UPS battery midpoint: what it does, when you need it, and how to connect it
UPS battery midpoint: what it does, when you need it, and how to connect it
In a UPS battery cabinet, you may find a cable connected to the middle of the battery string. It may be called the battery midpoint, center tap, or MID connection. Some UPS systems use this connection, while others do not.
The important thing is to find out what the MID connection does on the particular UPS. It may be used only for voltage sensing, or it may be part of the DC power circuit. The wiring method is different for these two applications.
1. What is a UPS battery midpoint?
A UPS battery bank is normally made up of battery blocks connected in series.
For example:
40 × 12 V = 480 V DC nominal
The midpoint is the junction between the 20th and 21st battery blocks. Depending on the manufacturer, the terminal may be marked Battery Midpoint, MID, M, or Center Tap.
The midpoint divides the battery string into two nominal sections:
Battery + → 240 V → MID → 240 V → Battery −
On some UPS systems, the midpoint is used to measure the voltage of the two sections separately. This can help identify an imbalance that may not be obvious from the total battery voltage.
For example, if one section of the string has a problem while the other section has a higher voltage, the total voltage can still appear normal. Looking at the two sections separately gives the UPS more information about the condition of the battery string.
The battery installation and monitoring arrangement should always follow the UPS and battery manufacturer's documentation.
2. Two types of battery midpoint connections
A MID terminal can serve different purposes depending on the UPS design. Before choosing a cable or making the connection, determine which type is used.
Sensing midpoint
In some UPS systems, the midpoint is connected to a voltage-sensing or battery-monitoring circuit.
The current in this lead is very small compared with the main battery cables. The UPS uses the connection to measure the two sections of the battery string.
The installation manual may specify a fused sensing lead, including the cable size and protection.
If this connection is left open, the main battery circuit may still be complete, but midpoint monitoring may be unavailable or an alarm may be generated, depending on the UPS design.
Power-circuit midpoint
In other UPS designs, the midpoint is part of the DC power circuit.
Some three-level inverter designs, including certain NPC and T-type implementations, use a split DC bus with positive and negative rails and a midpoint. Where the battery midpoint is connected directly to this power circuit, the midpoint conductor may carry battery charging and discharging current.
In this case, the midpoint cable is not a sensing lead. Its size, terminals, protection, and routing must be selected for the current it carries and according to the manufacturer's requirements.
Sensing midpoint vs. power-circuit midpoint
| Item | Sensing midpoint | Power-circuit midpoint |
|---|---|---|
| Purpose | Voltage sensing and battery monitoring | Part of the DC power circuit |
| Current | Very low | May carry battery charging/discharging current |
| Cable | Sized according to the sensing circuit | Sized according to the current carried |
| Protection | Usually specified as a sensing/fused circuit | Specified as part of the DC power circuit |
| If disconnected | Monitoring may be lost or an alarm may appear | May affect UPS operation |
| Small sensing cable | Only when specified by the manual | Not suitable |
The two connections can look similar from the outside, so cable appearance alone is not enough to identify the circuit.
3. How to tell which midpoint your UPS uses
The UPS wiring diagram is the first thing to check.
Look at three items.
Wiring diagram
Trace the MID terminal in the electrical schematic.
If it goes to a monitoring or control circuit, it is likely a sensing connection.
If it is connected to the DC bus or inverter power circuit, it may be a current-carrying connection.
Protection device
A sensing circuit normally has a small fuse or other protection specified for the measurement circuit.
A power-circuit midpoint will have protection suitable for the current in that circuit.
The rating shown in the manufacturer's diagram should be used rather than a value taken from another UPS.
Cable specification
If the manufacturer specifies the midpoint cable together with the main DC cables, treat it as a power connection.
If a small sensing cable is specified, follow that requirement.
Do not identify the midpoint only from the cable size, but use the cable specification together with the schematic.
On TAFENG three-phase UPS platforms, a MID terminal is used only on models where the design requires it. The battery wiring diagram for the specific model shows how the connection is used.
If the diagram is not available at the installation site, obtain the correct drawing for that UPS model before making the connection.
The same principle applies when sizing a UPS for inductive and motor loads: the actual UPS design and its specifications are more useful than a general rule of thumb.
4. Five checks before connecting the midpoint
Isolate the battery string
Follow the UPS manufacturer's battery shutdown procedure before working on the battery circuit.
Switching off the AC input does not necessarily remove the DC battery voltage. A 480 V DC battery string can remain energized.
Use a suitable meter and follow the required electrical safety and lockout procedures before making the connection.
Locate the correct midpoint
Identify the battery positive and negative terminals first, then locate the junction specified in the wiring diagram.
For a 40 × 12 V string, the midpoint is between battery No. 20 and No. 21.
Do not determine the midpoint simply from the physical position of the batteries in the cabinet. Follow the actual series connection.
Check each parallel string
For parallel battery strings, do not assume that the midpoint connections can be connected together.
Follow the manufacturer's battery monitoring and wiring diagram. If individual midpoint connections are specified, wire each string separately.
Size the cable and protection correctly
Use the cable size, fuse or breaker, terminal type, and other requirements shown in the UPS documentation.
Do not copy the midpoint cable size from another UPS just because the battery voltage is similar.
Verify the connection
Before closing the battery breaker, check:
| Check | What to confirm |
|---|---|
| Terminal | Correct MID / Battery Midpoint terminal |
| Diagram | Midpoint connection required and function identified |
| Battery layout | Correct number and arrangement of strings |
| Junction | Midpoint is at the specified battery junction |
| Cable | Correct size for the circuit |
| Protection | Correct fuse or breaker |
| Polarity | Positive, negative and midpoint correctly identified |
| Terminals | Correct connection and tightening torque |
| Voltage | Section voltages checked before energizing |
TAFENG provides a battery wiring diagram with its UPS battery cabinets so that the required connection can be identified for the specific model.
5. Why the battery midpoint is not AC neutral
The word "neutral" can sometimes cause confusion when working with a battery midpoint.
A typical three-phase AC system may have:
L1 + L2 + L3 + N + PE
A battery string has:
Battery + + MID + Battery −
The battery midpoint is the junction between two sections of a DC battery string. It is not automatically an AC neutral.
Many UPS battery systems use a floating DC arrangement, but the grounding method depends on the UPS design and installation requirements.
Do not connect the battery midpoint to AC neutral or protective earth unless the UPS manufacturer's wiring diagram specifically requires it.
6. How to check the midpoint after installation
Connecting the cable is not the end of the check.
Static check
Under the specified test condition, measure:
-
Battery positive to MID
-
MID to Battery negative
-
Battery positive to Battery negative
For a 40 × 12 V string, the two sections are nominally 240 V each.
The actual readings will depend on battery state of charge, charging condition, temperature, and battery condition. The acceptable difference should be taken from the UPS or battery manufacturer's specifications.
Dynamic check
If the midpoint is used for monitoring, check the two sections during charging and, where appropriate, during discharge.
A string that looks balanced when idle may show a larger difference during charging or load operation.
If the readings are abnormal, do not judge the battery condition from the midpoint voltage alone. Check individual block voltage, internal resistance or conductance, temperature, terminal connections, charging voltage, and previous measurement records.
7. What can happen if the midpoint is connected incorrectly?
The result depends on the type of midpoint connection.
Sensing lead connected to a power circuit:
The cable may be too small for the current and can overheat. Protection may operate, and the UPS may generate a DC bus or other fault.
MID connected to the wrong battery terminal:
The monitoring circuit receives an incorrect voltage and may generate an alarm or provide incorrect battery information.
MID connected to AC neutral or earth:
The battery grounding arrangement may be changed from the manufacturer's intended design and can cause an insulation monitoring alarm or an unintended current path.
Midpoints connected together on parallel strings:
If the UPS is designed for separate midpoint monitoring, commoning the connections can create an unintended connection between strings and affect the monitoring circuit.
MID taken from the wrong battery junction:
The two measured sections will not represent the intended battery configuration, so the UPS may report incorrect voltage information.
The midpoint follows the wiring diagram
A MID terminal does not have the same function on every UPS.
On one model it may be a low-current voltage-sensing connection. On another, it may be part of the DC power circuit.
The correct cable size and connection method therefore cannot be decided from the terminal label alone.
Check the UPS wiring diagram first, identify the function of the MID terminal, and then select the cable and protection required for that circuit.
FAQ
What is a battery midpoint on a UPS?
A battery midpoint is a connection taken from a specified junction in a series-connected battery string. On a 40 × 12 V string, it is normally between the 20th and 21st battery blocks, creating two nominal 240 V sections.
Is a UPS battery midpoint always required?
No. Some UPS systems do not use a midpoint connection. Others use it for battery voltage monitoring or as part of the DC power circuit. Follow the wiring diagram for the specific UPS model.
Is the battery midpoint the same as AC neutral?
No. The battery midpoint belongs to the DC battery system, while AC neutral belongs to the AC distribution system. Do not connect them unless the UPS manufacturer's design specifically requires it.
Can I use a small wire for the battery midpoint?
Only if the midpoint is a sensing connection and the UPS manufacturer specifies a sensing lead. If the midpoint is part of the DC power circuit, the conductor must be sized for the current it carries.
What should I check if the two battery sections have different voltages?
First check the midpoint connection and confirm that it is connected to the correct battery junction. Then check the individual battery block voltages, battery condition, temperature, terminal connections, charging voltage, and previous measurements. A difference between the two sections does not by itself identify a faulty battery block.
