14 | SAMLEX AMERICA INC.
SECTION 4 | Protections
4.1 OVER LOAD / SHORT CIRCUIT CURRENT PROTECTIONS
4.1.1 Battery Backup Function is not Used - External Battery is not Connected
and the Unit is Used as a Power Supply
In this case, the entire load current will be supplied by the Power Supply Section and will
be limited to a maximum of 25A by its Current Limit Circuitry. If the load tries to draw
a higher current than the current limit value of 25A, the output voltage at the Load
Terminals (3, 4 in Fig 2.1) and the Battery Terminals (5, 6 in Fig 2.1) will not be regulated
and will drop below 13.8V± 0.2V. If the load impedance is further reduced, the current will
remain limited at 25A but the voltage will drop further. In case of short circuit, maximum
limited current of 25A will continue to be supplied into the short circuit but the voltage
will drop to < 2V in case of a near dead short (Load impedance will be very low – say
< 100 milli Ohm). If over-load / short-circuit current of 25A continues over prolonged
period (> 100 sec), the external 25A load side Fuse (F2, in Fig 2.1) will blow and will
disconnect the load. If the overload / short circuit is removed before the external 25A
load side Fuse (F2, in Fig 2.1) blows, the output voltage at the Load / Battery Terminals
will automatically recover when the load current drops to less than 25A.
4.1.2 Battery Backup Function is Used - External Battery is Connected
If the load tries to draw current higher than the current limit value of 25A of the Power
Supply Section, the output voltage of the Power Supply Section will not be regulated and the
voltage at the Load Terminals (3, 4, Fig 2.1) will drop. Portion of overload current beyond 25A
will now be fed from the battery and the battery will start draining at this differential current.
For example, if the overload current was 40A, the Power Supply Section will provide 25A and
the battery will provide the balance 15A. The battery will start draining at 15A. The voltage
at the Battery Terminals (5, 6, Fig 2.1) will start dropping and will be equal to the voltage
corresponding to its actual State of Charge. The voltage at the Load Terminals (3, 4 in Fig 2.1)
will be up to 0.4 VDC below the voltage at the Battery Terminals (5, 6 in Fig 2.1) because of
forward voltage drop across diode D2 (Fig 3.2). This drop will depend on the current being
supplied through this diode (TABLE 3.3). External 25A Fuse (F2, Fig 2.1) on the load side will
blow only on sustained current ≥ 25A for > 100 sec but will not blow at higher short duration
surge currents determined by its Time Current characteristics. For example, based on the Time
Current Characteristics of 32V, 25A fuse Type ATC-25 from Cooper Bussmann, the fuse can pass
extremely high currents for shorter durations is as follows:
• 550A for 10 ms
• 170A for 100 ms
• 40A for 1 sec
• 25A continuous (for > 100 sec)
In case of short circuit on the load side, the external 25A Fuse (F2) on the load side will
blow because of very high additional current supplied by the battery (Additional battery
current supplied into the short circuit on the load side = Short circuit current - 25A from
the Power Supply Section). For example, if a short circuit current of 170A tries to ow
for > 100 ms, 25A will be supplied by the Power Supply Section and the balance 145A
will be supplied by the battery. As the external 25A Fuse (F2, Fig 2.1) on the load side
will see 170A and the external 25A Fuse (F1, Fig 2.1) on the battery side will see 145A,
the external 25A load side Fuse (F2, Fig 2.1) will blow rst.