I have a 7.5 VDC power supply that charges 4 aa batteries in a tool.
Is the "Power Supply" a power supply or is it a battery charger?
That may seem a silly question, but a power supply and a battery charger have different requirements.
Also, the requirements of the battery charger will depend on the battery chemistry.
I am going to make some big assumptions here, they may be wrong...
The AA batteries are either NiCd or NiMH.
They are intended to be charged with a 100mA constant current.
The charger is "cheap", it is simply a resistor in series with a voltage source which is greater than that of a fully charged battery.
Four charged NiMH batteries will have a terminal voltage of about 4 x 1.4x = 5.6v
12v - 5.6v = 6.4v That excess 6.4v will be dropped by a current limiting resistor which is in series with the output to the batteries.
Resistance R = 6.4v/100mA = 64 Ohm.
So, my best (worst) guess is that your PSU is an unregulated 12v PSU with a 68 Ohm resistor in series with the output to the batteries.
JimB