IN-FLIGHT SMOKE ? TECHNICAL
The battery involved was an EarthX Model ETX-680 Lithium-Iron-Phosphate aviation battery with a dual-redundant Battery Management System (BMS). The EarthX battery was the aircraft?s sole main battery. The EarthX battery was hard-mounted in the RV-8A?s lower forward baggage compartment in place of a Concorde lead-acid aircraft battery; in the same location.
There is no record or evidence of an EarthX battery physical installation problem; however, EarthX?s installation manual says: ?Installation of the battery in the cockpit is not recommended, unless the battery is properly vented over-board.? Technically, the battery was not mounted in the cockpit, but it was mounted in an enclosed compartment internal to the fuselage, aft of the firewall, and adjacent to the pilot?s right foot and leg. The battery compartment had a top cover, but unfortunately did not include an overboard vent.
The EarthX battery?s remote, discreet warning output (LED panel light or EFIS input) was not installed as recommended by EarthX. Since this installation seemed to be optional, the pilot/owner delayed the installation in favor of testing his new Dynon HDX system. The Dynon Skyview HDX EFIS was equipped with its own internal backup battery plus an advanced aircraft/engine instrumentation system which continued to record all flight, engine, and electrical data from takeoff until landing.
The highest recorded peak voltage was 29.1 volts although it is very likely that the voltage greatly exceeded 30 volts as there was a two-second voltage data drop-out at that time. There was also an initial, half-second voltage data drop-out about 4 minutes earlier as the recorded voltage increased above 20 volts and fluctuated. The highest recorded peak amperage was 44.8 amps. The Dynon data shows that a simultaneous, average voltage / amperage level of 21 volts / 42 amps (approximately 880 watts) was applied to the aircraft electrical bus (and EarthX battery) for a total of approximately 5 minutes although all other components connected to the bus were switched off (disconnected from the aircraft electrical bus) as soon as the fluctuating excessive voltage (19 to 25 volts) and high amperage (40 to 45 amps) readings were initially observed. The master was left on, because the pilot was focused on a panel component failure; not an aircraft electrical power system failure.
Unfortunately, the aircraft had no automatic overvoltage protection circuit as ?strongly recommended? by EarthX's installation manual (at the time of installation). When the RV-8A was purchased, the new pilot/owner was unaware that the aircraft was equipped with an automotive alternator/regulator with no overvoltage protection. Also, lacking a detailed aircraft electrical system schematic, the pilot/owner was unaware that the aircraft electrical system had no inherent, built-in overvoltage protection. Also, the installation of overvoltage protection seemed to be optional at the time (?strongly recommended?), so the pilot/owner did not inspect the alternator/regulator and aircraft electrical system to see if overvoltage protection was installed.
After the incident, the automobile-style alternator, a 35-amp-rated Bosch AL204X with integrated regulator/rectifier, was removed for bench testing and found to be non-functional. There was no output from the alternator. The Dynon data shows the alternator output starting to drop at 24 minutes after takeoff and the alternator output failing completely 1 minute later (2 minutes before landing). Please note that this bench test was only a functional test. There was no additional testing or detailed failure analysis to pinpoint the cause of alternator (regulator) failure. Also, there was no attempt to duplicate the high voltage and amperage recorded in-flight. The alternator was simply ?dead.?
The alternator output breaker was rated at 35 amps, but did not trip during the incident despite the Dynon-recorded high voltage (21+) and amperage (42+) levels. The breaker was tested after the incident and tripped at less than 36 amps with 14.3 volts. After developing an aircraft electrical system schematic and reviewing it (including the Dynon shunt location in the alternator output between the alternator and the 35-amp breaker), the authors have no definitive explanation relative to why the breaker did not automatically trip in-flight and thus save the battery. There are several possible answers, but there is no information clearly pointing to one answer. The authors have decided not to pursue a more detailed analysis.
The damaged EarthX battery and the Dynon instrumentation data were sent to EarthX for analysis. Also, the authors subsequently communicated with EarthX to determine what EarthX discovered about the battery and the incident. Based on the Dynon data and inspection/analysis of the battery, EarthX concluded:
The battery was forced into thermal runaway for two reasons:
(1) The alternator/regulator failed resulting in the application of sustained, excessive voltage and current to the battery which was above the rated limits of protection for the battery?s Battery Management System (BMS). Battery inspection clearly showed physical evidence of extremely high voltage being applied to the battery which was above the rated limits of the BMS.
Authors? Note: According to the Dynon data, the power applied to the battery was sustained at about 880 watts; peaking at well over a kilowatt (1,240 watts).
(2) The pilot/owner should have shut-off the master switch as soon as the fluctuating, excessive voltages and amperages were observed.
To quote EarthX:
?The aircraft voltage regulator failed and the battery was subjected to voltage greater than 20V charging with high amps for more than 7 minutes which caused the cells to reach thermal run-away. At a couple of points, the voltage spiked so high that the Dynon didn't record. Based on feedback from Dynon technical group, the voltage must have been above 30V. Our Battery management system indicated that the voltage exceeded 70V.?
?An over-voltage protection circuit in the alternator regulator would have shut-down the alternator within 100ms in the event the voltage exceeded 16V, but this equipment was not installed on your aircraft.?
Authors? Note: THE EARTHX BATTERY DID NOT CATCH FIRE. Although the battery over-heated in thermal runaway, the heat given off during thermal runaway event was not sufficient enough to burn the pilot?s leg or foot through the thin aluminum battery compartment wall. The pilot does not recall detecting excessive heat from the battery compartment. The main effect of the thermal runaway event was the eventual emission of smoke as the battery overheated. The smoke was emitted though wiring grommets in the battery compartment wall as well as small gaps at the edges of the battery compartment. Smoke emission would be expected from any battery, lithium or lead-acid, experiencing a thermal runaway event.