Due to USI updating their website, the direct link for the S-1810 often 404s (page not found). However, you can find archives of the manual on third-party sites or request it from USI support.
A: As temperatures drop at night, battery voltage sags slightly. A battery that is just barely alive during the warm day will dip below the threshold at 2 AM, causing the low battery chirp. Replace the battery. Usi Electric Smoke Detector S-1810 Manual