Methods of charging of batteries

Methods of charging of batteries

•         The batteries are charged  generally by two methods, namely

 (a) Constant Voltage Method

 (b) Constant Current Method

Constant Voltage Method : In this method, the voltage of the source is kept constant and current passes through the battery is regulated.

•         This method is preferred owing to the fact that the charging current would automatically taper off due to the growth of emf of the cell.

•         This method needs lesser attention compared to Constant current method

Constant Current Method

In this method, charging current is maintained  constant till the battery is fully charged.

•      Figure shows the method of charging a battery.

•      The battery to be charged is connected to the DC source.

•      The charging current can be maintained constant by a resistor connected in series with battery as shown in figure.

•      The charging current is measured with the help of a ammeter connected in series with the battery.

•      Care is to be taken, such that the temperature of the battery should not to exceed 40°C

•      If the temperature exceeds 45^oC, the charging process should be stopped for a while.

•      This method of charging takes longer time than that of constant voltage method, but the life of battery is increased.

•      This method is safe and causes less damage do the plates.

Efficiency of a battery

The efficiency of a battery is expressed in two ways

          (i) Ampere-hour efficiency or Quantity efficiency    

           (ii) Watt-hour efficiency      or  Energy efficiency

Ampere-hour efficiency  (or) Quantity efficiency

Ø  It is the ratio of out put in ampere hour during discharge to the input in ampere hour during charge

Ø  It is denoted by η_AH

Ø  %η_AH = [(Ampere hour during discharge) / (Ampere hour during charge)] X 100

Ø  If I_d is the average current  in amperes during
        discharging,

Ø        T_d is the time of discharge in hours,

Ø        I_c be the average charging current in ampere

Ø        T_c be the time during charge in hours

Ø        Then, %η_AH =[(I_d  X T_d)  / (I_c  X T_c)] X 100

Ø  The ampere hour efficiency varies between 90% to 95%.

Ø  It means that if lead acid cell provides 90  AH on complete discharge, then 100 AH must be put back into the cell to restore it to original condition.

Watt - hour efficiency  (or) Energy efficiency

Ø  It is the ratio of out put energy in watt hours during discharge to the input energy in watt hour during charge

Ø  It is denoted by η_WH

Ø  %η_WH  = [(Output in watt hours during discharge) /  (Input in watt hour during charge)] X 100

Ø  The Energy or Watt hour efficiency can be determined from the ampere hour or quantity efficiency

Ø  If  V_d  &  V_c is the average terminal voltage of the battery during discharge time & charging time respectively  in volt,

Ø              T_d and T_c  is the time of discharge in hours during discharging and charging respectively.

Ø        I_{c  and}  I_d   be the average charging current and discharging current respectively  in ampere.

Then,           

      %η_WH  = [(V_d X I_d  X T_d)  / (V_c  X  I_c  X T_c)] X 100

                       = {[(V_d ) / (V_c) ] X [ ( I_d  X T_d) / ( I_c  X T_c)] } X 100

               = [ (V_d ) / (V_c) ] X % η_AH