Question about Gigaset Sl450 Hx

Does anyone know how long the batteries last? I have had a Gigaset with 2 handsets for some time and have to replace the batteries (Varta, Eneloop) about every 12 months because the handsets then "drop dead" in the middle of conversations. An investigation showed that the batteries then still have practically full capacity, so the bug is in the Gigaset.