Get it out! First signs of leakage are enough for me to grab my plyers...
The easiest way to take out the battery is to cut the pin with small plyers (or a nailcutter) on the side of the battery where there's just one pin, and then start wiggling until the two pins on the other side break. If you want to put in a new battery is easier to desolder the old one, so you will have open pinholes on the mainboard..
