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Calculating charging time

Calculating the charge time of a battery should take into

account the following:

The first consideration is the efficiency of the battery. In

a standard wet battery, this is around 80%. This means

that if 100 Ah are discharged from the battery, 120 Ah

need to be charged in order to be able to extract 100

Ah again. With gel and AGM batteries, the efficiency is

higher – 85 to 90 % – so there is less loss and the charge

time is shorter in comparison with wet batteries. In

Lithium Ion batteries, the efficiency is as high as 97 %.

Another thing that needs to be kept in mind when

calculating charge time is that the last 20 % of the

charging process (from 80 to 100 %) takes around four

hours with wet, gel and AGM batteries (this does not

apply to Lithium Ion batteries). In the second phase, also

called the absorption or after-charge phase, the type of

battery determines how much current is being absorbed,

independently of the capacity of the battery charger.

The after-charge phase phenomenon again does not

apply to Lithium Ion batteries, which are charged much




= charging time


= capacity drawn from the battery


= efficiency; 1.1 for a Gel battery, 1.15 for a AGM battery

and 1.2 for a flooded battery


= battery charger current


= consumption of the connected equipment during

the charging process

The formula below is used to calculate the

charging time of a Gel or AGM battery:

The formula below is used to calculate the

charging time of a Lithium Ion battery:

Lt =

+ 4h

Co x eff

Al - Ab

Lt =

+ 4h

Co x eff

Al - Ab