Okay, let's break down what's happening with your 135V battery bank system, interpret the readings, and try to determine if you have a ground fault.

Understanding the System 135V Battery Bank

This is your backup power source for critical equipment (lube oil systems) in case of AC power failure.

Primax P600 Battery Charger

This serves two essential functions:

Charging

Keeps the battery bank at the correct state of charge, ready for an outage.

DC Power Supply

When AC power is available, the charger can also simultaneously supply DC power to the lube oil systems, bypassing the battery. This is more efficient as you're not constantly discharging/recharging the battery under normal conditions.

Grounded System

The reference to 67V readings means the system is probably grounded. Often one "side" of a DC battery bank is connected to earth ground through a high impedance to prevent any damage from a full short to ground but still provide a reference so faults are easier to detect.

Interpreting the Voltage Readings Battery Terminals

(+) to Ground: 2.6V (-) to Ground: 125V Difference: 125V - 2.6V = 122.4V (This should be near the expected 135V open circuit value if everything is ok, and the battery voltage is normal.)

Battery Charger (Output Terminals)

(+) to Ground: ~67V (-) to Ground: ~67V Difference: ~0V (Not exactly 0V, the difference can be accounted for with a voltmeter, but will be essentially the same)

Analysis and Possible Ground FaultYou most likely have a ground fault

Here's why:1. The Battery Terminal Voltage Readings Indicate a Ground Fault

In a healthy, grounded DC system, the voltage from either terminal to ground should be roughly half the total system voltage (close to 67V in your case). The battery terminal readings strongly suggest a ground fault on the positive (+) side of the battery bank.2.

How to Test Further? Disconnect System

Try disconnecting the battery charger completely from the battery bank. Then, re-measure the voltages at the battery terminals to ground. If the readings change drastically (and become more balanced), it's more likely that the charger is somehow contributing to the ground fault (or being affected by a ground fault downstream).

Load Disconnect

Try disconnecting the DC loads (lube oil pumps, etc.) from the battery bank. Then, re-measure the voltages at the battery terminals to ground. If the readings change drastically, it's more likely that the load is causing the ground fault.

Troubleshooting Steps1. Safety First

Before working on any electrical system, always ensure the power is safely isolated and follow appropriate lock-out/tag-out procedures.2.

Visual Inspection

Carefully inspect all wiring, connections, and components associated with the battery bank, charger, and DC loads. Look for signs of damage, corrosion, loose connections, or insulation breakdown. Check for any evidence of moisture or contamination in or around the equipment.3.

Isolation

As mentioned above, disconnect the battery charger and loads, one at a time, and observe the voltage readings at the battery terminals to ground. This will help isolate which part of the system has the fault.4.

Insulation Testing

A megohmmeter (insulation resistance tester) is the best tool for finding ground faults. Disconnect equipment and wiring one at a time to isolate where the ground fault lies.5.

Check Charger Settings

Consult the Primax P600 manual to confirm the charger is configured correctly for your battery bank type and voltage. Verify the charger's grounding connections are secure and properly made.

Which Readings Are "Correct"?

The readings at the battery terminals are the ones indicating the abnormal condition (the ground fault). While the charger readings might be normal, they could also be misleading due to the fault elsewhere in the system.

Important Considerations Ground Fault Protection

Does your system have ground fault protection? If so, it should have tripped, or at least given an alarm. Investigate why this didn't happen (if present).

Battery Health

A severe ground fault can damage the battery bank. Check the individual battery voltages.

Recommendation

Given the importance of your emergency lube oil systems, do not delay* in investigating this issue. Ground faults can lead to equipment failure and potentially dangerous situations. I strongly recommend involving a qualified electrician or electrical technician who has experience with DC battery systems and ground fault detection.Good luck! Let me know if you have any other questions. Flag for review