How Long a 4.0 Ah Battery Lasts

A 4.0 Ah (Ampere-hour) battery can theoretically supply 4 amps for 1 hour or 1 amp for 4 hours before it’s fully discharged . However, real-world runtime depends on the device’s power draw, voltage, and operating conditions.

Calculating Runtime

The basic formula is:
Runtime (hours) = Battery Capacity (Ah) ÷ Device Current Draw (A) 

For example:

• If a tool draws 2 A at light load: 4 Ah ÷ 2 A = 2 hours.

• If it draws 8 A under heavy load: 4 Ah ÷ 8 A = 0.5 hours (30 minutes)