On my F2 I ran the batteries down to no bars at all and they both tested "replace" on my tester. On my F 70 I replaced at 1 bar. Only 1 battery was in the "replace" and the other three were right on the line of good and low. I think next time I will replace just the one and see how that goes. The performance on both did not seem to change with low bars. The battery life on the F 70 is about double of that on the F2, 4 AA's vs 2 -9 volts.