Hi columbus,
personnal experience with a battery strimmer was pretty useless. They usually cut ok for a few minutes and then start to fade very quickly. The revs needed to keep the cutting head turning and clear just cannot be generated.
The best bet really is to buy a small petrol strimmer (i never thought i would say this but for light grass and nettles the smallest petrol McColoch is great).
I personnally picked up a lower end Stihl machine brand new of ebay brand new for just £65, cheap as chips.
TT