Understanding Line Loss in UPS Battery Packs

The UPS (Uninterruptible Power Supply) power system relies on a critical component known as the battery pack. However, these battery packs, although vital for backup power, often present challenges due to their bulkiness and weight, making them increasingly unsuitable for modern room setups with limited space. While battery packs are essential for ensuring backup time, the limited number of batteries they contain cannot always meet daily power demands. This dilemma has given rise to the need to relocate the battery packs outside the equipment room.

To maintain operational convenience, UPS units cannot be moved outside the equipment room along with the battery packs. If the batteries are placed far from the UPS host, a phenomenon known as line loss comes into play. Line loss refers to the loss of voltage that occurs when direct current (DC) travels through a specific length of cable. Understanding this concept and its implications is essential for the stable operation of power supply systems.

Line Loss and Its Impact on UPS Power Systems

It is a well-known fact that DC current flowing through a cable of a certain length results in line loss, which is essentially a voltage reduction. This voltage reduction, or line loss, significantly affects both the UPS unit and the connected load. Therefore, the ability to calculate line loss accurately is crucial for maintaining the stability of a power supply system.

For example, let's consider a 12V battery. Its discharge starting voltage is 13.5V (determined by the UPS float voltage), and its critical discharge voltage is 10.5V (determined by the UPS inverter termination voltage). In practical terms, this means that a single battery's terminal voltage should not drop by more than approximately 3V. If this 3V drop is entirely attributed to cable loss, a power interruption will result in a complete shutdown of the load, rendering the battery pack ineffective. Some may wrongly assume that the battery pack is malfunctioning, leading to unnecessary resource waste and failed problem resolution. It's crucial to note that the UPS DC bus terminal voltage minus the UPS DC bus terminal voltage (with the charger or DC input switch turned off) yields the line loss value. In general, the line loss value should not exceed 3V.

Key Factors Affecting Line Loss

To minimize line loss, several factors need consideration. The primary factors impacting line loss include:

1. Terminal Contact Resistance: The use of high-quality power connectors, specifically designed for DC terminals, and a well-thought-out installation design can reduce terminal contact resistance significantly.

2. Cable Material: The choice of cable material can affect the value of DC cable loss.

3. Cable Diameter and Length: This is the primary determinant. When calculating DC cable loss, selecting an appropriate cable length is crucial in reducing line loss.