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The Case of the Abused Accumulator
Myron J. Boyajian, President, Engineering Consultants -- Design News, June 26, 2006
While watching a Discovery Channel special on the Hindenburg disaster, I was reminded of the explosive power of hydrogen when carelessly handled. A few years prior, I was retained to investigate the cause of a battery explosion that resulted in flash and acid burns to a maintenance mechanic who was charging a battery he removed from a diesel-powered forklift.
Scene of the Crime
My client represented a trading company that imported the lift truck (in motorcycle parlance, a "metric" forklift), as well as the automotive-type battery used to start the forklift's engine. My investigation started with a review of a package of photos of the exploded battery, the charger and the general shop environment. The photos showed a battery well and truly blown apart. I soon received a copy of the plaintiff's expert's report in which the expert opined, without examining the battery, that the explosion was the result of a hydrogen explosion triggered by an electrical arc caused by a faulty internal battery connection.
My client hoped for an early resolution to this case and asked me to expedite the inspection of the subject battery. I wanted to consider all sources of a spark or flame and asked to have the charger used by the plaintiff available for inspection. The plaintiff claimed to have not been smoking at the time of the explosion, so at least for the moment I discounted a lit cigarette or match as the ignition source.
The Investigation
If a lead-acid battery could discharge and charge with perfect electrochemical efficiency, there would be no emission of hydrogen or oxygen gas, just the quiet conversion of the plate material, one of lead and the other of lead oxide, both to lead sulfate while the sulfuric acid electrolyte changed to water while discharging; and the reversal of this process during charging.
One hundred percent efficiency, like perfection, is only to be hoped for. Excess or rapid charging/discharging, plate age and condition, excessive temperature, and other reasons may cause hydrogen and oxygen gas to be emitted from a lead-acid battery.
In this case, there was no question that an internal explosion caused the battery case to break apart, scattering fragments and acid onto the plaintiff. I proposed a protocol, accepted by all parties, to test the battery and charger. At a storage facility, I set up a field lab. I used a resistor circuit to discharge an exemplar battery prior to the charger tests. I used that time to make a close visual inspection (magnifier, etc.) and resistance tests of the internal, intercell connectors on the exploded battery. I found no evidence of any break or any point where connectors of opposite polarity could have touched with a resulting spark.
I connected the charger to the exemplar battery and recorded charging current and voltage, and component temperatures. There was no abnormal operation or sparking during charging. Static tests demonstrated correct charger diode operation. The plaintiff said he pulled the 120VAC-charger plug, over 10 ft from the battery explosion, before he disconnected the alligator clips from the battery terminals making that an unlikely spark source. As he pulled the clips, he said the battery exploded. This implied that there was some stored energy in the charger that could have produced a spark. A review of the charger schematic and visual inspection confirmed there were no energy storage components (capacitors or inductors) that could have produced an arc, but I had to test the charger to prove this point.
I set up a mini darkroom around the alligator clips connected to the exemplar battery's terminals. An assistant disconnected the charger line cord and I then disconnected each alligator clip, testing both the positive and negative clip. A camera loaded with high-speed film and open shutter was inside the mini darkroom to record any arcing, but I ultimately found no evidence of arcing.
The Smoking Gun
I returned to my office, prepared and issued my opinion report, having concluded there was neither a faulty battery nor faulty charger. During this time, my client had been busy with depositions and other inquiries. Enough witnesses came forward to say that the plaintiff, a multi-pack per day smoker, was in fact, smoking at the time of the accident. Having (to a reasonable degree of engineering certainty) eliminated the charger and battery as a source of ignition, and with witness statements regarding smoking, the plaintiff's case withered and he settled for a minuscule fraction of the original demand.
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