Key fob low battery on dash but new batteries?

[Freedom55]

I'm trying to find out the proper load resistor value and got various results searching the web...One guy said 4.7K for a CR2032. I have a 5.6K and used it on a spare CR2025 I have for the garage opener. 3.28V no resistor 3.08V with the resistor. I also tried an old CR2032 and it read 2.78V no resistor .45V with the resistor. At least, it gives me a good idea of the battery condition. The chart below shows 3K; I have another 5.6K so I could put two in parallel resulting in 2.8K.

 

[AVC]

Well, any tech school graduate knows to test a battery under appropriate load, so not much strain on my Phd.

My salient point here, is that testing with a typical DVM alone won't (can't) reliably weed out good from bad button cells, measured when removed from the device or a device switched completely off. They'll measure within a few HUNDRETHS of volt when fresh, or when exhausted.

I bought this (or a version of this) battery tester some time ago. Not perfect, but very affordable, and it susses the living from the dead. Save the DVM for appropriate high Z measurements.

https://www.amazon.com (commissions earned)

The chart above seems reasonable test loads for various battery voltages and mAh (capacity) batteries. Simpler testers, like above won't load AAA ~ D cells like the chart shows as they don't have switches to match the battery type. There are testers with individual slots, where they provide a custom load for each battery type.

BTW, for lithium button cells, 2.25v under test is fine, in my experience, for a typical IR remote to work reliably, but for many auto FOB's I've found they need to stay above 2.7v under test (my Pilot FOB, 3.0v) for them to work reliably.

Last, beware that there are MANY dodgy button cell sources out there, where the batteries measure fine with a DVM but fail under load within days to weeks. In my less frugal moments, I stick with name brands (Panasonic, Toshiba, Sony, Energizer, Duracell...) and from trusted, sources (Mouser, Digikey, Target, Home Depot, Costco, Sams...) who likely turn over their stock frequently.

[EDIT] Many have reported good experience with Amazon Basics button cells.

 

[rzc]

So.... I have a 2019 CX5 Signature... does the keyfob go in "sleep mode" like the newer, bigger, remote?

 

[Budgetta]

Mazda Key Fob Low Battery Issue

It means it has gone into the power saving mode. Usually when the old battery is about to die, the Fob turns on the Power Saving mode itself. So, from there on you need to get the fob so close to the ignition button to start engine + your keyless entry won’t work.
So, after you change your old batteries and put a new battery, the fob will continue to be in the Power Saving Mode. There are simple steps to take the Fob out of Power Saving mode.

1. Turn on the Engine
2. Use your fob and press your “Lock” button 4 times
3. You’ll see the indicator (red light) on your fob light up
4. Immediately after you see the red light on your fob, press and hold “Lock” button until you see your indicator lamps (green) on your car dashboard flash a couple of times. This means you have toggled the POWER SAVE MODE.
5. Try your keyless entry and try turning on ignition with the keys in your pocket -

This should work

This is the solution! If you have a cover on your key fob remove it so that you can see the little red light that will blink above the lock button.