Wireless AC extension cord

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AA??? I'd guess AAA at that capacity, not AA.

They must be very old. If so, I'd guess they have lost much of that rated capacity due to aging.

Yep -- AAA has much less capacity than AA. NiMH would give you much higher capacity (up to 1000 mAh), and thus longer life, even with the higher self-discharge, and even assuming your NiCd are fully up to snuff.

I'm skeptical. Most such devices, including the old Logitech Cordless Wheel Mouse I used to own, are designed to tolerate a wide range of battery voltage, and only complain when the voltage drops below about 1.0V. While NiCd and NiMH have a lower starting voltage than alkaline, they have a very flat discharge curve as compared to the discharge slope of alkaline, and usually give proper life in digital devices. I can only think of one device where this isn't true, a very old cheap digital camera where the built-in flash was the likely culprit.

I'd be willing to bet that the best NiMH AAA would give you at least 50% more runtime than your NiCd AAA even with higher self-discharge.

They're both old and don't have much capacity. My guess is about

10-12 years old. No manufacturers labels. They came from a battery pack manufacturer that didn't want to ship such junk in their products. Most in the pile are AA but there are a few boxes of AAA. I use them as needed.

Of course. However, I have a box of NiCads and only a few NiMH. My cameras and most of my handheld radios use NiMH batteries.

True. I'll stick a power supply on the mouse and see how low in voltage it will run. 1.0V sounds a bit low. Alkalines are usually considered almost dead at 1.1VDC. However, the discharge curves vary somewhat by brand. See graphs at bottom of page:

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once had a Garmin GPS-45 that absolutely insists on 1.3VDC minimum and would therefore only work with alkaline batteries.

You might be correct. I'm just too lazy to test or calculate the relative merits. At my present rate of consumption, my pile of under capacity NiCads should last perhaps another 10 years. By then, we'll have fuel cells or thermo nuclear battery packs.