I believe you have the module, not the chip.

There is no way to measure the battery voltage on the module. In the SFR's relevant to the DS5000, there is no specific bit which can be checked or monitored to determine the condition of the lithium cell.

Dallas says the battery should last at least 10 years. Your power may have been applied in reverse polarity to the module (module plugged in backwards) or a negative spike on any pin. Your external circuitry may be the culprit. What are you powering this with and what are you driving?

Read Application Note 93 at the Dallas/Maxim website. It's also in the back of the "Secure Microcontroller User's Guide". If experimenting with the DS5000 you really should have this book.

Maybe it's time to try one of the ISP variants.