almost proved...one way or the other) is that this particular situation might result in the full battery being dragged down to charge up the empty battery, but it's not clear what the resulting current draw from the copter would do. I suspect you would have seen a lower-than-16.8V voltage in your telemetry, too, which should always be a sign of trouble. The more dangerous situation might be a dead or defective (bad cell, short, etc.) battery paired with a good one...
Any idea what might have caused that imbalance on the pair, Josh? I think what would have happened, if you use a Y-cable, is that there would have been a pretty big inrush current from the charged LiPo to the not-charged LiPo as the voltages tried to equalize. I don't think the copter would have suffered as it would just be a "bystander" to the in-rush current. If you have two LiPo connector leads to the power distribution board, that would be a different story as the inrush would have flowed through the board! Can't tell you whether the inrush current would trigger a fire, but it might! Andy.
I'm using the 6S batteries and have them wired as one continuous wire with the DEANS power distribution deal i built sending voltage to the ESCs and FC. Not 100% sure what would have happened either... I don't really want to find out... HOWEVER, I do have some old crappy 4s batteries that I may test this on, just to see what happens... Josh
It sounds like something internal shorted out a bit to cause a drain of current like that. There most likely be a prappid transfer of current to the battery. Could have been a dangerous situation for both batteries. I will be checking mine a little closer. I will be doing the 6S in parallel in a few weeks when my heavy lift is done.