hp dv6000t dies soon after startup when on battery--please advise!

I own an hp dv6000t, bought in 2006-09.

Normally, my laptop sits at home, where I power it via electrical outlet. This has always worked fine.

About 2 weeks ago, I tried using it while on a long train commute. After initially turning on and booting into Windows, it soon died and would never restart at all.

I figured that the battery must have gone, so I ordered a replacement battery. Looking around the web, I came across the Amstron LHP-62. This attracted me because its capacity, 5200mAh, is 800 greater than the 4400 stock battery capacity. So, I ordered one from Directron.

It arrived this last Monday, and I tried it on my Tuesday train commute (50+ minutes each way). It worked perfectly; in fact, 60+% of the battery capacity was remaining when I got home at night. I recharged it to about 100%, and then took it out of the laptop (since I did not do that with my previous batteries, and I have been told that leaving laptop batteries into a plugged in machine kills them faster).

Wednesday, I tried to use it on the commute again. Got on the train, started up the laptop, it booted, and I logged into Windows. But then it died within a minute. Pressing the power button always causes it to boot. I get past the BIOS, and see the windoze xp logo come up. It usually dies around that point, altho sometimes it lets me log into windoze before soon dying.

Anyone have any idea what happened?

Is my new battery a dud? I am not sure. On the one hand, the laptop still works perfectly fine when plugged into an outlet. On the other hand, it always powers up and gets past BIOS--its only when windoze loads that it crashes. When my last battery died, pressing the power button did absolutely nothing.

Thanks much for any advice that you can give.