Charging of lead acid batteries is not an oft-spoken or documented topic. So here I would like to describe how to charge lead acids properly using the 3-stage method (Bulk, Absorption and Float) or the CC-CV method as shown below.

Particularly, the “Charging current during Bulk phase” and the “Time for Absorption phase” that needs to be provided would be the focus of this blog.

Bulk Phase

The first phase of the charging process is the bulk charge. This is when the maximum amount of current flows into the battery bank until a desired voltage is reached. The recommended charge current is 10%-20% of the AH capacity of the battery bank, based on the 20 Hr AH rate (C20). This is also specified by the manufacturers in their datasheet. Snapshot of Exide and Amara Raja datasheets are provided below:


The above table is from Exide LMS Solar Battery datasheet. If you check the rightmost column (Current Settings) in the above table, it says Maximum – 20% of battery Ah capacity. This means that if you have a 100Ah battery, then the Max current you can provide for charging is 20A.

Similarly, if you check the above table from Amara Raja, it says that during Boost charge, the Maximum Charging Current is 0.2C. So here too, for a 100Ah battery, the Max current for charging is 20A.

Higher current levels may cause the battery bank to overheat. A lower current may be used; however, this will prolong charge time and increase the potential for sulfation build-up.

How to set the Max charging current in Charge controller or Inverter?

If you check the user manual of Studer Charge Controller, it is mentioned in section 7.2.1 Bulk phase, “This current can be limited with parameter {10002} “maximum output current”.

So, there is a setting available in Studer charge controller to set the Max current as shown below:

Similarly, in Flin Energy Inverter too, this parameter is available as shown below:

Let’s also talk about the Bulk phase voltages to be applied. You can generally use voltages that are a little bit higher than the manufacturer’s values for flooded batteries. This helps to keep them de-sulphated a little better and make it more likely for them to see a full 100% charge on a regular basis. The trade-off is slightly higher water use:

  • Bulk/Absorb = 59.5/29.8/14.9 Volt for 48V/24V/12V batteries
  • Float = 52.8/26.4/13.2 Volt
  • Equalize = 63.2/31.6/15.8 Volt for 2.5 hour

Absorption Phase

Here, we will answer the question, “How long should I keep the batteries in absorb phase?

The most important part of the charge cycle is the Absorption phase. The Bulk phase typically recharges the battery bank to approximately 80% state of charge, the Absorption phase then completes the charging cycle. Most chargers include an Absorption time setting which allows the user to program the duration of time needed to reach a full state of charge. To set the Absorption time, a simple calculation is required. Using the 20 Hr AH rating of the battery bank (C20) and the charger output, it is possible to determine the time needed to fully charge the battery bank using Roll’s equation.

As stated previously, the Bulk phase brings the bank to approximately 80% state of charge. The remaining 20% charge is a function of time and current. The charger will maintain current level until the Bulk set point has been reached. The charger will then switch to the programmed Absorption voltage and timer. As the battery charge nears completion, the internal resistance in the battery increases and charge current begins to decrease. It is assumed that over the time of the Absorption phase that 50% of your maximum charge current will be available (this is factored into the equation).

0.42 = (20%/50%) + 5%. 5% is added due to losses.


Where: T = 0.42 x C/I


C = 20 hr RATED CAPACITY (total battery bank)

I = Charging Current (Amps) (recommended 10% to 20% of C20 discharge rate)

0.42 = (20%/50%) + 5% (5% is added due to losses)


2 strings of 6 Volt 820Ah batteries

20 hr AH rate = 820 AH x (2 strings) = 1640 AH

I = 10% of 1640 AH = 164 Amps. If charger output is 120 Amps max, 120 is used

T = 0.42 x 1640/164 = 4.2 hrs OR T = 0.42 x 1640/120 = 5.75 hrs

How to set the Max absorption time in Charge controller or Inverter?

In Studer Charge Controller, it can be set using the Parameter 10011 as shown below:

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