In fact, I don't know if this battery was bad or the one that came with the vac was bad. In any case, someone else wrote here about a battery failure and Hoover replacing it, so bad batteries may be common. Here's my battery experience with two batteries.
The vac has indicator lights showing charge status: 3 lights full or nearly full, 2 lights lower, 1 light lower still, 1 blinking light-almost dead. Hoover's official position on battery life: 12-15 minutes. My test results with vac only, no beater brush running:
Battery 1 goes from 3 lights to 2 in 5 minutes, 2 to 1 in 3 more minutes, and 1 to blinking in 4 more minutes. Total running time 12 minutes.
Battery 2 goes from 3 lights to 2 in 1 minute, 2 to 1 in 2.5 minutes, and 1 to blinking in 3 more minutes. Total running time 6.5 minutes.
I just spent 30 minutes on the phone with Hoover. The first CSR treated me like I was a moron, and then when I explained my tests, she said, "OK. Send it to us and we'll check it out." Gave me an address. Then put me on hold, came back, put me on hold, came back and said, "I'm transferring you to another department." (It was like that light company ad on TV.) After another 10 minutes on hold, a guy came on the phone with no knowledge of why I was calling, heard the story again, and then said, "Ship it to us, and we'll check it out." Frankly, I'll be surprised if I ever see that battery, much less a replacement battery, again.
Followup 8/22/2011: I just had a call from Hoover. The CSR says, "As a courtesy we're going to send you a new battery, but the battery you sent us was out of warranty. The Mfrs code says it is over two years old and not covered by warranty." I bought this in May, sent the bad battery in August. Hmmm.
My advice. Charge your new battery. See my test results above. If you can't duplicate the better results, send it back right away as defective. If it's the battery with the vac, send the whole thing back.