How come one batt shows 4.1V and the other shows 3.5 V after using a few days/times after both batteries are fully charged in the beginning??
In a serial configuration cell voltage may drift a bit depending how close their specs are matched, but not that much…
Are your cells the same brand, model and age?
I'm using the Sofirn ones. Actually I might add that I measured the voltage differences not after using the flashlight but after sitting unused for a couple of weeks. Maybe if I used it more, then the voltages should even out maybe?
Charge both and store both separately for a few weeks. See if they have the same voltage. If not, this is another incident of Sofirn self discharge.
Using it more should not make a difference.
They are in series, so they won’t equalise.
Mark the questionable battery.
If you have a charger that can test the cells do that.
Sounds like one of the batteries is not keeping a charge.
After testing, or charging, keep them sitting outside the light and see if the one battery drops voltage.
All the Best,
Jeff
Okay good tips…will try them all. Yeah I gonna fire up the BT-C3100 and give errr! 
are the cells protected? i wouldnt use non protected in such light.