E T X T S O L I T H I U M B A T T E R I E S
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The ETX900-TSO BMS has the following features; over-charge protection, over-discharged
protection (completely draining the battery), excessive cranking protection, high temperature
protection and short-circuit protection. The BMS was designed to Design Assurance
Level (DAL), C (major).
The BMS disconnects the battery from the load if it is drained to less than 5% remaining
charge (an over-discharge condition). An over-discharged battery typically has a voltage less
than 11.5V. If the BMS disconnects the battery, the voltage reading of the battery will be
zero volts. Excessive cranking protection logic includes temperature monitoring to limit
“high current use” (engine cranking) to 10 -30 seconds in any 60 second period. If the
battery terminals are “shorted” (or a low impedance load is connected across terminals),
which causes the battery volts to instantaneously drop to a very low level, the battery will
disconnect from the load to protect the cells and BMS from damage (short circuit
protection). If the BMS disconnects due to excessive cranking protection or short circuit
protection, the BMS will automatically reconnect after a cooldown period (typically 1-3
minutes). The BMS is designed for short circuit protection > 1000 Amps.
All components associated with main electronic battery disconnect are redundant. The
built-in redundancy ensures that no single point failure results in the battery unintentionally
disconnecting. The battery also includes a thermal run-away containment system. The
design aligns with the requirements for a FAA approved lithium battery as per RTCA
performance specification DO-311A and DO-160.
In the event of a charging system failure where the voltage increases to above 15.5V, the
resistance to charging current increases, and above 16V the charging current is completely
blocked. The time delay for this feature is 2 seconds to allow the aircraft alternator’s over
voltage protection to activate first (typically less than 100ms). This design offers charge
voltage protection greater than 60V. The discharge current (current out of battery) is
unaffected in this situation. EarthX requires having automatic over-voltage protection
(crowbar or other means of shutoff) for alternator type charging systems.
The battery’s micro-controller monitors all failure modes, and reports failures with a built-in
LED indicator and discrete output. The discrete output for external fault monitoring is a
single wire connection with a ¼” quick connect terminal. The output is a “current sinking”
type circuit (see diagram below) that can handle 100mA (connects the discrete output to
battery ground if a fault is present). This output can be connected to an external 12V LED
or general purpose discrete input of an EFIS. The fault output has three states; fast flashing
(2 seconds on/ 2 seconds off), slow flashing (5 seconds on/ 5 seconds off) or solid.
The fast flashing fault is an indication of high temperature; temperature exceeding the
normal operating or storage limits of the battery.
The slow flashing fault can indicate an improper state of charge or a problem with the cells
internal to the battery. The solid fault indicates a BMS hardware failure.
