I have a tire pressure monitoring system on my bike. It’s integrated with my GPS and it’s a handy little thing that Garmin brought to the table that incented me to upgrade to their latest technology.

Each of the sensors runs on a single CR1225 battery.

Of course they rundown after a while… and I had to replace them. I ordered up a pack of them from eBay.

I was figuring that both the BR- and CR- batteries were basically the same. Both are specced to roughly the same overall energy density and the same voltage. Both are 3V nominally.

Installing them they seemed to work just fine.

Except that after just a few days I was getting a low battery alert again. Could it be that I got some counterfeit batteries or something? It’s possible but not likely. I replaced the batteries again with another set from the 10 that I got after they had gone flat. Same thing.

Then I started looking at the spec sheets (or another) for these batteries.

The BR- series are Lithium Poly-Corbonmonoflouride cells and the CR- are Lithium Manganese Dioxide cells. They are both lithium and have the same voltage potential. But the internal resistance and the discharge curves are different.

Different enough that the internal resistance of the battery makes it read low under high-load (relatively) situations. (Transmitting RF is considered high load in cases like this)

The more you know…