IEEE Power Electronics Magazine - March 2022 - 50

8-10 years. The largest factor that
affects the life of a Li-ion battery is the
average ambient temperature of the
battery, closely followed by the depth
of discharge.
Discharge Rate and Run Time
In general, there are two typical use
models for UPS/BBU solutions. The
first application is when the data center
operator plans to use the UPS/
BBU for a 5 to 10 minute interval, specifically,
the interim period from
power loss until the back-up generator
is started. In this scenario, the
UPS/BBU is sized to deliver all available
energy at a very high rate. The
second application is when the data
center operator plans to use the UPS/
BBU for a 1 to 8 hour discharge and
avoids starting the backup generator
until it is absolutely required. In this
application, the UPS/BBU is the primary
source of power for a much longer
period.
LFP has a very low internal
impedance and can be discharged at
very high rates without generating
much heat. NMC has an internal
impedance about 10x higher than
LFP and will generate more heat
internally when discharged at the
same rate of a comparable LFP battery.
Power delivery is a critical factor
in the UPS/BBU configuration. To
deliver a specific amperage profile,
multiple NMC batteries may need to
be connected in parallel to reach the
needed amperes (A), while a single
LFP battery may deliver the required
A. Given a specific volume constraint,
this means that a NMC battery
is more appropriate for applications
that can tolerate lower current
delivery over a longer discharge period
(i.e., one hour or more), while
LFP can also accommodate a higher
current delivery over a shorter discharge
period (i.e., 15 minutes or
less). Unlike typical lead-acid batteries,
LFP batteries are not affected as
significantly by the discharge rate,
and will deliver nearly the same
amount of energy regardless of the
discharge rate. The expectation for
back-up run time is a critical factor
in selecting the battery chemistry
and sizing the battery.
Ambient Temperature
The final dimension that affects the
longevity of Li-ion batteries is the
ambient temperature. Both, LFP and
NMC Li-ion batteries can safely operate
between −20 and +60°C, but the
optimal temperature range to maximize
the battery calendar and cycle
life is between 10 and 30 °C. High current
discharges will create internal
heating within the cell, but a stable
temperature environment maximizes
the battery's life. Active cooling within
the BBU ensures that internally generated
heat is exhausted into the equipment
rack and data center. As detailed
in Figure 2, the LFP battery that operates
at 23 °C has a much longer cycle
life than the same LFP battery operating
at 45 °C.
Safer Battery
LFP Batteries have a more stable cathode
material than NMC batteries and
do not decompose to provide an oxidation
source at higher temperatures.
In effect, LFP batteries provide the
best thermal and chemical stability,
which results in superior safety over
NMC batteries. As detailed in Table 1,
a LFP battery will only enter a thermal
runaway condition at 195 °C and
release minimal energy during thermal
runaway. A typical NMC battery
can enter thermal runaway condition
as low as 170 °C and will release more
energy and burn at a much hotter
temperature. Properly designed Li-ion
batteries with a quality battery management
system are safe, but LFP is
one of the safest Li-ion battery chemistry
available.
As data center operators consider
LFP Cycle Life and Ambient Temperature
120
110
100
90
80
70
60
50
40
30
20
10
Lithium-ion batteries as an alternative
to Lead Acid batteries for their
UPS/BBU, they should scrutinize
the major performance differences
between LFP and NMC-energy density,
cycle and calendar life, back-up
duration and discharge rate, ambient
temperature, and safety.
23 °C
45 °C
0 250 500 750 1,0001,2501,5001,7502,0002,250 2,500 2,750 3,000
Cycles
FIG 2 Cycle life is shortened by higher ambient temperatures.
About the Author
Jeffrey VanZwol is the chief marketing
officer with Green Cubes Technology,
Kokomo, IN, USA. His range of
experience includes engineered
power solutions for OEMs and Fortune
1000 enterprise customers. Prior
to joining Green Cubes, he was a
senior marketing executive at Inventus
Power and Micro Power Electronics,
as well as Alcatel (Newbridge Networks
division) and ADC Telecom.
50 IEEE POWER ELECTRONICS MAGAZINE z March 2022
Rated Capacity (%)
IEEE Power Electronics Magazine - March 2022

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2022
