I am not saying this will work for you but sometimes I use to switch off battery function, what I mean by that is, I use to use the battery interface to switch off battery charging whenever my AC adapter was plugged in since I had it plugged in 100% of the time. I would have the battery charged full but use only AC power.
So see if you accidentally turned off battery function and have your laptop working on AC power only. that way if your battery is not fully charged and you unplug the AC adapter your laptop will turn off.
Otherwise, as Alex asked, how do you know it is working? There should be a function allowing you to test your battery, to see what its charge rate is, to see how well it is holding charge and how much charge it is actually holding.