HP Laptop - new battery - does not turn on

Registration date
Sunday September 9, 2012
Last seen
September 9, 2012
I have just replaced the battery to my daughter's HP laptop G61-424CA. I recharged it for 6 hours without being in use and fully shut down during this period. I then turned on the laptop and fine, the battery was fully charged. I then shut down the laptop and, without using the power charger, "tried" to turn on the laptop. It would not turn on without the power charger. What is happening here and how do I fix this?

Almost the same thing has happened to my Dell Vostro 1700. New battery and power charger but in this case the battery does not charge. I unplug the power charger and straight away, the low battery warning comes up.