Like a "9V" battery, for example: why does the "volts" metric get the almost exclusive highlight in advertising them rather than "9V / 500 mAH"? Or 12V and 2 aH? Or maybe even a watt-hours designation instead?

 

Simple -- most people don't know what the voltage means, so adding anything more would be meaningless confusion for them.

For most applications, from the user's standpoint, what really matters is the form factor -- they need three AAA batteries, for instance. But in some cases you can have significantly different voltages in the same form factor, so you need to specify that. Once you have the correct voltage in the correct form factor, most other things are going to be pretty well nailed down, particularly if you have also specified the battery chemistry.

It's the designer of the device that needs to know and deal with all of the other information and they know how to get the information they need.

 

The key might be "advertising." It is not that uncommon to see new members ask about running larger motors with "9V " batteries. Second, the "9V battery" has a specific connotation through common use among the public.

Once you get away from that example, particularly with newer battery chemistries, it is more common to see the voltage and capacity given.

 

why does the "volts" metric get the almost exclusive highlight in advertising them rather than "9V / 500 mAH"?

Because generally voltage is more critical than the AH.
A circuit is typically designed to run with a particular voltage and, as long as the battery has the voltage, it will operate properly, whether the battery capacity is 500mAH or 2000mAH.
It will just run four times longer with the 2000mAH one.

But if the circuit is designed for 9V, it will likely not operate with 3V, even if the AH ratings of both batteries are the same.

Another factor is that the mAH ratings depend upon how you test the battery (test current, low voltage cutoff, etc.) whereas the voltage is just the open-circuit voltage.

But it would be nice if the AH ratings of batteries was more prominently shown in the battery specs.

 

In addition, the usable amp-hours depends a lot on the current draw, and so it would be wrong for all except certain applications. Also, it would be one more thing to tell lies about.

 

You missed out internal resistance .... Voltage ... Capacity (A or W Hrs) ... and Internal Resistance

These three parameters tell you everything .... Internal resistance is most important , it determines how quickly you can get the power out . (max current) ...

Oh ... nearly forgot 'C' rating ... mostly used for lithium ... 20C means it can discharge at 20 times the 1 hr capacity (20C =3min to completly discharge) without getting too hot!

 

I have one instrument that takes AA Ni-Cd batteries. The ones with the raised button won't fit. really subtle. The company won;t talk to me about the product either. Zero information. Says it's industrial.

Well, it is. It's a portable expensive thermal gas detector. I got the old one when when my company upgraded. I can use it for refrigerant and natural gas leaks. EPA certified for refrigerants for Automotive and small appliance.

You can't even buy a battery pack that says 3.6 V 2000 mAh and a JST xyz connector with leads 5" long exiting "this way". You buy a battery pack for "some device". I buy the packs and the connectors separately and buy packs that have about 3x the capacity. Application: Cordless phones.