Lithium Batteries for UPS Applications: Battery Management and Chemistry Choices Pave the Way

Overview

UPS duty is constant-power discharge with typical times of 1 to 10 minutes and high current; batteries are sized to the runtime plus a small margin. The paper compares LMO, LFP, NMC and NCA cathodes for rate capability and safety (oxygen release in thermal runaway). Thermal runaway can reach 300-500 C; LFP does not release oxygen at those temperatures and is the safer choice.

Expected float service life of lithium in high-rate UPS duty is 7-10 years (estimated from cycle testing), with recharge in about five to six hours or less at about 0.55C. The paper also discusses float life, charge time, shelf life and intolerance to over-charge and over-discharge.

BMS functions cover balancing, voltage, current, case temperature, SOC/SOH and disconnect limits. Without balancing, a 12S3P LFP pack showed cells below the 2.0 V minimum after eight cycles and a 0.59 V charge spread, while balanced packs stayed tightly around the 3.65 V float. Cell case temperature should be measured directly because internal air temperature may respond too slowly to detect thermal runaway onset.

Heat can be managed by adding parallel strings, adding series cells (lower current at constant power) or raising the cut-off voltage at high load or ambient temperature.

Key points

1. UPS duty is constant-power with typical 1 to 10 minute discharge at high current; batteries are sized to runtime plus a small margin.
2. Thermal runaway can reach 300-500 C; LFP does not release oxygen at those temperatures and is the safer choice.
3. Expected lithium float service life in high-rate UPS duty is 7-10 years (from cycle testing); recharge in about five to six hours or less at ~0.55C.
4. Without balancing, a 12S3P LFP pack had cells below 2.0 V after eight cycles and a 0.59 V charge spread; balanced packs stayed around the 3.65 V float.
5. Cell case temperature should be measured directly because internal air temperature may react too slowly.
6. Heat can be managed with parallel strings, more series cells (lower current at constant power) or a higher cut-off voltage at high load or ambient.

Using it in design

• Add cathode chemistry, oxygen-release flag, float life, maximum charge C-rate and BMS disconnect limits (V, I, T) to the battery catalogue.
• Verify that UPS charger and discharge current stay within BMS limits, and require case-temperature sensing for Li-ion cabinets.
• Prefer LFP where fire risk is weighted.
• Calculate runtime at constant power with cut-off voltage, and show BMS trip-before-UPS-cutoff warnings.

Who should read it

Designers specifying UPS battery chemistry and BMS requirements, and fire-safety assessors.

Related standards

• IEC 62619
• UL 1973
• UL 9540A
• IEC 62040-1

Publication details

Authors

Webb Burgess

Publisher

Eaton

Year

2018

Document number

WP162003EN

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation