My mom’s smoke detectors started beeping, which usually means the batteries need to be changed. These are wired into the electrical system, but they have battery backups just in case the power goes out. I took the batteries out so the infernal beeping would stop. And so my mom would know what to get for replacements. These were 9 volt batteries. You remember the boxy-looking ones that you can put on your tongue to see if they still have juice in them? I don’t know the last electronic device that took this size battery. I don’t know if it’s a cost or a power issue, but everything seems to have switched to AA and AAA. As devices have gotten smaller, you barely encounter a need for C or D batteries any more.