First know that, eventually, every rechargeable battery will need to be replaced. How soon your battery will wear out depends mostly upon your usage, your care of the battery and the quality of the battery itself. It makes sense that extensive use of the battery will reduce it’s running life faster than an average amount

of use and a better built battery will probably enjoy a longer life span than one built with lower quality materials.

On average, a battery will begin to show a normal decrease in power after 18 to 24 months. With extensive use, a reduced run time might be noticeable before 18 months.

There are 3 main indications that it might be time to replace your battery:

Your laptop battery is simply not charging. First, remove and then reconnect your battery to make sure the battery has just not loosened from its proper connection. Second, start your laptop with another charger to see if the AC-adapter might be damaged. Adapters don’t usually wear out that quickly but it’s certainly worth giving it a try. If neither of the above solve the problem, the battery has worn out and needs to be replaced.

Your laptop is not connecting with the battery. Try cleaning all the connectors on the battery and the laptop and if that doesn’t solve the problem, most likely, your battery is dead.

The vast majority of which, including me, feel that there will be no detrimental effect to the battery by leaving your laptop plugged in 24/7.

Some "experts" feel that this practice reduces the number of charge/discharge cycles that your laptop will be capable of. While this may be true, think of how many cycles are skipped whilst it's plugged in (rather than if you tried the disciplined 40%-80% regimen they espouse)?

Leaving the laptop plugged in lets you enjoy the "plugged in" power profile (Windows) that you can set to optimize performance over battery life… Who knows, the extra performance may make you keep the laptop another year…Laptop batteries are made of 2–3 LiIon cells in series. When a cell dies, available charge will typically plummet from hours to minutes, as the internal average voltage will be 7.4v instead of 11.1v in case of 1 dead over 3.

When this happens usually PC itself warn the user to replace the battery, either with a screen message or beeping a code before boot. It makes sense that extensive use of the battery will reduce it’s running life faster than an average amount of use and a better built battery will probably enjoy a longer life span than one built with lower quality materials.