What is broken on my battery?

Around January (2018) my phone (Huawei P9 Lite) broke the charging cable. Well, I'll order a new one, I thought to myself (the power adapter is still working today). I ordered from the Internet a charging cable (without power supply (!)), Which somehow Quick Charche allows (called it, as this may be relevant) It was all super, until my phone suddenly no longer loads. So, it is displayed as it loads, but I have it load for example 10h (inside flight mode, Wi-Fi, gps, bluetooth all off) but after 10h it has charged 3% (sometimes while i load and use the phone, I even lose percentages!). I want to clarify that the charging cable is not broken, since it 1. Charges my power bank without any problems 2. Also charges my phone when it's on a power bank's turn and I might just be on the way 3. That these dropouts are not the norms, because sometimes the loading works smoothly (!). I'm currently charging my phone with a JBL charging cable, which works fine, and I have often changed the power supply (tried it with 3 others), but it does not work and I desperately despair!

Everything sounds like a loose connection or cable break. Maybe the power supply does not have a flaw, if the error never occurs when loading via the power bank?

Thanks, the guess with the power supply has been confirmed.