The operating characteristics of rechargeable batteries make them more difficult to evaluate. Not only do they start out at a lower voltage, but they die very rapidly at the end of their charge. My tests show that NiCds start at 2.5v and very rapidly fall off below 2.2v. This can cause a significant problem for the battery management circuit. It was not unusual for me to never see the low battery warning. My 95LX would drop right into the backup mode, which means that I would lose any files that had not been saved. (This is why HP does not recommend rechargeables.) However, by faithfully adhering to a regimen of swapping batteries at 2.3v, you can safely use them. The new high capacity NiCds, such as the Gates batteries marketed by Millennium, tested to 12 hours of useful life on the 95LX.
The hottest rechargeable technology on the market is nickel-metal-hydride. These batteries are rated at almost twice the capacity of NiCds -- 1100 milliamps-hours and a 0.1 volt higher average operating voltage. Unfortunately, NiMHs also have a very
steep fall off rate below 2.3 volts. They did yield 21 hours of average life, which provides a good compromise since they can be charged 500 times. Even though they have a high initial price of $18 per pair, that translates into 0.2 cents per hour compared to 3 cents per hour for alkalines.