General query on batteries in parallel
Batteries can be viewed as an ideal voltage source and a small resistor. An alkaline AA battery has an internal resisitance of about 0.15Ω. You didn't specify the chemistry of your rechargeable batteries, but NiMH AA's are about 0.02Ω. Source: http://www.learningaboutelectronics.com/Articles/Battery-internal-resistance. If I recall correctly, NiCd are about the same.
If you're using NiMH, when you first connect the batteries in parallel to charge, you have a circuit with two opposing voltages, and two 0.02Ω resistors. If the voltages of the batteries are different by a mere 0.1V:

I = V/R
I = 0.1 / 0.04
I = 2.5A

The more powerful battery drives 2.5A through the circuit! And that's without the charger applying any additional power.
Related: always wear safety glasses as an absolute minimum if you're testing out charging circuits. Even better if the charger is behind a transparent shield. The last thing you want is a face full of super-heated battery chemicals.