So I got a lenovo z580 3 weeks ago and I just found out I can use it without the battery plugged in.I use it as a desktop replacement and I rarely have to move it.Is it better to remove the battery ? I heard this will make it last way longer..but I also heard that if I just left it plugged in fully charged the whole time there would be no harm.What do you think ?
I would just take it out. Keeping a battery fully charged at 100% can actually decrease the battery life. I think by default Lenovo laptops only allow the battery to be charged to 80% unless you manually override it.
In any case, since you are using it as a desktop replace, there is no real need for the battery to in the laptop. One thing though, it is not recommended you allow the battery to full drained. Once a battery's charge goes below a certain level the battery is "forever asleep". No amount of charging would "re-awaken" the battery.
Having said the above, I have completely drained my laptop's battery several times on purpose and had no issues getting it to recharge.