Although the life of these chips is spec'd at 10 years,  their probable life at normal storage/operating temperature is probably 30+ years.  I talked to a designer of these devices and he said the limiting factor in most cases is the self-discharge rate of the internal batteries.  The actual lifetime is determined by statistical luck of the draw.

If the batteries go,  you will need to replace the chips and do a full cal.  If your chips are old and you are having a full cal done,  it would be best to replace them.  A full cal will set you back around $500.  The chips should be around $50...  cheap insurance.

I have some Tek 1503B/C TDR's that are over 20 years old.  The Dallas Semi batteries have never been changed and still measure 3.6V.

Windows Live™: Keep your life in sync.
http://windowslive.com/explore?ocid=PID23384::T:WLMTAGL:ON:WL:en-US:NF_BR_sync:082009

Mark - a few questions that might be helpful to the rest of us...

1. How do we measure the internal battery voltage for the DS1230Y and
   DS1220Y chips? ...Across which pin numbers?

2. Where can we buy the 3 chips for around $50? (That sounds like a good
   deal to me.)

BTW, my first 3458A NVRAM battery failure occurred at 15 years, right in the
middle of a critical 6 month-long data collection experiment. So this was
catastrophic. In a second 3458A, the NVRAM battery failure occurred at 16
years. Over their lifetimes, I estimate those two units were powered up only
10 percent of the total time. So if someone powers-up their 3458A more of
the time (e.g. running 24/7 all those years), maybe they stand a better
chance of longer battery life? (These chips automatically disconnect their
internal batteries when Vcc rises above 3Vdc.)

The 3458A first went into production 21 years ago. Time sure flies when
we're having fun!

-Greg