Dear IFI-Team, this was the fastest reply I ever got in my live, thanks a lot! Also thank you very much for the great support!
Your answer makes it much more clear for me ("I do not have to care about the battery"), and that is great. However, I have one additional question: I will use the DAC only at home, therefore I think I do not even need the battery (maybe except for the - I forgot the name - extra boost I could get for my headphones..). If I do not charge the battery (I am very lazy and I know that I will forget to switch on/of the DAC in order to charge it), will there be any problems?
Simple answer - a big nope!
Worst case, you get a new battery from us but only contact us in 3-4 years' time because your battery wont need to be replaced until then.