Speaking of laptop batteries as a UPS source, some laptops come with battery management features that keep the battery healthy even when plugged in full time, usually exposed as a setting in the BIOS/UEFI. I've found that business/enterprise type laptops like Thinkpads and Probooks have this as standard, for example Thinkpads from 2010 already had this, assuming you're lucky enough to find one with a usable battery of course.