THEMA:

What happens if you disconnect also your DS16 transmitter? Keeps the battery its charge?

Gernot

Finally found that the battery is guilty: disconnected from the transmitter, it goes from 4v to 3.5 v within 24h.
There is no power on the internal PPM/EX plug when transmiter is off so no chance to drain power from here while on off.
The current drawn by the transmitter when of in some microA only.

First time I found this problem on a battery but I'm more familiar of Lipo and Life than Liion, maybe a normal behavior when it's get's old ?

I've ordered a new 5200 and will open the 3300 one to understand what is this electronic inside and if it is safe to charge it directly from a Liion charger.
Will let you know

Well, nothing is save from faults, even batteries.

I am pretty sure there is not going to be any electronic device inside of the pack !

Do a capacity test of that defect battery and compare it to your newly purchased 5,2Ah battery.