I’ve had an Olight S15 for many years, but it was refusing to work. An online source said to rotate the spring to improve the contact. Yup, it now worked, but only the two lower modes. Maybe it is broken. I wore out the LED in an old Fenix light by regularly running it on max for an hour. I decided to dig out my Maha Powerex MH-C9000 charger and discovered that most of my AA cells have high internal resistance and hence they cannot provide high current. After selecting two good AA cells, the S15 worked perfectly. Well I never, but I guess some of these batteries are 15 years old, so it is to be expected.
I will keep the worn cells for low drain uses, and buy fresh made in Japan AA cells.
As expected, the made in China Panasonic, and high capacity made in Japan Duracell, batteries were the first to wear out.
Is there any way to reduce the internal resistance? A search on t’internet would suggest not.