First of all, do not cheap out and buy your replacement batteries from eBay. Instead, buy them from someone reputable, like Digikey. I tried the cheap route and despite some new 1620 batteries arriving with 3.27 volts, they lack adequate amperage to power our Mazda3 keys.

Battery sourcing aside, this YouTube video does a good job of showing how to take a Mazda3 key apart. If you happen to snap any parts off and the keys don’t want to stay together, just dab some super glue blots, one each corner, and let the key dry. It will still come apart the next time you need to replace batteries.

When it comes to reprogramming, this guide may be helpful.

** Update 3-10-2026 **
Researching this with Digikey, it appears Murata (now owned/manufactured by Maxell) can put out over 1,000µA and Jauch can put out 650µA, whereas most other brands indicate approximately 100µA. I am ordering the Murata/Maxell (lithium manganese dioxide) model.