You are trying to charge a 6V battery (this unit is 12V only).
The battery is too damaged to hold a charge. Sparking Prevention motomasterbatterycharger111567manual full
Use this for automotive, marine, and deep-cycle batteries that need to be charged quickly (usually 3–5 hours for a standard car battery). You are trying to charge a 6V battery
Compatible with Lead-Acid (Standard, AGM, Deep Cycle) Operating Instructions: Step-by-Step 1. Preparation motomasterbatterycharger111567manual full