3.3V Lithium NanoPhosphate Batteries (LiFePO4)
These are the rechargeable batteries used in electric vehicles & have a lifetime of 10 - 15 years. They do not suffer memory effects & hold their 3.2 - 3.3V voltage output steady until depleted. More importantly for powering clocks & digital circuits they seem to have very low noise.
These batteries have been tested by a user against some very highly regarded regulators & found to be easily superior:
I compared one of the A123 batteries that Jkeny uses to a number of other highly regarded regs, including a PFM flea, and the battery was easily superior to any of them. Kinda sad really as I've spent years building and designing discrete regs. I'm not sure what actually makes the batteries better, so I need to run some more tests, but I suspect it's either self noise from the regs or earth leakage currents causing problems. Good batteries solve these problems, but obviously charging/maintenance becomes the issue.
Have a look at 4 of these batteries starting a car at -21C in Quebec