Jake,

As someone noted, most of these wall warts are just a transformer, a diode to convert to sort of DC, and a resistor to limit the current. If it's a 12 volt ( for instance ) and the internal resistance of the transformer, the resistor, and the battery is 140 ohms, the diode gives a .5 volt drop, and the battery is discharged to 4.5 volts; that means you got 7 volts (12 - .5 - 4.5 )dropping across 140 ohms which will give you 50ma. This goes down as the battery voltage goes up, but that's an estimate to start with.

Since you don't really know what the internal resistance of the wart is, (as others have said) or it may even be a more complicated constant current supply, plug in an amp meter and measure the current when plugged in. If the current is less than twice the rated current of the wart, you're probably OK.

Joel