Okay, this is a fairly common scenario and you're on the right track. You want to provide normal power and seamless battery backup. Here's how to approach this, breaking down the methods and the considerations

The Problem:

Directly wiring the battery backup output to the transformer input, as it's currently done, is a hack and likely not ideal, potentially unsafe, and may not be providing conditioned power.

Solution 1: Using a UPS (Uninterruptible Power Supply) - RECOMMENDED

This is the simplest, safest, and most reliable approach. It provides conditioned power and automatic switchover. Replace the existing battery backup with a proper UPS. Sizing

Determine the VA (Volt-Amps) or Wattage requirement of the BMS panel. Check the panel's documentation. Oversize the UPS slightly (20-30% is a good rule) to account for potential future additions.

Voltage and Output

Choose a UPS with a 120V input and 120V output. Ensure the UPS output waveform is compatible with the BMS panel's transformer. Sine wave output is generally preferred, but consult the panel's documentation.



Runtime

Select a UPS with sufficient battery runtime to keep the BMS panel operational for the required duration during a power outage. Consider the BMS's critical functions and how long they need to run on battery.

Wiring

1. Connect the 120V feed from the electrical panel to the UPS input.
2. Connect the UPS output to the 120V input terminal block on the BMS panel (where the original 120V feed was supposed to connect).3. Remove the existing battery backup and its wiring completely.

Advantages



Clean Power

UPS units typically provide filtered and regulated power, protecting the BMS panel from voltage spikes and sags.

Automatic Switchover

Seamless transition to battery power during outages. The BMS panel won't even notice the power loss.

Battery Management

The UPS manages the battery charging and health, extending its lifespan.

Safety

Properly designed and certified UPS units provide better safety compared to a DIY approach.

Monitoring

Many UPS units offer monitoring features, allowing you to track battery status, load, and power events.

Disadvantages



Cost

UPS units can be more expensive than a simple battery backup.

Solution 2: Using a Contactor and a Separate Battery Backup (More Complex)If you absolutely cannot use a UPS, this is a more complex solution that requires careful implementation. It involves using a contactor to switch between the normal power feed and the battery backup output.

Components



Contactor

A normally closed (NC) contactor with a 120V coil. The contactor needs to be rated for the voltage and current of the circuit. Crucially, the contactor must be rated for inductive loads if the transformer is an inductive load.

Separate Battery Backup

(The existing one, presumably) Double check voltage is 120V

Enclosure

A suitable enclosure to house the contactor and any wiring.

Wiring Supplies

Appropriate wire, connectors, etc.

Wiring


Normal Power

Connect the 120V feed from the electrical panel to the normally closed (NC) terminal of the contactor. Connect the other side of the NC contact to the 120V input terminal block on the BMS panel's transformer.2.

Battery Backup

Connect the output of the battery backup to the normally open (NO) terminal of the contactor.3.

Contactor Coil

Connect the 120V feed from the electrical panel (the same feed that powers the NC contact) to the contactor coil.

Operation



Normal Operation

When the power is on, the contactor coil is energized. This opens the NC contact, disconnecting the normal power feed. The BMS panel is powered by the battery back up, but since it is plugged in and is the source of power to the Contactor’s coil it is also constantly being charged.

Power Outage

When the power goes out, the contactor coil is de-energized. The NC contact closes, connecting the normal power feed to the BMS panel.

Advantages

Potentially lower cost than a UPS (depending on the cost of the contactor).

Disadvantages



Switchover Time

There will be a brief interruption of power to the BMS panel during the switchover. This may or may not be acceptable depending on the panel's sensitivity.

Complexity

More complex to wire and troubleshoot than a UPS.

Safety

Requires careful wiring and adherence to electrical codes to ensure safety. Proper grounding is essential.

Battery Management

Requires a separate battery charger for the backup battery.

No Power Conditioning

Does not provide any power filtering or regulation.

Important Considerations for Both Solutions



Local Electrical Codes

All wiring must comply with local electrical codes. You may need a permit for this type of work.

Grounding

Ensure proper grounding of all components.

Overcurrent Protection

Verify that the circuit feeding the BMS panel has appropriate overcurrent protection (circuit breaker or fuse). The UPS or contactor setup should not compromise this protection.

Labeling

Clearly label all wiring and components for easy identification.

Testing

Thoroughly test the system after installation to ensure it functions correctly. Simulate a power outage to verify the switchover to battery backup.

Documentation

Provide the customer with documentation of the wiring and operation of the system.

BMS Panel Documentation

Absolutely essential! Consult the BMS panel's documentation for specific power requirements and recommendations. Some panels might have specific requirements for backup power systems. This is especially important regarding the type of AC power (sine wave, modified sine wave, etc.) that the panel can accept.

Recommendation:

Using a UPS is the strongly recommended* solution. It's safer, more reliable, and provides better power quality. While it may be slightly more expensive upfront, the long-term benefits and peace of mind are worth it.Before you start any work, DISCONNECT THE POWER and verify that the circuit is de-energized. Consult with a qualified electrician if you are not comfortable performing this work yourself. Flag for review