P0AA6 – High Voltage Isolation Fault

This code is set when the BMS measures an isolation breakdown between the high voltage battery and the 12 volt system. A breakdown in isolation can be caused by ruptured or leaking cells, high voltage cabling insulation that has rubbed off and come into contact with low voltage systems, condensation, use of non-isolated equipment, by an intentionally non-isolated design, or by other causes. This error code may indicate an unsafe condition that exists in the battery pack and care must be taken to avoid risk of short circuit and risk of personal injury from shock while investigating the error as simply touching a cell could cause a shock. While this error tends to indicate a real problem, there are certain situations that can cause a false positive.

This error code is an informational code only and does not change the behavior of the BMS. It will not cause the BMS to stop charge or discharge in any way (but other external controllers may be setup to do so).

The isolation fault detection circuit in the Orion BMS applies a very weak, slow (about 1 Hz) AC signal on the negative wire on the total pack voltage sensor and measures the amount of signal degradation to determine if a breakdown in isolation has occurred. The fault is triggered when the 60 second average of the “Isolation Shortest Wave” parameter drops below 3.2 (unless ordered differently from the factory). For most systems, this indicates the BMS is measuring less than about 150k ohms of resistance between the high voltage battery and the low voltage system, but external factors such as parasitic capacitance between the high voltage and low voltage systems can artificially increase or decrease the measurement.

Note: A false positive may be triggered if a large amount of capacitance is present between the battery pack and the low voltage system causing the signal to degrade. Some DC:DC converters or inverters may have large filtering capacitors which can cause false positives and other equipment designed to measure breakdown in isolation may also cause signal degradation.


Resolving the issue:

Step 1. Determine if the system is supposed to be isolated and if other isolation detection circuits are connected at the same time.

If the overall system is intentionally designed to be non-isolated, isolation fault detection should be disabled. Other isolation fault detection circuits operating on the same battery pack may interfere with each other and cause false readings.

Step 2. Check the instantaneous isolation “shortest wave” reading.

The instantaneous value for the isolation fault detection can be monitored in the Orion BMS utility by looking at the “Shortest Wave” measurement on the Live Text Data screen and selecting the “Advanced Parameters.” Every BMS unit has some variation for the maximum and minimum readings, but the readings are generally between 1.4 and 4.8, with the higher number meaning better isolation. Over 4.5 generally indicates insulation over 1M ohm and below 1.5 indicates a likely dead short, but external factors can change the exact numbers.

Step 3. If isolation is degraded only when cold, it may be due to condensation.

Water may condense on cold cables, battery terminals or other exposed connections and create a path for current to travel to the chassis or low voltage system. Additionally, conductors and cells may expand and contract and physically come into contact with other parts.

Step 4. If the isolation is degraded according to the shortest wave value, attempt to isolate various systems while monitoring the shortest wave measurement in the Orion BMS to see if removing a system improves the isolation number.

For example, if a battery charger can be disconnected or fully isolated from the battery pack, disconnect it and look to see if the isolation improves. If isolation improves after removing a part, investigate the cause of isolation breakdown or replace the defective system. Care must be taken to prevent personal injury and prevent shorting cells since a cell may be shorted to the chassis causing the chassis to be “hot.” Simply touching a cell and the chassis may cause a shock. If safe to do so, disconnect any device which may have extra capacitance to ground (chargers, inverters, DC:DC converters or any cabling which may have excessive parasitic capacitance.

Step 5. If the shortest wave measurement is still showing degraded isolation after the above steps have been performed, look for isolation faults within the battery pack.

Isolation faults may occur due to leaked electrolyte, liquids shorting to the chassis, battery terminals shorted to the chassis, chafed wiring, conductive debris or other causes. As before, care must be taken while inspecting the battery pack to prevent personal injury and prevent shorting cells since a cell may be shorted to the chassis causing the chassis to be “hot.

Step 6. Verify correct operation of the isolation fault detection circuit on the Orion BMS.

Disconnect the total pack voltage sensor connector (the 8 pin connector with 2 wires) and ensure that the “shortest wave” measurement reads over 4.5.


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Ewert Energy Systems, Inc
161 East St. Charles Rd
Carol Stream, IL 60188

Phone: (630) 868-3173
Fax: (866) 657-5667

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