Become a Battery Sizing Guru

UPS 101

What is a UPS?Per Wikipedia:An uninterruptible power supply, also uninterruptible power source, or UPS is an electrical apparatus that provides emergency power to a load when the input power source fails. A UPS differs from an auxiliary or emergency power system or standby generator in that it will provide instantaneous protection from input power interruptions, by supplying energy stored in batteries. The on-battery runtime of most uninterruptible power sources is relatively short (only a few minutes) but sufficient to start a standby power source or properly shut down the protected equipment.

This is a UPS

Why is a UPS Needed?

• A UPS is needed for Critical Computer Systems that CANNOT lose power.• Power loss to a computer can be catastrophic.• Lost billing• Lost sales• Lost productivity• Equipment damage

Simplified Diagram of UPS

Normal UPS Power Flow

Loss of Utility - Power Flow

Battery

Sizing

Batteries Are Fun!

Rules for Battery Sizing

Battery Sizing Rules1. UPS battery systems CANNOT be sized for less

than (5) minutes of run-time. NO EXCEPTIONS!

2. The Battery End Cell Voltage is 1.67VPC (volts per cell) when sizing VRLA batteries for 5 to 15 minutes of run-time.

Coup De Fouet EffectUPS battery systems CANNOT be sized for less

than (5) minutes of run-time. Why?

* If you size the battery system for less than (5) minutes, there’s a very good possibility the UPS will shutdown prematurely at this point.

End Cell Voltage Rules UPS systems are TYPICALLY designed to provide 15

minutes of battery run-time at full load. For applications needing run-times in excess of

15 minutes, the “RULES” change. For 15+ minutes of run-time, the end cell

voltage raises to 1.75VPC.

What You Need to Know to Properly Size a UPS Battery System

1. Output kVA rating of the UPS

2. Output power factor of the UPS

3. DC link voltage of the UPS

4. Number of battery strings on the UPS

5. Minutes of battery run-time needed

Customer Supplied Information

1. Battery Manufacturer’s Run-Time Chart(s)

2. Formula to Calculate Battery Run-Time

3. A Calculator

Have These Handy

Battery Run-Time Chart

UPS kVA * power factor = kWB

“The Magic FORMULA”

# of battery cells in 1 string kWB * 1000 = WPC

WPC# of battery strings

= WPC per String

UPS inverter efficiency (90%)

40kVA * 0.8 power factor = 35.6 kWB

Small UPS Example

240 # of battery cells in 1 string 35.6 kWB * 1000 = 148.3 WPC

148.3 WPC(1) # of battery strings

= 148.3 WPC per string

UPS inverter efficiency (90%)

C&D High Rate Max Battery Run-Time

Chart

1.67VPC UP TO 15 MINUTES148.3 WPC per string(1) # of battery string

500kVA * 0.8 power factor = 444.4 kWB

Large* UPS Example

240 # of battery cells in 1 string 444.4 kWB * 1000 = 1,851.9 WPC

1,851.9 WPC(4) # of battery strings

= 463 WPC per string

UPS inverter efficiency (90%)Yep! The Form

ula is the SAME!

* 150kVA and larger

C&D High Rate Max Battery Run-Time Chart

1.67VPC UP TO 15 MINUTES1.75VPC OVER 15 MINUTES

463 WPC per string(4) # of battery strings

C&D High Rate Max Battery Run-Time Chart

1.75VPC OVER 15 MINUTES463 WPC per string

(4) # of battery strings

You Are Now Officially a

Battery Sizing Guru!