|All laptops have two batteries.|
- a tiny cmos battery that is required to retain the time and date and other custom settings the user has made in the bios Setup when the computer is not running, even when all other sources of power are disconnected - that's always inside the case and may or may not be easy to change. That may be a single one piece unit, or multiple cells connected together. That may or may not be inside some sort of casing. That usually lasts at least 5 years - it may last a lot longer than that. Some are rechargable and charge when you plug in the AC adapter; some are not rechargable. There is often no info about that cmos battery in the User's or Owner's manual for the laptop, it's usually not on a parts lists for the model, but there is usually info about that in the Service and/or Maintenance manual for the model.
- a much larger battery, or usually, it's actually a battery pack because multiple individual cells are connected together inside of it, that provides the major power to the laptop even when the AC adapter is not connected to the laptop, if the battery is charged up enough. I can that the main battery, others just call it the battery because they're not likely to be talking about the cmos battery. That battery, or battery pack, is always inside a casing of some sort, and has contacts on it that connect to contacts inside the laptop in the socket where you install it.
Both batteries are internal, technically, but, of course, the main battery is always easy to remove from the outside of the case.
The main batteries are never warrantied for longer than a year.
If your main battery is a lot older than about two years old, you're likely to have problems with it.
If there's something wrong with the AC adapter or it's cord or the plug on the end of the cord, or the power jack that plugs into on the laptop, the battery may not be getting charged properly when the AC adapter is plugged in .
If the battery is not charged to at least a minimal capacity, for whatever reason, whenever the AC adapter is not connected, the computer will not start up normally - either it won't start up at all, or it starts up but you see no video and the hard drive is not flashing it's led.
Most recent or more recent laptops cannot start up normally if only the AC adapter is connected to it - the battery must be installed as well, and be capable of being charged to at least a minimal capacity. If the main battery is really old it may have become defective and is no longer be capable of that, and in that case, the computer will not start up normally even when both the main battery and the AC adapter are connected to it.