I’d say, you didn’t use them as you should have. So use them more! 
But to put it in perspective: of all the devices I have been using since the start of the development of this app (mid 2017), non have ended up with an empty battery yet. My 2 temperature sensors, continuously reporting over 1.5 years, show a level of 73% and 87% and those are the lowest among my devices.
Perhaps good to explain the way the battery value is determined (as also used by several other home automation controllers):
As part of the lifeline report (0xFF01), the Xiaomi / Aqara device also sends the actual voltage of the battery.
The battery percentage is determined based on the reported voltage, as:
Battery value (percentage) = (reportedVolts - minVolts) / (maxVolts - minVolts) * 100
with maxVolts = 3.0 (V) and minVolts = 2.5 (V)
