I use the 40 volt Ryobi stuff too and it runs for a long time on one charge... at least as long as I want to use it at any one time. Huck didn't mention how much of an area he has to work or how tough the weeds but any of the higher voltage modern electric trimmers will handle most home/yard situations with.. as they say... gas like power without any of the fuss.
I have a fairly small yard,for which I have an 18 volt Black and Decker trimmer,but my yard backs on to a 27 acre Church property and once or twice a year we have to drive over and clear any weeds and thick grass that runs along their chain link fence,which is also my rear fence and sometimes if I let it get too neglected,the old B@D can go through two fully charged batteries......the batteries run at full power,but run down fast when chopping large weed and thick bladed grasses.
I see... well if the B&D does the job at all then I can say with full confidence that a 40 volt Ryobi with at least a 2.5 amp hour battery will go right through it. I think that combination will cost more, especially higher voltage electric using higher capacity batteries, but to me the trade off is well worth it in terms of lighter weight and practically no maintenance... no oil, gas, mess. I keep the batteries inside during the cold of winter. Ryobi is Home Depots house brand and might be the best bargain but not necessarily the best... Echo makes good ones for example. It took me a while to go Ryobi but having had good experience with a few of their products and realizing nearly all of their equipment is backwards compatible with battery packs I decided the company was thoughtfully building a customer base and went with them.
That's a higher end unit then the Ryobi and I think has a thicker string but not sure, Ryobi does not have variable speed which might provide extra battery time. That EGO will do the job, you'll like it... I was amazed at the power, batteries have come a long way.
That is the question. I've been good with all of the Li ion batteries I've had going on several years now, haven't had a failure, same tech as what's in phones just higher capacity, or hybrid cars. Looking at high prices for replacements, I am thinking the more prolific this battery technology becomes the cheaper they will be... seems to be the pattern as competing companies start producing and packaging them as general replacements. At any rate, given the ease of use and lack of mess and fuss with fueled options, I kind of don't care in the sense I'll take my chances, the expense is worth the trade off. Now if, as above, 400 hundred yards of trimming serious weeds was involved, I want a powerful noisy gas weed whacker... a hand held version of a wheeled saw blade brush hog... no doubt.
