> When plugged in, the battery is not in use. The laptop’s power circuitry bypasses the battery unless it’s needed. Depending on how smart the charger is, it may occasionally poll and “top off” the battery if its charge decreases to a certain threshold below 100%, but this is rarely needed in practice.
Not strictly true.
If you plug in your macbook, your battery is still being used. Evidently the power supply actually cannot meet the peak power demand of the system, so the battery needs to be there to provide for that [1] [2]. Apple also feels that the conditioning the battery receives while it interacts with a powered system increases its longevity.
Generally the power adapter provides more power than your machine is using, so obviously it can charge. But if you remove your battery and benchmark your system, you'll find that your system is limiting itself back about 20-30% to account for the lack of battery.
References: [1] http://support.apple.com/kb/HT2332 [2] http://www.tomshardware.com/news/apple-macbook-pro-battery-b...