Strange battery pack behavior

I had built two 9.6V circa 2500mAh external packs for FT-817. One, the older, started misbehaving.

Once charged it shows the usual 9.6+ V. If I connect the FT-817 to it, the displayed voltage is 9.6V rather than 10+V as before. If I hit the morse key the radio shuts down, except for 0.5W output power setting.

I need to check the power cord resistance, because I notice a 0.4V drop on the displayed voltage even with an external 20A PSU.

Then I suspect one or more AA cell gone bad. Now, how do I test them? I am thinking of a resistive load and voltmeter. At 5W keydown the FT817 draws 2.5A or so, that's about 0.5 ohm/3W resistor (testing once 1.2V cell at a time). I might start with a lighter 1.2A load, with a 1 ohm/1W surplus resistor. Resistors with a higher power rating are obviously fine. Let's see what I have in the junk box or recycle bin... Any better idea?