User Manual

– 19 –
Two Stage Constant Voltage Charging
Two stage constant voltage charging is a recommended
method for charging valve regulated lead acid batteries
in a short period of time and then maintaining them in a
fully charged float or standby condition. Figure 20
illustrates the characteristics of a two stage constant
voltage charger.
The characteristics shown in Fig.20 are those of a constant
voltage, current limited charger. In the initial charging
stage, the current flowing into the battery is limited to a
value of 0.25C Amps. The charging voltage across the
battery terminals rises, during the charging process, to a
value equal to the constant voltage output of the charger,
which is set to 2.45 volts per cell. Whilst continuing to
charge, in stage 1 (A-B), at 2.45 volts per cell, the current
will eventually decrease to point “Y”, where the value of
this decreasing current is “sensed” causing the circuit to
switch into the second stage (B-C), reducing the charging
voltage from 2.45 volts per cell to a constant voltage,
float/standby, level of 2.3 volts per cell. The switch to
stage two, where the constant voltage level of 2.3 volts
per cell is applied, occurs after the battery has recovered
about 80% of its rated capacity. This is one of the most
efficient charging methods available as the recharge time
is minimised during the initial stage whilst the battery is
protected from overcharge by the system switching to
stage 2 (float/standby) charge at the switching point “Y”.
When this charging method is used, the output values
will be as follows:
Initial Charge Current . . . . . 0.25C Amps (max).
Charge Voltage:-
1st Stage . . . . . . . . .2.45v/cell (2.40 to 2.50 v/cell, max.)
2nd Stage . . . . . . .2.27vpc ± 0.005
Switching Current From
1st Stage to 2nd Stage . . . . . . . . . . 0.05C Amps
(0.04C to 0.08C Amps)
Note: This charging method cannot be used in applica-
tions where the load and the battery are connected
in parallel.