I recognize that gas trimmers will be less expensive and won't have the battery life issues cordless electric trimmers have (and corded is impractical - too many obstacles in the yard.) BUT! I would prefer to avoid the noise, smell and maintenance hassle of yet another small engine. I have a half acre, but there are many rock outcroppings, trees, shrubs and flower beds - it's more than just four sides each for house and property line. On the other hand, it's pretty tame - no heavy grass or weeds. Are the newest high-end cordless trimmers up to this? I'd like to hear from anyone who has one of the new fast-charge models.
My lot is only 60x110 but I bought a Ryobi lithium cordless trimmer and it works great. I spend about 15 minutes trimming the entire yard and can get around 3 trims on one charge. Sounds like that might work for your 1/2 acre (charging after every trim). It's powerful enough to cut down numerous weeds, it's quiet and the battery works in other ryobi devices. The battery also has a charge indicator button which is useful. You could also just buy a second battery and rotate through them.
edit: you could also go for more volts
I have 2.5 acre hobby farm. I bought a 58 volt Jonsered (now Snapper) weed wacker and blower from Costco as a test knowing I could return it for $250 as the last one marked down from $300. With that much voltage it is as strong as a medium size Echo or Stihl. Found a chain saw with battery from Sear for $150. These tools do the job almost as strongly as gas. No pulling, mixing, running dry to protect the carburetor. Run and stop as much as you like. All batteries drained go do something else for 30 minutes or so. And they are way quieter or just buy an extra battery for $100. I do not want to run a weed wacker for hours. Weed wack and then mow is my way. One area is totally done. Then on to the next. I am never going back to smelly noisy gas. If you need to run these tools many hour a day, most days, then gas is the way, but ease of start and never replacing a gummed up carburetor is a pleasure. Just use gas for my mowers now and no mixing for them.