Cordless electric string trimmers are another low-cost choice for smaller trimming jobs.
Battery-powered and cordless, you aren't restricted by an extension cord.
Electric string trimmers are great for cutting errant grass and weeds, but they lack the power to mow down hardy weeds and brush.
Batteries
Rechargeable batteries for lawn tools typically run for 45-60 minutes. If you'll be doing large projects, consider a backup battery so you won't have to wait for the battery to recharge.
Ni-Cad Batteries
Ni-Cad battery-powered string trimmers are generally the least expensive cordless models. Ni-Cad batteries are relatively heavy, and you will see a noticeable drop-off in power and performance as the battery runs down to the end of its charge.
Lithium-Ion Batteries

Lithium ion batteries are the most recent innovation to power string trimmers. Lithium-ion batteries are available in several different voltages -
18-volts,
20-volts or
24-volts.
Lithium-Ion batteries are extremely popular for 3 main reasons:
1. They are light weight – up to 50% lighter than a comparable Ni-Cad battery – so the string trimmer won’t weigh as much when in use
2. They do not experience any drop-off in power or performance as the battery runs down. Power output stays consistent through the entire cycle of use
3. They can be recharged up to 3 times as many times as a Ni-Cad battery before the battery needs to be replaced
Trimmer Shaft
Curved Shaft string trimmers are good for grass and weed trimming in open areas. Because of the curved shaft design, you will not have as much maneuverability trimming in tight spaces, such as under a fence.
They are considered easier to use than straight-shaft trimmers.
Straight Shaft string trimmers are more functional in tight spaces. You can trim in hard-to-reach corners or areas farther away from yourself.
Many models feature a telescoping shaft, meaning you can make the shaft as short or long as you need to comfortably trim.
2-In-1 Capability

Some cordless string trimmer models are a 2-In-1 style, meaning you can use them for multiple-purposes.
Use it as a string trimmer, or turn the pivoting cutting head to use as a grass edger.
The video above describes how to use a 2-In-1 string trimmer.