There is another possibility aside from an intentional limit to protect the battery. Laptops often (I can confirm for some Dell and HP models) have a specific signal from the charger, which informs the laptop of the charger's maximum capacity.
Sometimes, the wire that carries the signal or the corresponding part of the socket is damaged, and the laptop can no longer receive this signal. When this happens, a laptop might assume the charger is rated lower and refuse to charge the battery, ostensibly to protect the charger from overloading (which can cause a fire). Some will even throttle the CPU. Normally, the laptop will alert you both within Windows and at boot time with rather obvious messages - so, as long as you are not getting those messages, you should be fine.
If you are getting warning messages concerning the inability to detect the charger rating, the charger may have to be replaced, and/or the laptop may have to be repaired.
Some possible BIOS messages:
Power Adapter not recognized.
The AC power adapter type cannot be determined. Your system will operate slower and the battery will not charge. Please connect a Dell 90W AC adapter or higher for best system operation.
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More information is available here.