You are not thinking it out correctly.
The longer the run time, the less beer to drink while charging before resuming the chore.
I don’t have an answer to your question—just an observation. I have a 40v chainsaw. I see two great advantages: There is no issue with starting it, and more importantly, the saw runs out of energy about the same time I do.
Good luck with your quest for the perfect string trimmer.
Two 3 Ah batteries will give you 20% more charged run time than a 5 Ah battery, but would require one change out of a spent battery.
2 * 3 Ah = 6 Ah
6 Ah > 5 Ah
Plus with the two, one can be on the charger while the other is in use. 20V 3 Ah could charge very quickly depending on the charger and battery or really slow too. Remember do not charge a battery while it is hot from use unless your charger senses battery temp and manages all that.
If they weren’t so expensive, I’d buy a $400 self-propelled battery powered lawn mower and fire my gardeners. We’ve come a long long way I’m here to tell you!
https://www.amazon.com/BLACK-DECKER-BDCAC202B-Lithium-Charger/dp/B010BFHORK
This will charge a 3 Ah in 1.5 hrs
https://www.amazon.com/dp/B074V7KGVT/ref=emc_b_5_t
This will charge same in about 9 hrs
for 5 Ah in about 3/17 hrs
I’ve never found a string trimmer in whose string doesn’t break constantly when I use ut. Is there such a thing as a chain trimmer or a metal guitar string trimmer?
40V Home Depot special (Ryobi) used here. One charges gives about 20 minutes of continuous cutting on grass. Tearing through thick weeds, blackberries and wild rose cuts it down to about 15 minutes.
I’ve bot the same battery in a pole saw and slap it in when the first one dies. After that it’s time for a beer.
Charge time is about and hour and a half from dead zero.