Considerations for Selecting a Lithium-ion Battery System for UPSs and Energy Storage Systems

Overview

The paper explains the variables that drive lithium-ion battery system decisions: cathode chemistry (NMC, LFP, NCA, LTO), C-rate, depth of discharge, cycle life, calendar life, round-trip efficiency and temperature. The common cathode chemistries for UPS and energy storage systems are named as NMC, LFP and NCA. The text aims to prepare buyers for vendor trade-offs rather than to give a design procedure.

An important section shows that short-runtime UPS discharges extract only a fraction of a lead-acid battery's nominal energy. Lead-acid energy extraction is about 100% at the 20-hour rate but about 33% at the 5-minute rate. A lead-acid battery priced at $100/kWh with 33% extraction is therefore effectively about $300/kWh, which changes simple cost comparisons. Comparisons across chemistries and vendors therefore need to use the same assumptions throughout, or they can mislead.

The end-of-life definition also affects comparison: typically 80% of initial capacity is used in automotive applications and 70% in stationary ones. A UPS loaded at 50% gets more than double its full-load runtime (for example more than 20 minutes). A limit is that the paper gives no numerical design method for a specific product or system, so results must be checked against chemistry and manufacturer data.

Key points

1. End of life: typically 80% of initial capacity (automotive) and 70% (stationary).
2. Lead-acid energy extraction is about 100% at the 20-hour rate but about 33% at the 5-minute rate.
3. Example: $100/kWh lead-acid at 33% extraction is effectively about $300/kWh.
4. A UPS loaded at 50% gets more than double the full-load runtime (for example more than 20 minutes).
5. Common cathode chemistries for UPS and ESS: NMC, LFP, NCA.

Using it in design

• Use C-rate-dependent usable energy (Peukert-like) in the battery runtime calculator rather than nameplate kWh.
• Add catalogue fields: chemistry, maximum C-rate, cycle life, calendar life, round-trip efficiency, end-of-life percentage.
• For dual-use UPS/BESS, make cycle life a required input.

Who should read it

Buyers and designers selecting a lithium-ion battery system for UPS and energy storage.

Related standards

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

Publication details

Authors

Scott Daniels

Publisher

Schneider Electric

Year

2020

Document number

WP 284

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