How to Calculate Battery Charging Time and Charging Current? – Example

Electrical Technology

Simple Battery Charging Time and Current Formula for Batteries (with 120Ah Battery Example)

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.

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Below are the formulas for calculating the required battery charging time (in hours) and the necessary charging current (in amperes):

Charging Time of Battery = Battery Ah ÷ Charging Current

t = Ah ÷ A

and

Required Charging Current for battery = Battery Ah × 10%

A = Ah × 10%

Where:

t = Time in hrs.
Ah = Ampere-Hour (Ah) Rating of a Battery:
A = Current in Amperes

Example:

What is the suitable charging current in amps and the required charging time in hours for a 12V, 120Ah battery?

Solution:

Battery Charging Current:

First, we will calculate the charging current for a 120Ah battery. As a general rule of thumb, the charging current should be ≈ 10% of the battery’s Ah rating.

Therefore,

Charging Current for 120Ah Battery = 120 Ah × (10 ÷ 100) = 12 Amperes.

However, considering losses such as heat and internal resistance, it’s common practice to use a slightly higher charging current (typically around 12 to 14 amps) instead of the exact 10% (i.e., 13 or 14 amps) of the battery’s Ah rating.

Battery Charging Time:

Suppose we choose 13 amps as the charging current.

Then,

Charging Time for a 120Ah Battery (Ideal Case) = 120 ÷ 13 = 9.23 hours

However, this is an ideal case without considering any losses.

In practice, it’s observed that approximately 40% energy is lost during the charging process due to heat, resistance, and other factors.

To account for these losses:

Losses = 120Ah × (40 ÷ 100) = 48Ah

So, the total effective capacity to be charged becomes:

120 Ah + Losses

i.e.:

120Ah + 48Ah = 168Ah

Now, to calculate the practical charging time:

Charging Time = Total Ampere-Hours ÷ Charging Current

Charging Time = 168 ÷ 13 = 12.92 hours (approximately)

Therefore, a 120Ah battery would take approximately 13 hours to fully charge when using a 13-Amp charging current, considering practical losses.