Help with RTC battery problem

I have a Compaq Armada E500 laptop, faithfully running WinXP Pro SP1.

Last week, the system time began to reset to 1980 whenever the computer was powered off and unplugged. In addition, my power-on password disappeared.

I assumed this meant that the RTC battery had finally run down after five years.

I bought a CR2016 watch battery - compaq uses solder tabs on its batteries, so I scraped off a bit of the coating on the negative side so I was able to solder the wires - Red to + and black to -, and reinserted the connector. Nothing.

Thinking I might have shorted the battery I tried with a new one. Again, nothing.

Any advice? The E500 works fine except for having to reset the BIOS and date/time.