If part of the ring starts to flash slowly or lights up permanently, you know that the batteries will soon be flat. In any case, it's good to have a house key somewhere for emergencies. I have found very good lithium/ion AA batteries with a capacity of 3600mWh. They last a really long time. You can also find an indication of the charge level in the NUKI app.
You can also see it on the illuminated ring, which then briefly lights up red. I used quite a lot of batteries and therefore recently bought the rechargeable cylinder, which I think is great.