Hi.
I'm planning to use the PCA2129 in a battery powered circuit, since it's basically perfect for what I need.
What I do not understand is this: can I power it from 3 AA batteries? The "absolute maximum ratings" seem to agree with it, but the normal electrical characteristics make me doubt it since rate the component to a maximum of 4.2V
Running any device at its absolute maximum ratings isn't recommended. It may work, but may lead to lifetime shortening or other premature damage of the device. This is what nearly every datasheet says regarding the abs max ratings. Above the abs max ratings, instant damage may occur.
So if you want to build a reliable product, you better not exceed the recommended operating range.
Since the chip has a operating range from 1.8V ... 4.2V, one could run it from three batteries using a series diode to drop the voltage somewhat below 4.2V (the diode drop will be rather small because of the low operating current), and still have enough reserve for depleted batteries.
IMO the specified operating range implies one shall be able to run this chip off a single cell Li-Ion battery.