GMDSS Battery Voltage Collapse Under Transmit Load
During a monthly GMDSS battery test on a 5,000 GT offshore supply vessel, shore power is removed and the emergency switchboard is fed from the dedicated 24 V GMDSS battery. The bank consists of two 12 V, 200 Ah AGM batteries in series, commissioned four years ago. The charger float voltage is 27.2 V. With the VHF/DSC transmitting on 25 W, the terminal voltage at the battery posts falls from 26.4 V to 18.9 V for about two seconds, and the VHF resets; the MF/HF radio also drops out. The positive terminal lug is warm and the battery case of the port battery is slightly swollen. The antenna VSWR meter in the VHF shows 3.5:1. A clamp meter reads 38 A during transmit. The vessel's SMS requires the GMDSS battery to support one hour of operation. The battery charger is a 24 V, 30 A three-stage unit.
1. Interpret the voltage drop and identify the likely fault or faults.
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A drop from 26.4 V to 18.9 V at the battery posts is far too large for a healthy 24 V bank. At 38 A, that is roughly 0.2 ohm of effective resistance, which points to a battery with very high internal resistance, a bad inter-cell or inter-battery connection, or both. The warm positive lug suggests a high-resistance connection, but the swollen port battery case is a strong sign of battery degradation. Measure the voltage directly across each battery while under load and again at the radio terminals. If the collapse occurs at the battery posts, the battery is the problem; if the posts hold up but the radio terminals collapse, the wiring or lug is the problem. Load-test each battery separately according to the manufacturer's procedure. Do not assume the clamp reading is meaningful without checking what loads are on and whether the meter is in the correct range. The VHF VSWR of 3.5:1 is a separate fault that should be corrected, but it does not explain a 7.5 V battery collapse.
2. What immediate corrective action is required, and can the vessel sail?
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The vessel should not sail on this battery bank as the sole reserve source. Replace both batteries as a matched set; replacing only the swollen battery in a four-year-old series string is likely to leave a mismatched bank and repeat the failure. Clean and remake the terminal lugs, replace any heat-damaged cable, and verify the charger's AGM setting and temperature compensation. After replacement, perform a capacity test at the manufacturer's one-hour rate or the load profile required by the SMS. Confirm that the VHF/DSC and MF/HF can transmit without resetting and that the battery voltage stays above the manufacturer's end-of-discharge limit for the required duration. Also correct the VHF antenna VSWR before treating the radio as reliable, because a high VSWR can cause the transmitter to protect or draw abnormal current. The vessel can sail only when the reserve source has demonstrated the required capacity and the failure has been logged and reported under the SMS.
3. How would you test the repair and the antenna system?
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After the battery replacement, recharge the bank fully and record the float voltage. Disconnect the charger and apply a known load, such as the GMDSS console with a dummy load or the manufacturer's test load, for the required period while logging voltage and current at intervals. Confirm that no terminal or cable becomes hot. Then test the VHF antenna system with a known-good dummy load at the transmitter: if the VSWR is normal into the dummy load, the radio is probably good and the antenna or coax is faulty. Inspect the antenna, mount, connector, and cable for water ingress or corrosion, and measure VSWR again on the antenna. Check the MF/HF antenna tuner and ground system separately, because the MF/HF dropout may be caused by the same low-voltage event rather than by its own antenna. Record all test results, including the battery serial numbers, capacity test duration, and VSWR readings, so the next survey can compare them.