标准答案：可以。

有电池时可不可以用电源线？
You can. When it is fully charged, the charger will be off. But, it does impact the life if you keep charge not to full and discharge not to zero and charge up again. Though battery manufactures won't agree, it is life experience.

好像电池电线一起用时 电池寿命会减少 因为电池一直不断在充放电 是不是这样？
Not really. The computer will use line supply instead of battery. However, there will be leakage, battery will be charged once a while.

还有电池电量剩多少时充电最好?
The ideal case, though no one would say this, is wait when it is almost fully discharged. The battery life is calculated by how many cycles. To do this will burden users hence no PC manufacture will state this. Besides, they want to sell you more batteries.

Also, to accurately charge a battery, charger will detec the fully charged point and the fully discharged point to calculate best charging curve, usually based on lab test from battery manufacture.

Of course, only good battery (meaning more expensive authentic ones) have charger smarter enough.