Ideal battery
Also called Ideal voltage source
An ideal battery is a model source that holds a fixed potential difference between its terminals however much current it delivers, because it is taken to have no internal resistance. AP problems assume batteries are ideal unless a question says otherwise.
One sentence in the CED carries the whole model: ideal batteries have negligible internal resistance, and ideal wires have negligible resistance (11.5.B.1, printed identically in AP Physics 2 and AP Physics C: Electricity and Magnetism).
What that buys you is a source whose terminal potential difference never sags. Draw 0.1 A or 10 A from it and the terminals still read the same value, and that value is the emf: the CED defines the potential difference a battery would supply if it were ideal as the potential difference measured across the terminals when there is no current in the battery.
The default is set for you, twice over. A boundary statement under Topic 11.5 says that unless otherwise stated, all batteries, wires and meters are assumed to be ideal, and adds that circuits with batteries of different potential differences connected in parallel will not be assessed. Both equation sheets repeat the assumption in their exam conventions: strings, springs, batteries, wires and meters are ideal.
So the model holds until a question names a resistance inside the source, or hands you data showing the terminal voltage falling as current rises. Then the battery is nonideal and you switch to internal resistance, which the CED models as a resistor in series with an ideal battery.
A battery does not store charge and does not push out a fixed current. It maintains a potential difference, and the charge already sitting in the conductors responds to it.