Checking NiMH cells?

I have a relatively cheap blood pressure tester that takes 2 AAA cells. I buy these cells "by the case" for less than $.25 each.
When I was "into" those little portable radios I purchased quite a few NiMH cells. They cost several $dollars each and it seemed that only after a few charging cycles would fail the test the charger put on them.
If I had to guess why your blood pressure tester isn't happy, it might be the voltage is just a little too low. I think the rechargables are 1.2 volts. The "cabon/zink" cells, of course, put out 1.5 volts and the alka cells put out something in the middle.

I should have said that the device uses 4 AA cells.
Bill