Why Do Data Center UPS Systems Use Multiple Battery Strings and What Is Their Maintenance Policy?

Uninterruptible Power Supply (UPS) systems are critical in data centers, ensuring continuous power during outages or fluctuations. You might notice that these UPS systems often have multiple strings of batteries, sometimes with 30 to 40 batteries per string and several strings per UPS. This setup raises questions: why so many batteries, and how are they maintained to guarantee reliability? This post explores the reasons behind multiple battery strings in data center UPS systems and outlines common maintenance policies to keep these batteries in top shape.

Multiple battery strings in a UPS room at a data center

Why Multiple Battery Strings Are Used in Data Center UPS Systems

Ensuring Sufficient Backup Time and Capacity

Data centers require a reliable power backup to keep servers and networking equipment running during power interruptions. A single battery string with a limited number of batteries might not provide enough energy to support the load for the required duration. By using multiple strings, the UPS system can:

• Increase total capacity: More batteries mean more stored energy.

• Extend backup time: Multiple strings allow longer power supply during outages.

• Distribute load evenly: Batteries share the load, reducing stress on individual units.

  
  

For example, a UPS system supporting a 500 kW load might need 5 strings of 40 batteries each to provide 15 minutes of backup power. Each string operates in parallel, ensuring the system can handle the load without overtaxing any single battery.

Redundancy and Reliability

Data centers cannot afford downtime. Multiple battery strings provide redundancy. If one string fails or degrades, the others continue to supply power. This setup reduces the risk of total power loss due to a single battery failure.

Manageability and Scalability

Using multiple strings allows easier management and scalability. If the data center expands or power demands increase, additional battery strings can be added without replacing the entire UPS system. This modular approach saves costs and simplifies upgrades.

Battery Types and Configuration in UPS Systems

Most data centers use Valve-Regulated Lead-Acid (VRLA) or Lithium-Ion batteries in their UPS systems. These batteries are connected in series within each string to achieve the required voltage and in parallel across strings to increase capacity.

• Series connection: Increases voltage by adding battery voltages.

• Parallel connection: Increases capacity by adding current capabilities.

  
  

This configuration explains why you might see 30 to 40 batteries per string and multiple strings per UPS.

Maintenance Policies for UPS Batteries

Maintaining UPS batteries is crucial to ensure they perform when needed. Battery failure is a common cause of UPS downtime, so data centers adopt strict maintenance policies.

Battery Monitoring Systems

Many modern UPS systems include battery monitoring systems (BMS) that continuously track battery health. These systems measure:

• State of Charge (SOC): How much energy remains.

• Voltage and current: To detect anomalies.

• Temperature: Batteries perform poorly or degrade faster if overheated.

• Internal resistance: Indicates battery aging or damage.

  
  

The BMS can alert technicians to potential issues before failure occurs, enabling proactive maintenance.

Regular Testing of Batteries

In addition to monitoring, periodic manual testing is essential. Testing methods include:

• Discharge testing: Simulates a power outage to check if the battery can deliver the required current and duration.

• Capacity testing: Measures how much charge the battery can hold compared to its rated capacity.

• Voltage and resistance testing: Identifies weak or failing batteries.

  
  

Technicians use specialized battery testers to perform these tests on each battery or string. Testing schedules vary but often occur every 6 to 12 months.

Visual Inspections and Environmental Controls

Technicians also perform visual inspections to check for:

• Corrosion on terminals

• Swelling or leakage

• Loose connections

  
  

Maintaining proper environmental conditions is vital. Batteries should be stored in temperature-controlled rooms, typically between 20°C and 25°C with low humidity to prevent corrosion.

Replacement Policies

Batteries have a limited lifespan, usually 3 to 5 years for VRLA and longer for lithium-ion. Data centers plan battery replacements based on:

• Age

• Test results

• Manufacturer recommendations

  
  

Replacing batteries before failure avoids unexpected downtime.

Practical Example of Battery Maintenance in a Data Center

A large data center with a 1 MW UPS system might have 5 strings of 36 batteries each. The maintenance team uses a battery monitoring system that sends real-time data to the facility management software. Every year, they perform discharge tests on each string and replace batteries showing more than 20% capacity loss. This approach has reduced UPS-related outages by 90% over five years.

Summary

Data center UPS systems use multiple battery strings to provide sufficient backup power, improve reliability, and allow scalability. Each string contains many batteries connected in series, and multiple strings operate in parallel to meet voltage and capacity needs. Maintaining these batteries involves continuous monitoring, regular testing, visual inspections, and timely replacements. These practices ensure the UPS system remains ready to protect critical data center operations during power interruptions.