Not-so-obvious CR2032 Battery Problem (and an excuse to buy a flashing bicycle tail light?)

This topic applies to the CR2032 coin cell ("watch battery"), which I use quite a bit of in cycling computers, sensors, flashing bicycle tail lights, and as the CMOS battery in personal computers.

I would use a multimeter to make sure that the voltage is at least a little bit over the voltage specified on the battery itself. But despite a valid voltage measurement, I was experiencing sychronization problems between a bike computer and its sensors, both taking CR2032 coin cell batteries. So I used a new battery that was not past the "use by" or "best before" year listed on its packaging, and the synchronization issues disappeared.

Yes, some batteries may still work fine after the date... so if you cannot rely solely on the voltage reading on the multimeter, how can you validate that the CR2032 coin cell battery can sustain that voltage close to what a new battery would?

I found a way, using a Planet Bike Spok bike tail light (NOT the USB one) that I already have.

NOTE: This YouTube video (English only) shows how easy (once you know how) it is to open its battery compartment.

Once I put a questionable battery in the light, I turn the light on, and ask myself: "Would the light that comes off of this LED be rather blinding if it were to be aimed at my face?" (knowing from personal experience)

• If YES (ouch!) then the battery is still good.
• If NO (light being rather weak/faint), then the battery cannot sustain the voltage it indicated in the multimeter. I would not put that battery in a sensor or bicycle computer (or a personal computer).

QUESTION: I have a dedicated battery tester at home already. This should be OK, right?

ANSWER: You would have to have a tester whose instruction manual shows you "load current drain" ratings.

• If you have a battery tester with a digital display (EXAMPLE), I have not purchased one of those, but have not seen anything specified during shopping that it has resistors or that the device does any sort of load test; it could be giving you only the voltage as a multimeter would.
• If you have a battery tester with an analog display (EXAMPLE), resistors (which can provide some load) are needed as part of that display's design. However from one device to another, the resistor values vary, meaning that one device may provide more optimistic/pessimistic values than another when reading the same battery.

Your mileage may vary, but a dedicated battery tester that can simulate a load can be helpful. Some testers either specify at the back of the tester or in their manual what the voltage ranges are for "Good" and "Replace" for the different batteries supported if you don't want to have to use both a multimeter and a battery tester just to test a battery.
I personally would use both a multimeter (for voltage) and a tester (for load). But if you have neither or are in the field, the LED light trick above can also be helpful.