As a general rule of thumb, the charging current should be ≈ 10% of the battery's Ah rating. It's typically measured in amperes (A). What Is Charging Time? Charging time refers to the duration it takes to fully replenish a battery from a. In this simple tutorial, we will explain how to determine the appropriate battery charging current and how to calculate the required charging time in hours. To make it easy to understand, even for non-technical users or beginners, we'll use a basic example of a 12V, 120Ah lead-acid battery. Below. Battery charging calculations ensure safe, efficient, and reliable energy storage performance across industrial, renewable, and transportation applications. Charging Time (h): The duration required to charge a battery fully.