Any ideas why this happened?
Shooting yesterday in bright sun, lot of contrast between people in shade under portable shelters and sunny backgrounds. So I was using a 270EX II for fill flash (hi-speed synch). I didn't shoot a lot, fewer than 100 pictures over three hours.
Before starting, I tested batteries. I have some AA lithium batteries I bought years ago (2021 expiration date) so I'm using them until they run out. The two in the flash had been used previously (perhaps 25 shots), and when I tested them yesterday morning on a load meter, they were both 100%. Eneloop backups I brought along also tested 100%.
Two hours into the morning, the flash stopped working. I took the batteries out, very hot. I put Eneloops in and continued on with no problem. When I got home, here's what I found:
Lithium one: 80% on load tester and 1.51 on voltmeter
Lithium two: 20% (red) on load tester and 0.97 on voltmeter
Eneloops: Both still at 100% although they probably weren't in for more than 20 shots.
I don't think I've ever seen one battery in a pair fail. Battery issue? Flash issue?
I'll keep monitoring it. I especially want to see what happens to the Eneloops over time -- this is first I've used them. If one of them goes to 20% in service and the other doesn't, that will tell me something.