Well, I am a big fat liar. Tried out both the 18v and 40v at the store, and decided the 18v was handier and sufficient for my needs. Did a quick trim and edged my yard and decided I made the right decision.
I can't answer your question about charging while traveling, but I won't even consider the 18V trimmer.
Especially since I have a small yard and the 40V trimmer uses my lawnmower's battery.
I see that Ryobi has added a new line of HP brushless tools.
https://promo.ryobitools.com/hp
The legacy batteries will still work in the HP machines, but with less output.
I think I am gradually going to transition to the HP batteries, starting with the angle grinder.
It sounds great, but I'm moving to a .14 acre lot, and really don't need the range.
I'm considering going with the 40W weed trimmer mainly so it will use the partially depleted battery from my lawn mower when I mow. Ryobi has a package where you get the 40W weed trimmer with a leaf blower...
A couple years back, a friend turned me on to Ryobi One cordless tools.
Since that time, I've added tool after tool, until I've got Ryobi One cordless everything.
A lawnmower, belt sander, radial saw, 3/8" wrench, drill, fan and shop light.
And I'm looking to add a weed trimmer, leaf blower...