« on: November 23, 2014, 02:57:15 PM »
The batteries are in series and get used equally, but there are some possibilities.
1. A bad battery. Remove the bad one. This would be a 99% + probability
2. Bad battery charger not fully charging batteries.
3. Current leakage in the battery compartment or related bus bars of the flash that affects only one battery. Look for any contaminants.
4. Excessive heat in the flash right next to one cell that is causing a problem with it.
That's all that immediately comes to mind.
I agree with the #1.