How to Calculate the Charging Time and Current for 12v Battery?

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Example based on a 120 Ah battery (This information is available on the label of the battery on the top side)

First of all, we will calculate the charging current for 120 Ah battery. As we know that charging current should be 10% of the Ah rating of the 12v battery. This is because a higher rate may cause the battery acid to boil.


So charging current for 120Ah Battery = 120 x (10/100) = 12 Amperes but due to losses, we can take 12-14Amperes for charging purpose. Suppose we took 13 Amp for charging purpose, then charging time for 120Ah battery = 120 / 13 = 9.23 Hrs. 


but this was an ideal case…

Practically, it is noted that  40% of losses (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
168 / 13 = 12.92 or 13 Hrs ( in real case)

Therefore, a 120Ah battery would take 13 Hrs for completely charging ( with 13A charging current).