Hi there,
So you are in good hands with the Power Ninja @msfujino. I use a 3.85vDc battery So my results are based around same, AT the lower end it appears the voltage regulator switching modes is indeed the culprit I have found so far to effect if it will even go to sleep let alone , wakeup properly. We are out on the edge so expect some cuts… 
HTH
GL 
PJ 
FWIW the BSP’s also can have an effect overall current so Beware.