|Providing the charger/mains adapter is OK - producing the correct volts and current, the it ought to charge the battery. It may take a little while to fully charge; which is pretty normal/standard.|
There are some guru who advocate running down a laptop battery to dead flat; and then recharging. Similar advise too for iDevices, tablets various and other phones.
Incidentally does the laptop work - without the battery installed? It ought to - if the charger/mains adapter is OK.
Being an Aspire you may have to prop up the case a little in one area, as the battery when installed usually provides one of the "feet" on the base.
And to answer your post header question:
What Does Caution Battery Critically LOW! Mean?
It means just that. The system is designed to warn you when the battery is low; and logically ought to be recharged. This will definitely happen if running on battery only - for obvious reasons... If the charger unit/mains adapter is connected and powered up then the battery will charge up whilst you use the laptop; and it will take a wee while.
message edited by trvlr