Do not put batteries in parallel. Their charge level and voltage behavior is not identical event if they are of the same model. This is due to the manufacturing process. Thus, if you connect them in parallel, your design impose that that their voltages are equal.
When the design is switched off, the battery with the higher voltage will discharge into the one with the lower voltage. It the chosen battery technology doesn't allow recharging, this energy is lost. After some time the voltage will be equal and the loss of energy will stop. But if an external disturbance appends, such as an increase of temperature, the voltage of the battery cells will differ again and some energy is lost, again.
This is why you should never connect battery cells in parallel. And if you reverse-engineers battery powered devices, they are always connected in series.