15 Year Ni-Cad Battery Life

i have an Hp45 calculator with the original batter pack that still functions after 42 years.

I've had plenty of NiCads that lasted 10 years or more. In fact, I've got a drawer full of them left over from underwater photography in the 80's that still will take a charge, although probably not as much as when they were new. 

Most AED (automatic defibrillator) devices use NiCads.  One of the positive points about NiCad's are they can hold a charge on the shelf for a long period of time (the AED's usually need replacing on a five year cycle).  A rechargeable battery has a number of factors that affect its longevity.   You only get so many charge cycles out of a battery and then it needs replacing.  In additiona, a NiCad's achilles heel is that it has "memory" problems, aka if you only partially discharge and then recharge it will latch onto that partial charge as "full".  

This is probably what lead to the three clanks of death issue with the PS-1 equipment.  The batteries were only need to store the state machine's settings from shut down to startup.  The more you actually ran the trains the more you were abusing the battery (aka exposing it to a short cycle) and doing so repeatedly.

While Ni-Cad has a lower self-discharge than NiMh, they're still pretty high, typically around 20% a month.  So, after a few months, I hope you don't need that defibrillator, because it's likely not to work!

I have not seen that discharge rate with NiCad.  I also think cell type matters.  The AA which are 1.2V cells but with more stored energy last longer than the higher voltage cells with less stored energy.  Part of the reason the 2.4V AA NiCad last longer.

The 2 4.2V cells in the 8.4V battery are only rated at 120mahs, even newer models don't get much higher, maybe 200mah.  While a 2 1.2V cell AA in the 2.4V battery are rated at 700mahs.  Easier to keep the voltage at 1.2 or 2.4Vs than 8.4Vs.

Now if they used a 7 cell AA I am sure you would never have a problem, you would just need an aux tender to carry the battery:-)  G