GMDSS Batteries: Tests and checks mariners must know

GMDSS batteries provide power to GMDSS equipments in case ship’s main as well as emergency power fail. The requirement of GMDSS batteries is governed by Regulation 13, Chapter 4 of SOLAS.

As per SOLAS, GMDSS batteries should provide power to operate GMDSS for

1. 1 hour in case GMDSS has the power from emergency generators
2. 6 hours in case GMDSS does not have the power from emergency generators.
3. Batteries must be recharged to the required minimum in less than 10 hours.
4. The capacity of the batteries must be checked at interval of less than 12 months.

Maintaining the GMDSS batteries in excellent condition is important to have them ready in emergency. There are typically three type of tests/Maintenance done on GMDSS batteries.

1. Daily on Load/Off Load test
2. Yearly Capacity test
3. General Maintenance required on the battery.

1. Daily On Load / Off Load test

On load / off load test is done to ensure that

1. GMDSS equipments can have the power from battery. This ensures that all the connections from the battery to the GMDSS equipmemts are intact.
2. Battery is able to provide power during operation of the GMDSS equipments. That is when load is put on battery, it does not drain out too quickly.

For the on Load / Off load test, Following procedure should be followed

1. First Switch off the AC power to the GMDSS station. The power button is usually under the GMDSS panel or on some ships in the radio room (if you still have one). If you have different AC source for charging the battery, that too need to be switched off. This is because if the battery is on continuous charge, this will not show any drop in voltage which we intend to find out with On Load/Off Load test.

However most of the time, GMDSS power source flow is AC power to batteries to GMDSS equipments. Switching off the AC power automatically ensures batteries are not on continuous charging during test. So after the GMDSS equipments are on battery power, note down the voltage of the batteries.

2. Press the PTT button to transmit on a non-distress and idle R/T frequency. Note down the on-Load voltage while the PTT is pressed.

3. The drop in voltage should not be more than 1.5 volts.

2. Capacity Test

All batteries have a life span. The capacity of a battery can reduce with age. There has to be a way to measure the capacity. And with Capacity test we do exactly that.

We can understand the capacity test with the comparison to a water tank. We do not know how much water it can hold. The one way of measuring its capacity would be filling the water tank to full and then measuring it by draining. Measurement can be with flow meter or with smaller measuring buckets/container.

With capacity test, we measure the capacity of battery by same method. We charge the battery to full and then measure it by discharging. To discharge the battery, we apply a known load to measure its capacity. Capacity of the battery is measured in Ampere hour (Ah). So 200 Ah means the battery can give a current of 200 Ampere for one hour or 20 Amperes for 10 hours and so on.

Before we proceed further, let’s agree on these two staements

1. The voltage is not the measure of capacity of the battery.
2. A battery 100% full (Fully charged) does not mean that it will or can produce the rated capacity.

Looking at the first statement, if the voltage is not the measure of capacity, what is ? As we already discussed, The measure of capacity of a battery is “how much current it can produce for how many hours”. With capacity test, that is what we aim to measure.

It would be easier to understand the second statement with example of a laptop. A new computer with 100% battery might last for 8 hours . After few years same battery 100% charged would only last 4-5 hours. This is due to the ageing of the battery.  With age, battery looses its storage space.

Another analogy to understand the second statement would be the same water tank example. If the capacity of the water tank is 2000 Litres and if it is 100% full, does that mean it contains 2000 litres of liquid. We cannot be sure of that as we do know know what is in the bottom of the tank. There could be number of stones at the bottom. This figure can better explain what we are referring to here

Avoiding Deep Discharge

There is another SOLAS requirement about deep discharge of the battery while performing capacity test. In the simplest of the terms, deep discharge means the least voltage a battery can be brought to. If we discharge the battery below this voltage, the battery can loose its capacity to a level where it cannot be used again. For Nickel based batteries this voltage is 1.0V per cell. So for 24V battery pack (1.2V x 20 Cells), the deep discharge voltage will be 20V. While performing capacity test, we should never allow the battery voltage to go below 20V or 1V/Cell.

Now we all must be knowing this basic physics

 Power = Voltage x Current

The GMDSS battery is usually in the range of 200 Ah which is required to give 24V.

We need to test if it still has 200Ah left in it. For this we need to remove the batteries from charging and the existing load (connections to GMDSS station) and attach some known load to it. Usually a rig which consists of number of 100W bulbs in series is attached to the batteries terminals. Say, if 6 bulbs (600W )are attached to the battery, it would draw 25A of current from the battery bank. This is because

600 watts / 24 Volts = 25 Amp

Once the load is attached to the battery bank, we need to measure the voltage and the current across each battery bank terminal. We need to do this at least every hour. This will continue and we shall stop the test only if

1) The one battery cell is failing. That is drop of voltage in one battery cell is different than others. In this case we need to isolate this failing cell and then continue the test.

2) The voltage has reached the deep discharge voltage. SOLAS requires that while performing the capacity test, deep discharge of the battery shall be avoided. We have already discussed what deep discharge is. During the test we need to measure the voltage of each cell. The voltage should not go below 1V in any cell or 20V for the battery pack.

3) The test has performed for sufficient time to show that battery has 100% of its rated capacity. Say it has been 8 hours since the test started. And for 8 hours the current measure was 25A. So the battery has already delivered 200Ah (25A x 8 Hours). This shows that battery’s capacity is still 100%. In this case, this would conclude the test.

If we stop the test because of 3rd point, the battery is fine. If we stop the test because we have come to deep discharge voltage. We need to measure how much Ampere Hour has the battery delivered at this point. If it is less than 80% of its rated capacity, the capacity test has failed.

Charging the battery after capacity test

If the measured capacity is more than 80% of the rated capacity, we can move to next step where we measure the time required to charge the battery. There are two parameters to show the percentage level a battery is charged. These are terminal voltage reading or the specific gravity of the electrolyte. A 24V battery when fully charged would show a voltage of around 25.4V and specific gravity of 1.265. Determining the state of charge with voltage can be tricky as voltage can change with temperature. Specific gravity of electrolyte is considered more accurate way to determine state of charge of battery. So after the capacity test, we need to measure the time a battery takes to charge to 100%. SOLAS require this time to be less than 10 hours.

3. General maintenance required on GMDSS Batteries

The onboard batteries do not need much of maintenance as such. There are two things that need to be checked. First is the electrolyte level and second is the specific gravity of the electrolyte. If you have maintenance free batteries onboard, you do not need to check anything on that except the conditions in which it is stored.

Level of the electrolyte

The level of electrolyte can reduce due to various reasons but it is important that the level is maintained as per the manufacturer’s instructions. If the level is low, the battery cell must only be refilled with distilled water.

 Checking specific gravity of the electrolyte

Specific gravity of the electrolyte is considered more accurate measure of state of charge of the battery. Specific gravity must be checked daily and recoded in battery log alonwith voltage. Specific gravity can reduce because of sulfation which causes the charging plates to be deposited with crystals. This causes reduction of holding charge and thus the capacity of the battery is reduced. BCI (Battery Council Internation defines specific gravity of 1.265 as 100% state of charge. 1.225 is considered 75% state of charge. It is still better to refer to manufacturer’s instructions as some manufacturers can go upto specific gravity of 1.280 as the sign of 100% state of charge.

Conclusion

GMDSS batteries are important equipment which ensures that emergency equipments gets power in real emergency. It is thus important to maintain these batteries in an excellent condition. Various tests/checks ensure that GMDSS batteries would provide the required power. Daily on load/off load test ensures that all connections are intact and when on load batteries do not drop voltage too quickly. Annual capacity test measures the capacity of the battery in Ampere hours. We should replace the battery if capacity is less than 80% of the rated capacity. Finally we should daily check the batteries state of charge by measuring the specific gravity and level of electrolyte.