In my opinion, and it is qualified because I have used all the sprayers, Birchmire makes a really good battery backpack but for 1/2 the money this is the one you will be really happy with.  I love the choice of tips that come with the sprayer.
Product Information
Smith Multi-Use 18V Lithium-Ion Powered Backpack Sprayer (190576)
The Multi-Use 18V Lithium-Ion Powered Backpack Sprayer will allow up to 4 hours of spray duration or 50 gallons of liquid. It provides a consistent chemical spray output that ensures optimal application rates saving time and reducing chemical waste. Included with this sprayer are six nozzles for a wide range of applications: Brass Adjustable, Low, High and Max volume fans, jet stream and foaming (spot treatment) nozzles. It has viton seals, heavy-duty kink resistant hose, a stainless steel wand, and powder coated steel frame for chemical resistance and durability.
Specs
Tank Capacity: 4 Gallons
Extension: 21" Stainless Steel
Charger: 3.5 hour charger included.
Hose Length
50" Nylon Reinforced
Nozzles: Six nozzles included for a wide range of liquid application: Brass Adjustable, Low, High and Max volume fans, jet stream and foaming (spot treatment) nozzles.