The 6 volt 12 amp hour battery will give you around 14 hours of runtime, depending upon the size coil and whether you are using an amplifier and speaker.
These batteries are suitable for the Minelab SD2000, SD2100, SD2200, GP series including the GP Extreme, the GP3000 and the GP3500.
2. 6v SLA battery charger 800ma for dome top batteries