While I scanned a few shortwave bands tonight, the E5 shut itself off and flashed the empty battery icon. I finally dug out my multimeter to test the battery levels for the point that the E5 was shutting itself off. The voltage on the batteries that just came out of the E5 are: 1.12, 0.89, 1.21, and 1.02. That's a total of 4.24 volts, which is about 11.6% below the best-case scenario of four full rechargeable batteries at 4.8 volts.
These are Energizer 2500 mAh NiMH batteries. The E5 charges NiMH batteries at a rate of 100 mAh per hour for a maximum of 23 hours, which means that it cannot fully charge these batteries if they are empty.
I have twelve of these batteries; the E5, the SW7600GR, and my Olympus digital camera each take four batteries. I should come up with a way to label the batteries so I can measure voltage on each one when I am about to recharge them. This could help indicate if there's a battery that is draining much quicker than the others.