Emergency Diesel Generator (EDG) starting battery explosion
Thermal runway is a common operational issue for any battery that can lead to incidents on-board. Learn more about this issue, and how crew can address and resolve it.
The Emergency Diesel Generator (EDG) starting batteries exploded during the routine auto-start test in several recent incidents including one on our ship.
In most cases, the maintenance records neither showed any battery abnormality nor deterioration. The average age of the battery in these incidents was approximately 3 years.
Probable Causes of Explosions
Thermal runaway is the condition where the rate of heat generation within a battery component exceeds its heat dissipation capacity. Thermal runaway can have many causes such as overcharging, high ambient operating temperatures, etc. It can lead to a catastrophic or destructive failure of the battery cell.
Thermal runaway is a situation where the current flowing through the cell causes the cell temperature to rise. This can happen during charging or due to overcharge or over discharge. The rise in temperature increases the current which further increases temperature.
Another potential reason for battery explosions is the use of Valve Regulated Lead Acid (VRLA) and Absorbed Glass Mat (AGM) batteries. These are lead acid batteries, with a different construction than the standard ventilated lead acid batteries. These batteries tend to generate more heat if combined with mismatched chargers and when the battery racks and boxes have poor ventilation.
These batteries are then unable to dissipate the extra heat generated. Prolonged float charging is also a cause of heat generation. The EDG starting batteries are under float charging for long periods until used for occasionally starting the EDG.
Thermal Runway
Several factors can contribute to thermal runaway, though the most common being a combination of high temperature and high float voltage. Thermal Runaway occurs when more heat is generated within the battery than what can be dissipated through its case. Heat is generated internally by exothermic chemical reactions from excessive charge current pumped into the battery.
This heat generation increases as the applied voltage and / or the battery’s internal temperature increases. Power, in the form of heat transfer is dissipated from the battery through its exterior case into the ambient environment. If the battery cannot dissipate as much heat as it generates, its internal temperature rises. As the battery’s temperature rises, its internal power rises.
ISC= Internal Slot Circuit
This can go out of control if the rate of increase of the generated heat is greater than the dissipated heat. This heat / current cycle eventually causes the battery to bulge. At 90°C the plastic case becomes soft, and, with just a little pressure, bulges out. The separator membrane within the battery, which prevents the positive and negative electrodes from touching, can be damaged triggering short circuits. Internal short circuit is the most common outcome of all the abuse conditions. Side reactions of thermal runaway can cause hydrogen release through the safety vents, causing a possible explosion.
How to Address Thermal Runway
There are no requirements at present to monitor the condition of the battery charger during charging / discharging. Also, there is no requirement for an alarm to be triggered when abnormal conditions occur within the battery bank. Hence, it is important to educate the crew of such dangerous situations which can occur without one’s knowledge.
Take at least following precautions prior to entering the EDG compartment and before or during EDG testing:
- Prepare the EDG compartment, open the doors, ventilate the space using the forced draft fan.
- Find out and display the battery installation date. Be more cautious if the batteries have been in service for longer time e.g., 2.5 years or more. There can be aging and potential deterioration of the cells and the membrane separation between the anodes and cathodes. This can lead to a volatile condition within the battery. The recent incidents mentioned above did show a trend. From the time the battery bank was renewed till the explosion occurred, the age of the batteries was between 2 – 3 years. It is prudent to renew the batteries within this period.
- Review records of the weekly onboard maintenance log. What are the inspection regimes and the conditions reported?
- Check the temperature of the outer casing of the battery, preferably use a laser thermometer. If it is abnormally high, take additional precautions such as stop charging, check if the charger cuts out once the full charge is reached or goes to trickle charge, the voltage and charging current are correct, cool down the battery bank with additional fans and replace the batteries at the earliest possible time.
- Visually inspect starting batteries for deformities. If any deformities are seen, replace the affected batteries.
- All personnel must stand away or on the opposite side from the battery bank location during the auto start test. This way the EDG acts as a shield and can prevent anyone from getting hurt in case of battery explosion.
- If the EDG fails to start on the 1st attempt, DO NOT ATTEMPT REPEATED STARTS. This generates further heat in the battery and may accelerate the thermal runaway condition.
- Check crew awareness of the possible causes and what to look out for during routine inspection and tests of the EDG.
Recommendations for Crew
Splash Protection Shield
Keep air circulation space between adjacent batteries. Consider temperature compensating chargers, particularly for EDG starting batteries. Specify charger voltage between 2.25 – 2.3 Volts per cell (VPC) for float chargers. EDG’s are starting only occasionally, normally for testing.
The batteries spend long periods of time in a float charge (trickle charge). For VRLA and AGM batteries, set the float charge around 2.25 – 2.3 Volts Per Cell (VPC). If set higher, then these batteries overheat. Create a protection shield to prevent exposure to a possible battery explosion. Ensure replacement batteries have the same exact specifications as existing ones.
