[MrFixr55]

Air Cells were 2.5V, non-rechargeable disposable batteries.  Since the tubes were designed for 2V rechargeable lead acid batteries, a 3 V Battery or a pair of 2 #6 dry cells put out too much voltage for these 2V tubes when new, so a variable or tapped resistor or ballast tubes had to be used.  Output voltage over the life of a 3V battery was not consistent over its life.  Eveready claimed that the air cel Model a600 did maintain its 2.5V over its life (1,000 -1,500 hours depending on tube count and total filament current.

A brochure on radio batteries is attached below:

Everready Air Cell A Battery Layerbuilt B Battery1937-2.pdf Size: 2.38 MB  Downloads: 404

Below is a link to an Antique Radio Forum thread. The author, Robert Lozier made 3 replicas of the Eveready A600 Air Cell and is making replicas of the Eveready 45V battery for his collection. An amazing build!
https://antiqueradios.com/forums/viewtop...?p=1754680

[Arran]

There is a bit of B.S in that pamphlet with regard to "C" batteries, without that bleeder resistor connected across them the "C" battery will pretty much live out it's shelf life as there is next to no current draw. I don't know how many farm radios actually used this bleeder resistor as it's not rare to find a very old "C" battery sitting in the back of the cabinet whilst the "A" and "B" batteries are long gone. In some later 2 volt tube sets they used Mallory button cells to supply the tube bias, whilst in others they used a self biasing arrangement, usually with a resistor network in the "B-" circuitry. But then again this pamphlet was for marketing, which is probably why they do not mention powers a 2 volt tube set from a rechargeable wet cell.
That being said I don't think that the Air Cells were all that popular, if that thread is anything to go by, as surviving examples seem to be few and far between. I'm guessing that the cases must have been molded from hard rubber unlike the typical dry cells.
Regards
Arran