The CR2032 coin-style battery in your heart rate monitor or power meter dies. No big deal, right? Put in a new battery and... it doesn't work? You try yet another new one and that one doesn't work either. So what's going on? Double-check if you happened to buy new batteries with a bitter coating safety feature - because they don't work well in many heart rate monitors, power meters, shifters, and other cycling products.
So what's going on here? Isn't a battery a battery? Apparently not. The bitter coating can affect the conductivity of the battery, and results can vary based on where the battery contacts are on your specific device. Your device might get intermittent electrical power, or none at all.
Batteries with a bitter coating may work poorly or not at all for cycling
Unfortunately, the bitter coating can interrupt the flow of current from the coin battery to your shifter, power meter, or heart rate monitor. This varies from device to device. Some may work with "bitter" batteries, while others don't, and it has to do with the exact placement of the electrical contacts in your cycling accessory.
Recognizing this problem, some brands have begun to re-design their electrical connections, so if you have a more recently manufactured device, you may never run into this issue, regardless of which coin batteries you buy. SRAM / Quarq for example, have switched to a different design which moves the battery contacts to a different spot, which doesn't touch the bitter coating portion of the battery.
However, if you have an older power meter or heart rate monitor with a design that predates the introduction of bitter-coated batteries, you may find that it doesn't work due to the placement of the electric contacts - if so, the fix is to use a different battery that doesn't have the bitter coating.
Why do these batteries have a bitter coating anyway?
CR2032 and similar coin-style batteries are compact and easy to swallow, making them a tempting target for babies and toddlers to put in their mouths. The bitter coating is intended to make that unpleasant, encouraging them to spit the battery out instead of swallowing it.
If you're a parent, this is a reasonable concern.
Duracell's Bitrex®
Duracell, one of the most popular manufacturers of CR2032 batteries use Bitrex®, which they claim is "the most bitter substance in the world" - I didn't test, I'll take their word for it. Other popular brands have also added similar substances to their own battery products.
Popular brands recommend against the use of bitter, child safety batteries
Manufacturers of heart rate monitors, power meters, and other accessories recognize the problems with bitter-coated batteries, and often recommend against their use as a result. Among the brands who caution against the use of CR2032 coin-style batteries with a bitter, child safety coating include Wahoo Fitness and 4iiii. I personally experienced this problem with the Wahoo TICKR heart rate monitor strap before I realized what was going on. After switching to a non-coated battery, the heart rate monitor strap works fine.
This problem is not unique to cycling accessories - Apple, for example, warns that the bitter-coated batteries may not work reliably in their AirTag trackers, which many cyclists use for anti-theft purposes.
Consider buying CR2032 batteries that don't have the bitter coating for use in cycling products
If you don't have babies or toddlers at home, the easy fix is to simply avoid buying batteries with the bitter coating. Non-coated coin style batteries are readily available in brick and mortar stores and online. They'll work more reliably and consistently across devices like heart rate monitors and power meters, but also in non-cycling devices too.
YMMV
For what it's worth, battery makers and cycling equipment manufacturers are both aware of the issue, and appear to be making progress toward fixing it. Cycling accessories released in the future will likely have a different placement of electrical contacts which don't touch the bitter coating.
It's possible that your specific device won't have this problem at all. That happens because the bitter coating is only on a portion of the battery - if the contacts on your device touch if, you've got a problem, if they don't, bitter-coated batteries will likely work without issue.