Notification low Battery

I first used my Nuki 2.0 with the normal batteries and even then I never received a low battery warning. At some point the lock opened very slowly and I then changed the batteries. But then I took rechargeable batteries with 2600mAh. When I came home yesterday, the Nuki just managed to turn the key with the last power, and after that nothing worked. I then looked at my phone and saw nothing. then I opened the app and first came also nothing, and then suddenly was above in the notifications (Android) a message “Critical battery level”. I have disabled all standby and battery optimization options on my phone, I also open the app at least once a week, and still the notification came when it was already too late.