IEEE Power Electronics Magazine - March 2022 - 48

Expert View
by Jeffrey VanZwol
Chemistry Options for
Lithium-Ion UPS and BBU
I
ndustrial rack-mounted uninterruptible
power supplies (UPS) or
battery backup units (BBU) are
used in data centers or telecom central
offices to provide backup power
for servers and switching equipment
in the event of power failure. Historically,
these UPS's have relied on
lead-acid as the predominant battery
type. Currently, lithium-ion (Liion)
batteries are gaining market
share over the incumbent lead-acid
battery technology. To differentiate
UPS from BBU power sources, we
define a typical UPS as double-conversion
with ac-dc-ac power conversion,
while BBU is the embedded
battery with dc voltage input and
output. While the input/output of a
UPS is more universal by providing
either single-phase 120-240 VAC or
three-phase 208-480 VAC, the BBU
dc output voltage is typically
matched to the equipment it is powering.
DC powered equipment is
gaining acceptance in multiple
industries for the simplicity, and
efficiency. More and more installations
are relying on BBU's and dc
plants to power their equipment
directly. Figure 1 presents a BBU
and a dc plant installed in a telecom
rack with space for other equipment
that needs a backup power source.
This article presents some of the
considerations and trade-offs when
Digital Object Identifier 10.1109/MPEL.2021.3139254
Date of current version: 17 February 2022
FIG 1 BBU's installed in a telecom
equipment rack.
48 IEEE POWER ELECTRONICS MAGAZINE z March 2022
selecting the Li-ion battery options
for UPS/BBU system for your data
center equipment. There are two
commercial ly viable chemistry
options-Lithium Iron Phosphate
(LFP) and Lithium Nickel Manganese
Cobalt Oxide (NMC).
Two Practical Lithium
Chemistries for UPS/BBU
For typical UPS/BBU applications,
there are two popular Lithium-ion
chemistry variants offered by vendors.
Lithium Nickel Manganese Cobalt
Oxide (NMC) is the most common
Lithium chemistry in the world, as it is
predominantly used in notebooks and
cell phones. Lithium Iron Phosphate
(LFP) is less common in consumer
applications but is used in high cycle
life or high-power applications such as
power drills or industrial electric
vehicles (buses, forklifts). While both
chemistries are based on Li-ion ex -
change, they have some significantly
different performance characteristics.
The five major differentiators for UPS/
BBU applications are energy density,
cycle and calendar life, back-up duration
(i.e., discharge rate), ambient
temperature, and safety.
Energy Density
As detailed in Table 1, NMC chemistry
has a higher gravimetric and volumetric
energy/density ratio than the competing
LFP chemistry. Additionally,
NMC chemistry has a lower cost/watthour
than LFP. As a result, the higher
energy density and lower cost makes
NMC the optimal chemistry for most
lower power consumer applications.
This means that, given a certain volumetric
space allowance, an NMCbased
UPS/BBU will deliver more runtime
than the comparably sized LFPbased
UPS/BBU.
Cycle and Calendar Life
Three main dimensions affect the longevity
of a Li-ion battery-number of
cycles (cycle life), the length of time in
service (calendar life), and the average
ambient temperature. Industry norms
measure the effectiveness of a battery
by comparing the actual capacity relative
to the original capacity of a fresh
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