Why do pole-top, recloser and ACR batteries fail so often?

Because of where they live. Pole-mounted ACR, sectionaliser and RTU cabinets are unventilated, sun-loaded and small, and routinely run internal temperatures far above the 20–25 °C at which design life is quoted. At roughly 50% life reduction per 10 °C, a ten-year design-life battery in one of those enclosures is a two-to-three-year consumable, which is why they are the highest-churn small-battery population on most distribution networks. The failure consequence is real: AusNet notes that low-voltage supply or battery failures in an ACR controller can prevent the ACR from operating. Addressing the enclosure thermal design usually returns more than upgrading the battery.