How to Calculate the Battery Charging Time & Battery Charging Current

Easy Battery Charging Time and Battery Charging Current Formula for Batteries. (With Example of 120Ah Battery).


Below is a simple battery charging current and battery charging time formulas with a solved example of 120Ah lead-acid battery.

Here is the formula of charging time of a lead acid battery.

Charging time of battery = Battery Ah / Charging Current

T = Ah / A


  • T = Time hrs.
  • Ah = Ampere Hour rating of battery
  • A = Current in Amperes


Suppose for 120 Ah battery:

First of all, we will calculate charging current for 120 Ah battery. As we know that charging current should be 10% of the Ah rating of battery.


Charging current for 120Ah Battery = 120 Ah x (10/100) = 12 Amperes.

But due to some losses, we may take 12-14 Amperes for batteries charging pur pose instead of 12 Amp.

Suppose we took 13 Amp for charging purpose,


Charging time for 120Ah battery = 120 / 13 = 9.23 Hrs.

But this was an ideal case…

Practically, it has been noted that 40% of losses occurs in case of battery charging.

Then 120 x (40 / 100) = 48 …..(120Ah x 40% of losses)

Therefore,  120 + 48 = 168 Ah ( 120 Ah + Losses)

Now Charging Time of battery = Ah / Charging Current

Putting the values;

168 / 13 = 12.92 or 13 Hrs ( in real case)

Therefore, an 120Ah battery would take 13 Hrs to fully charge in case of the required 13A charging current.

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