Odd that, the last one died during a hot spell too. It was the original the came with the 2011 mobo upgrade and lasted
7 years. The replacement Duracell one lasted 4 years. It measures 2.59 V and the RTC almost stopped when the M/C was powered off (which I usually do overnight).
This time I did a hot-swap, by wearing some disposable gloves and taping a bit of 38mm conduit cover under the battery holder to act as a chute as I depressed the spring clip.
Andrew