GMDSS batteries provide power to the GMDSS equipment in the event of a failure of the main and emergency power sources.
According to SOLAS, GMDSS batteries must provide power for GMDSS operation:

1. 1 hour if the GMDSS receives power from emergency generators.
2. 6 hours in case the GMDSS does not have power from emergency generators. 
3. Batteries must be charged to the minimum required in less than 10 hours. 
4. Battery capacity should be checked at intervals of less than 12 months. 

Keeping GMDSS batteries in good condition is important to ensure that they are prepared for an emergency. Generally, GMDSS batteries carry out three types of tests / maintenance.

1. Daily on / Off
2. Annual performance test. 
3. General battery maintenance. 

1. Daily under load / no load. On / off load, a test is performed to ensure that:

1. GMDSS equipment can be battery powered. This ensures that all connections from the battery to the GMDSS equipment are not damaged.
2. The battery is capable of providing power during the operation of GMDSS equipment. That is, when the load transfers to the battery, it does not discharge too quickly. 

To check the load, you must perform the following procedure:
1. First, disconnect the AC power from the GMDSS station. The power button is usually located under the GMDSS panel or on some ships in the radio room (if any). If there is another AC source to charge the battery, it must also be disconnected. This is due to the fact that if the battery is on continuous charging, the voltage drop that should be detected using the On Load / Off Load test will not be shown.


 Most of the time, the GMDSS power source is AC power from batteries to the GMDSS equipment. Disconnecting the AC automatically ensures that the batteries will not charge continuously during the test. Therefore, after the GMDSS equipment switches to battery operation, record the battery voltage.
2. Press the PTT button to transmit at R / T frequency without distress and in standby mode. Record the voltage under load while the PTT is pressed.
3. The voltage drop should not be more than 1.5 volts. 2. Performance Test All batteries have a lifespan. Battery capacity may decrease with age. And with a performance test, we measure this capacity



We charge the battery until it is fully charged, and then measure it by discharge. To discharge the battery, we use a known load to measure its capacity. Battery capacity is measured in ampere-hours (Ah). Thus, 200 Ah means that the battery can deliver a current of 200 Amps for one hour or 20 Amps for 10 hours and so on.
There are two statements:
1. Voltage is not a measure of battery capacity.
2. If the battery is 100% charged, this does not mean that it will or can provide rated capacity. Avoid full discharge



There is another SOLAS requirement that the battery be completely discharged when conducting a capacity check. Full discharge means the lowest voltage that the battery can be connected to. If the battery discharges below this voltage, it may lose its capacity to a level at which it cannot be used again. For nickel batteries, this voltage is 1.0 V per cell. Thus, for a 24 V battery (1.2 V x 20 cells), the deep discharge voltage will be 20 V. When performing capacity testing, you should never allow the battery voltage to drop below 20 V or 1 V / cell.
There is a formula:
Power = Voltage x Current
The GMDSS battery is usually in the range of 200 Ah, which is necessary to supply a voltage of 24 V.


If you need to check if there is another 200Ah left, you need to remove the batteries from the charge and the existing load (connections to the GMDSS station) and connect some known load to it. Usually, an installation consisting of several series-connected 100 W bulbs is connected to the battery terminals. For example, if 6 bulbs (600 W) are connected to the battery, it will receive 25A of current from the battery. This is because:
600 W / 24 V = 25 A

Once the load is connected to the battery pack, you need to measure the voltage and current at each terminal of the battery pack. You must do this at least every hour.
 Stop the test only if:

1. One battery cell is out of order. That is, the voltage drop in one battery cell is different from the others. In this case, isolate this faulty cell and then continue testing.
2. The voltage has reached a deep discharge level. SOLAS requires that a deep discharge of the battery is not allowed during the capacity test. During the test, you need to measure the voltage of each cell. The voltage should not be lower than 1 V in any cell or 20 V for the battery. 
3. The test was conducted for sufficient time to show that the battery has 100% of its rated capacity. Let's say 8 hours have passed since the start of the test. And for 8 hours, the current measure was 25A. Thus, the battery has already delivered 200Ah (25A x 8 hours). This shows that the battery capacity is still 100%. In this case, it will complete the test. 

If we stop the test due to the 3rd point, everything is in order with the battery.
If we stop the test because we have reached a deep discharge voltage, we need to measure how much A / hour the battery was charged at that moment. If it is less than 80% of its nominal capacity, a performance check is not passed. Charging the battery after checking the capacity If the measured capacity is more than 80% of the nominal capacity, you can go to the next step, where the time required to charge the battery is measured. There are two options that show the percentage of battery power. This is the voltage reading on the terminals or the specific gravity of the electrolyte.



A 24 V battery when fully charged will have a voltage of about 25.4 V and a specific gravity of 1.265. Determining the state of charge using voltage can be difficult, as the voltage can vary with temperature. The specific gravity of the electrolyte is considered a more accurate way to determine the state of charge of the battery. Therefore, after a capacity test, you need to measure the time required to charge the battery to 100%. SOLAS requires that this time be less than 10 hours.

3. General maintenance required for GMDSS batteries
On-board batteries do not require special care as such. There are two things to check: the electrolyte level and the specific gravity of the electrolyte. If you have maintenance-free batteries on board, you do not need to check anything except the conditions in which they are stored.

Electrolyte
Level The electrolyte level may decrease for various reasons, but it is important to maintain it in accordance with the manufacturer's instructions. If the level is low, the battery cell should only be filled with distilled water. Checking the specific gravity of the electrolyte


The specific gravity of the electrolyte is considered a more accurate measure of the state of charge of the battery. The specific gravity must be checked daily and re-encoded into the battery log, depending on the voltage. The specific gravity may decrease due to sulfation, which causes the charging plates to crystalize. This causes a decrease in the holding charge and, therefore, the battery capacity decreases. BCI (Battery Council Internation defines a specific gravity of 1.265 as a state of 100% charge. 1.225 is considered a state of charge of 75%. It is even better to refer to the manufacturer's instructions, as some manufacturers may rise to a specific gravity of 1.280 as a sign of a 100% state of charge. Conclusion


GMDSS batteries are essential equipment that provides emergency equipment with energy in a real emergency. Therefore, it is important to keep these batteries in excellent condition. Various tests / checks ensure that GMDSS batteries provide the required power. A daily test under load / no load ensures that all connections are not damaged and that when loaded, the batteries do not drop too quickly. The annual capacity test measures the battery capacity in amperes. The battery must be replaced if the capacity is less than 80% of the nominal capacity. It is necessary to check the state of battery charge daily by measuring the specific gravity and electrolyte level.