These wall warts have a rather high internal impedance. To see what the current really is when you are charging a battery is to put an ammeter in series with the charge leads. If you measure the voltage while it is hooked to the charger, you will find that the voltage drops appreciably as the current flows from the charger into the battery. Also, this voltage may be only a rectified AC (with no filtering) so the actual RMS (root mean squared) voltage as measured with the typical voltmeter will indicate roughly 1.414 times higher than the equivalent DC voltage when you measure without any load on the charger.