Button cell batteries look like foil-wrapped candy to small children. Thousands of children swallow them every year, and the batteries damage their stomach lining, sometimes killing them.
I purchased some pocket microscopes for grade-school-age relatives; these use three AG-12/186/LR43 button cells in series to power the built-in illuminators. I worry that their younger siblings may get at these button cells and hurt themselves.
So, I wrapped the three stacked batteries circumferentially with 1-cm wide porous Micropore medical tape, then painted the tape with Mavala Stop Nail Alert, which is intended to be painted on children's fingernails to discourage nail-biting and thumb-sucking.
Mavala Stop contains Denatonium benzoate , the most bitter-tasting chemical known to science. Swallowing a fist-sized lump of dentatonium could be fatal, but the tiny amount in the paint is only enough to taste awful. You will surely spit it out before poisoning is possible.
Cutting and painting and wrapping a tape strip is time consuming and messy, and Mavala Stop is very expensive. Manufacturing presized tape strips printed with Denatonium benzoate directly from the manufacturer (trade names include Bitrex ), with a removable cover, would make wrapping batteries much easier for parents and other end-users. This would be complicated to develop, but the manufacturing process could be very fast.
Researchers are encouraging manufacturers to incorporate denatonium into the batteries themselves, but this hasn't happened yet. Until then, be careful with button cells (in these microscopes, mini-flashlights, remotes, hearing aids, toys, and other common devices) and keep them away from children.