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