Six months ago, the battery in my 2010 Mazda 3’s key fob (aka “flip-key”) totally died and I needed to replace it. The battery in these things is a CR1620, which I couldn’t find anywhere in stores, so I ordered the cheapest ones I could find off eBay — a pack of 3 generic brand Eunicell (made in China) for $2.18 CAD shipped. Unfortunately, I didn’t have a good first experience with them. There’s an orange LED that should light up whenever you press a button on the key fob. On a working key fob that still contained the original Panasonic battery from the manufacturer, the LED was bright and solid, but on the one I had just replaced, it was flickering and dim. I actually couldn’t get the first battery to work, so I had to take a 2nd one from the 3-pack, and that eventually did work, but never 100% reliably. I also had to reprogram the key fob to be recognized by my car. I concluded that these batteries were just “weak” or underpowered, or that the eBay seller was giving me old/used stock (I now realize that this wasn’t true. EDIT: it turns out my original suspicions were probably correct; see update below!). Anyway, a week ago, I sought out a brand name and ordered a 5-pack of Sony batteries (made in Japan) from Amazon, which cost me $13 all in (including Ontario taxes).

Left: 5pcs Sony CR1620 (made in Japan), about $2.60 each.
Top-right: Original Panasonic battery (made in Indonesia).
Bottom-right: 3pcs Eunicell (made in China) from eBay, about $0.73 each.

Funny enough, today while I had the key disassembled and was about to replace the Eunicell battery with the Sony, I thought back to the time when I fixed a broken mouse button by simply cleaning the electrical contacts. In my Mazda’s key fob, there are two silver-coloured metallic prongs which contact the battery on the opposite half of the fob. I couldn’t see any visible dirt on these contacts, but I could feel a bit of grime on the battery itself, so I just scratched it off with my fingernails until it felt smooth. Sure enough, this fixed it, and the key worked right away! Each button press now produced a solid orange light, and this is with the same Eunicell battery that I had written off before. I didn’t even have to reprogram it this time. I guess the cheap stuff from eBay isn’t so bad after all. Wish I had thought of this before spending the money on new ones, but oh well. Anyone need 5 new, unopened Sony CR1620 batteries? 😛

UPDATE 2015-Nov-14: 2-3 months after I wrote this post, my key fob with the Eunicell started flickering again. I replaced it with the Sony battery I bought earlier and it’s been going strong ever since.

Conclusions

1. The Eunicell branded battery is several times cheaper and works perfectly fine. UPDATE 2015-Nov-14: I no longer recommend buying Eunicell batteries or any cheap batteries in general. This would be the second time I made this mistake! (The first was a replacement battery for a Samsung smart phone that ended up being just as bad as, or worse than, the 2-year-old original battery that it was supposed to replace. It was labelled as Samsung, but clearly a fake at 1/4 the price.) Buy a known brand from a trusted seller instead.
2. Cleaning electrical contacts can work wonders. It’s a free fix and works for all kinds of stuff like computer mice, batteries for small electronics, and even a Toyota Prius…

Oh, and just in case someone came here expecting a how to guide, here are the links that I found most helpful:

• Video guide: How To Replace A Battery In A Mazda Key Fob [youtube.com]
• Text with pictures: How to: Replace flip key battery [mazda3forums.com]
• Reprogramming guide: Spare Keyless Remote Self-Programming – How To Guide [mazda3forums.com]