Complete discharge

We have eight electric thermostat two batteries in it. After 6 to 8 months was at four (so far) are a completely discharged and the other full (we had previously used alkaline batteries, and were then in each case both for about two years and had to be exchanged empty). Here I had hoped to extend the cycle.

Addendum: Meanwhile, in another three the same thermostat. A battery is completely empty and the other full.
Pairwise seemingly useless.