VRLA Battery Explosion

Recently a client called for advice regarding a battery explosion. One of their customers had a small 100 Ampere Hour, 48VDC battery system blow the door off a NEMA 4 Cabinet used for Public Safety Radio.

I thought it important to share this story, but I will not mention product brands or company names.

The system was a simple 


48VDC UPS comprised of (4) 12 Volt, 100 Ampere-hour (VRLA)Valve Regulated Lead Acid batteries. Each battery is about the size of your car battery. It’s a typical telecom system with 48VDC rectifiers to power the Emergency Radio load plus power to float charge the batteries at 2.25 volts per cell (That is 54VDC for the system). This float assures the battery is at a full state of charge and ready for action in the event of a utility power outage.

A technician performing routine maintenance opened the cabinet and discovered one of the 12V batteries leaking. He removed the faulty battery and reconnected the charging system back to the remaining three batteries. You see where this is headed? You now have 54VDC across a 36VDC battery system.

Not long after he closed the cabinet and left the area there was an explosion. The cabinet door blew off releasing smoke into the room. All equipment in the cabinet was destroyed. 

There are multiple things that can cause VRLA battery thermal runaway. High charge voltage is one. The charge voltage was 3.0 Volts/Cell and the specification calls for 2.25 Volts/Cell.

VRLA batteries are very safe when operated within the manufacturer’s recommended specifications. However, if you increase the charge voltage from 2.25 Volts per cell to 3.0 Volts per cell you have a serious problem. The battery will generate excess hydrogen gas, increasing the pressure in the sealed battery. The battery safety vents will lift and release this hydrogen gas. The gassing rate jumps to over 1800 ml/hr. at this voltage rate. With this enclosure buildup of hydrogen gas, it doesn’t take long for concentration to exceed 4%, the hydrogen level that is explosive!

Fortunately, no one was seriously hurt in this case.

There is a misconception that small battery systems are not worth the cost of battery management. This story illustrates the danger of these systems regardless of size. A simple monitor would have alarmed the second the system was put back in service. It would detect there was a battery missing.

My client had originally offered protective features that would have alerted them of the mistake immediately, but the customer didn’t want to spend the extra $450 per cabinet. A small price to pay for safety.

I welcome comments and questions.

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