I bought the 40V Ryobi 12" string trimmer from Home Depot recently. It will haul you along a fence line if you're not careful. All the power of gas without mucking around with gas/small engines. The string dispenser has worked well so far. You can find these at Home Depot. Not sure about battery life, but cut my small yard twice and trimmed and it was somewhere around 75% power remaining. YMMV
Side note: Bought the $150 Ryobi 40V blower to go with it, rated at I think 110mph. Doesn't work for chit. Getting returned and buying the Black and Decker corded blower at Menards for $37. Borrowing my mom's B&D one right now and it really blows- 180mph. Lots of power.
