The 3.4 amps is at 120vac this is 408watts.
The current is probably under rated load and not stall.
Using the rated current would be worst case.
The motorcycle battery is 20 amp hour * 12v?
or 240 watt hours This would give
a grand total of 35 glorious mowing minutes.
HMMMMMM! This might be a problem!
We have not even considered drive wheel & CPU power consumption.
A deep cycle marine battery is rated 95 amp hours at 12v.
or 1140 watt hours.
That would give 2.8 hours of mow time. Still not close to our
target of 8hrs per day. :-(
The Shrubber is 108 watts (how wide are these?)
That would give us 10.5 hours! :-)
Somthing like this might just work.
Ron