Lidl Charger isn’t right?

Servus, I have a charger out of Lidl, this would like to C-tek. Since my car is currently not registered and I am installed and set up the music, I have the system very much in the stand running. If it has run longer I always hang over after the charger on it. Recently, the charger hung the whole night and showed by now “full” I had to run only a very short hour the music, then the battery was almost empty. Has just been enough to start. since then I have had I am a little afraid to unload the battery inadvertently and always let the motor run. today my voltage indicators have come, I installed them immediately and let the music run a bit: Battery in quiet: 12.3V net especially full, but actually ok. Music: 12V at high level then breaks the sane, but I also have a very small acid batt at the moment. Engine on: 14.4V with music still 14,4 only at very high level does the voltage fall below 14V So, 12,3V on the Batterie, I thought I could load them really full. Did I put the charger on it, which also indicates that it is loading, but the voltage remains kpnstand at 12,3V….wouldn’t have to be lifted when charging? I have the presumption that it is because it is constantly almost empty.