This test is targeted to measure the capacity of the battery as it tends to reduce with age.
To do this we should charge the battery to full, then measure its capacity by discharging and applying the known load.
NOTE:
The measure of battery capacity is not the voltage!
The capacity of the battery is measured in Ampere hour (Ah).
200 Ah means the battery can provide a current of 200 Ampere for one hour or 20 Amperes for 10 hours and so on.
Thus, the results of capacity test should answer the question: “How much current the battery can produce for how many hours?”
A fully charged battery doesn’t mean that it can produce the rated capacity.
To illustrate the last point, let's see at a new smartphone and the one that is being in use for some years. A new cell with 100% charged battery works much longer than the used phone with fully charged battery. It is because the battery storage space reduces with age.
Preventing the batteries from deep discharge
One of the SOLAS requirements concerns avoiding deep discharge of the battery while performing capacity test.
Deep discharge means the least voltage a battery can have.
If the battery is discharged below this voltage level, there can be a situation when the battery can lose its capacity to a level where it cannot be used again.
E.g. for Nickel based batteries this voltage level is 1.0V per cell. Thus, for 24V battery pack (1.2V x 20 Cells), the deep discharge voltage level will be 20V. So, while performing capacity test, we should prevent the battery voltage from going below 20V or 1V/Cell.
Let’s refer to the formula that tells us
Power = Voltage x Current
The GMDSS battery is usually in the range of 200 Ah which is needed to give 24V.
To ensure that it still has 200Ah, we remove the batteries from charging and connections to GMDSS station (the existing load), then attach some known load to it. Usually a chain of several 100W bulbs is attached to the batteries terminals. So, if for example 6 bulbs (600W) are attached to the battery, it would receive 25A of current from the battery bank.
600 watts / 24 Volts = 25 Amp
When we apply the load to the battery bank, then we need to measure the voltage and the current across each battery bank terminal. We need to do it at least every hour.
The cases when we may stop the test:
1) The voltage drop in one battery cell is different than others. In thiscase the failing cell should be isolated. Then we may continue the test.
2) The battery has deep discharge voltage.
To avoid this, as per SOLAS, we need to measure the voltage of each cell during the test. The voltage mark of each cell should not be below 1V or 20V for the battery pack.
In this case we need to measure how much Ampere Hour has the battery delivered at this point.
NOTE: If it is less than 80% of its rated capacity, the capacity test has failed.
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 this point is the reason of the stoppig of the test, then we make a conclusion that the battery is fine.