Battery charger is battery charging equipment which used float and boost charging techniques for battery charging. In every electrical substation or industries, battery charger panel vailable for battery set charging as per AH rating of batteries.
Difference between Float charging and Boost Charging of battery
The battery charging equipment comprises of float charger and boost charger. The float charger normally supplies the station load and trickle charging current to the Battery, while Boost charger is in off condition. During an emergency when AC power fails, the battery supplies the station load and it discharges to a certain extent, depending upon duration of emergency.
When AC power is restored, the battery needs to be boost charged. For boost charging the Battery, or giving an equalizing charge to the Battery, Boost charger is used. A battery tap diode connected to the 42nd cell of the Battery, maintains DC continuity at load terminal continuously. During boost charging the battery, float charger is also “ON” to take the care of station load.
Float battery charger
The charger is fed from single phase AC supply and gives a DC stabilized output at rated full load current. The variation in DC output voltage is limited to + / – 1 % for 0 to 100 % load variation and simultaneous AC voltage variation of +10% – 15 % from 240 Volts & frequency variation of +/- 8 % from 50 Hz.
The rectification is obtained by using a full wave bridge connected silicon rectifier stack comprising of two silicon controlled rectifiers (SCR) and two diodes with surge suppression RC network connected across each SCR and diode.
The circuit works on AC phase control principle. The SCR is a semi-conductor device with 3 terminals i.e. Anode, cathode and Gate. The load current is carried by Anode and Cathode, while the control current flows through Gate and Cathode.
The characteristic of SCR is that it blocks the forward voltage when Gate is not supplied with any current, while it goes into conduction when the gate current reaches a specified level. Therefore the instant at which the SCR goes into conduction, or triggers, can be controlled by changing the instant at which the Gate current or pulse is supplied.
Once the SCR is triggered it remains in conduction till Anode current is reduced to less than holding current or reverse voltage is applied to it. Thus the output voltage of rectifier bridge can be controlled by changing the instant of firing / triggering the SCR.
Shunt senses the load current. The signal proportional to this load current is fed to the controller. In event of load current exceeding the rated full load current, the output voltage starts dropping , thus limiting the out put current of float charger. This inherent protection is provided in float charger apart form the back up protection by HRC fused.
Four numbers indicating lamps shall be provided, one for AC supply ON indication and one for float charger ON indication. One ammeter with necessary shunt, for measuring float charger output current, shall be provided.
Boost battery charger
This charger is also fed from single phase AC supply and gives a DC stabilized output current. The variation in DC output current is limited to +/- 2 % for 0 to 100 % load variation and simultaneous AC voltage variation of +10 % – 15 % from 240 volts & frequency variation of +/- 3 % 50 HZ.
The rectification is obtained by using a full wave bridge connected silicon rectifier stack comprising of two silicon controlled rectifiers (SCR) and two diodes with surge suppression RC network connected across each SCR and diode.
The principle of operation is same as that of float charger. The only difference is that this charger can be operated either in constant current mode or in constant voltage mode.A series of 28 dropper diodes with four way selection switch is also provided to avoid excess voltage appearing across the load while boost charging the battery.
Shunt senses the load current. The signal proportional to this load current is fed to the controller. In event of load current exceeding the rated full load current, the out put voltage starts dropping, thus limiting the output current of boost charger. This inherent protection is provided in boost charger apart form the back up protection by HRC fuses.
Battery charger in electrical Substation
Under normal operating conditions the battery charger should give a D.C. output equal to the steady demand load for signal lamps, auxiliary relays etc. plus an output to trip coils and closing coils of the circuit breakers and relays as and when required as well as float charging current of the battery.
The charger shall be suitable for charging the battery and supplying the load simultaneously. The entire charger scheme shall be divided in two sections, “float charger section” and “floatcum-boost charger section”. The float-cum-booster charger shall be suitably operated either in float mode or in boost-cum-standby float charger mode.
Under normal operating condition, with the input AC supply present, the float charger section’ shall supply the DC load and also float the battery by trickle charging and the “float cum boost charger section” shall be kept off.
In the event of main AC supply failure, the battery shall supply DC power to the Sub-station DC Load. The battery thus discharged shall be charged after resumption of AC.
If there is any trouble in ” float charger section “,the float mode of float cum boost charger section “, through Float-Boost selector switch shall supply the D.C. load as well as trickle charge the battery and thus serve as a spare float charger. Suitable arrangement shall be provided at the charger output circuit for isolation of battery during maintenance.