How to calculate battery charging current and time?

Individuals who use batteries on large scale do care about battery charging current and time because batteries are delicate and need care. In this article, we’ll check out the way to calculate the battery charging current and battery charging time. For the sake of an instance, we are taking a 12 Volts 150 Ah which we use at the inverters.

Charging an inverter battery like 150 Ah requires some hefty time, any newbie may make questions like:

How much current (Amps) is required to charge a 150 Ah battery?
How much time it will take to charge a 150 Ah battery?

Note: Here we are going to read some technical terms related to electronics so clear the basics first.

Current is denoted by the alphabet “I”.
Amperage, Ampere, or Amps all are the same and denoted by the alphabet “A”.
AH is Amps Hours, which means we are holding a current capacity in hours.

Understanding C rating.

A battery’s C Rating is defined by the rate of time in which it takes to charge or discharge. The C rating is denoted by a number like C5, C10, C20, and so on… where C is Capacity and the number is time in hours.

For example, a 150AH C10 battery will charge and discharge optimally with a 15A current, we can calculate this simply by dividing the battery’s capacity which is 150AH by its C rating which is C10 means 10 hours.

Battery AH	150AH	150AH	150AH
Capacity (in Hours)	C5	C10	C20
Battery AH ÷ Capacity (in Hours)	150AH ÷ 5H	150AH ÷ 10H	150AH ÷ 20H
Charge & Discharge Current	30A	15A	7.5A

Calculating battery charging current.

Here we should look for the C rating of the battery, the C rating defines at what capacity (in amps) the battery can be charged and discharged of its total capacity which is rated in AH (ampere-hour).

I have a 150 Ah battery that has a C10 rating on it, so it should be: 150AH ÷ 10H = 15A.

AH is the rating used to tell consumers how much amperage a battery can provide for exactly one hour. This means you can drain a full 150A in 1 hour, 50A in 3 hours, and 15A in 10 Hours.

You can charge a battery using more current to decrease the charging time, but not all batteries are designed that way to handle more current.

Charging a battery with more than needed current may damage it or shorten its life.

So here formula is very simple, just divide the battery’s AH by C# ratings which are in hours.

I = AH ÷ C# (in Hours)

Put it in an example of a 150AH C10 battery.

I = AH ÷ C# (in Hours)
  = 150 AH ÷ 10H
  = 15 A (theoretical answer)

Practically you’ll need a little more like +1 A or +2 A to charge a battery.

Calculating battery charging time.

Calculating battery charging time is easy too, all you need is Ah rating and current needs which you can calculate from the above sum. Here we’ll see how much it will take a 150 Ah battery to get charged fully.

So here formula will be a battery’s Ah rating divided by its charging current.

Time = AH ÷ I

Time = AH ÷ I
     = 150 AH ÷ 16 A
     = 9.375 Hours (theoretical answer)

Practically there are some charging losses due to which it could be more (13.125 hours) but close to the above answer.

For a lead-acid battery, charging losses are 40%, so either adds an additional 40% value to the Ah rating or to the final hours’ value. I have shown both the ways, just check them out carefully.