A standard 19mm length Varta replacement with a CR1220 and charge protection.
This is a battery replacement for old circuit boards utilizing the 19mm length 3.6V Varta battery. The replacement is a 3V CR1220 with charge circuit protection. Low voltage drop is achieved with the MAX40200 "ideal diode" chip. Most boards will work with RTC/CMOS function at 3V, so check datasheets before you go through the hassle of building and installing this. Some motherboards have additional diodes that need to be bypassed. Clean up any corrosion from the pre-existing evil Varta before installation.
CR2032 version here: https://github.com/wiretap-retro/Not-A-Varta-CR2032
Open hardware design. Do what you want with it.