Calculation of Load current in GMDSS battery

How to calculate Drain Current and GMDSS battery life during GMDSS SRC

Under normal circumstances, it is but natural for the ship staff to get confused with the drain current of GMDSS equipment during survey. This often leads to complicated situation where  a proper explanation becomes crucial to convince the Class surveyor especially if the battery is more than two years old.

As per IMO, the GMDSS battery (reserve source) should be replaced once in 3 years irrespective of the condition of the battery after three years. The battery has to be subjected to a capacity load test (similar to your daily routine and weekly routine) once in a month. This is basically done by connecting 6 x 100W DC bulb in series connected to the GMDSS battery. Let us look into a  situation as follows:

1. the above load is connected and 0900 hrs (lt) - Specific gravity and the voltage is recorded at this time.

2. At an interval of 30 minutes, the specific gravity and the voltage has to be recorded and if the vessel has emergency generator it will be for 1 hour and if not, for 6 hours.  If at the end of the specified period, the voltage of the battery drops down to less than 10% of the total voltage, (24VDC) then the battery is either not charged or there is some mistake in the battery. In this condition the battery has to be changed - whether it is 6 months or 4 years and well maintained. 

3. The interesting part is that the drain current of the GMDSS equipment: For example - A MF/HF GMDSS console of 500 watts full power will consume around 30A of current during transmission. Since it is idle most of the times, the transmission period is more crucial to calculate the current drain. So this means, only when the PTT (or) Push to Talk switch is pressed and if the equipment is in full power and preferably in HF range, 

The following should give a rough idea on the calculation of the load current and operating hours of GMDSS battery.

From the table below, assuming the battery Ah is 180 and the total current is 29A, with a de rating factor of .85, it can be concluded that the battery will continue to supply the GMDSS equipment for 5.27 hours which is more than the required time of 1 hour in vessel fitted with emergency generator.

16.1	“PRIMARY EQUIPMENT”
16.1.1	VHF/DSC radio installation	3.0
16.1.2	MF radio installation	13.0
16.1.3	MF/HF radio installation
16.1.4	INMARSAT ship earth station	4.0
16.1.5	Gyro compass (if continuous heading information to the INMARSAT ship earth station is required (Reg. IV/13.8))
16.1.6	Electrical lighting for illumination of radio controls	1.0
16.2	“DUPLICATED EQUIPMENT”
16.2.1	VHF/DSC radio installation	3.0
16.2.2	MF/HF radio installation
16.2.3	INMARSAT ship earth station
16.2.4	Gyro compass (if continuous heading information to the INMARSAT ship earth station is required (Reg. IV/13.8))
16.3	“OTHER EQUIPMENT PERMANENTLY CONNECTED”
16.3.1	GPS,AIS	5.0
16.32
	Total current :	29         Amperes	Amperes
16.4	ENDURANCE OF RESERVE SOURCE OF ENERGY
	To allow for the reduced capacity of a battery when discharged over a shorter period than its rated specification (usually 10 or 20 hours) a de-rating factor of 0.5 for a 1 hour discharge and 0.85 for a 6 hour discharge should be applied. The result, in hours, should exceed the IMO minimum requirement.
16.4.1	Reserve source of energy (Item 3)
	All capacity of battery  X  de-rating                   180       X.85     ---------------------------------------------   =     -------------------------    =                      5.27           hours                            total current                                         29