Battery Technology - May 2021 - 9

Introduction
1000
Silent Mobility + 100 kW DE

Silent Mobility + 30 kW DE

Peak Discharge (C Rate)

Silent Mobility (All Electric drive)

100

10
Electric Compact Car: BMW 13

Electric Mid Size Car: TELSA S

1
1

100

Platform Weight (103 kg)

Figure 1. Battery capacity normalized with platform size to provide silent mobility and directed energy capabilities. (US Army CCDC Ground Vehicle Systems Center)

SElectrolyte

LiFePO4

Electrolyte

SEI

Li+
LixC6

The U.S. Army's pursuit of vehicle electrification is to realize benefits of significant fuel savings/range extension, increased silent watch/mobility, and new
capabilities in Electronic Warfare (EW),
high-power sensors, and Directed Energy
(DE) systems.
The discharge rates for silent mobility
- a 30-kW DE and 100-kW DE capability
using a Hybrid Electric Vehicle (HEV)
configuration - are shown in Figure 1
along with examples of commercial
systems. (Note: Standard industry practice is to define charging/discharging
by C rates. By definition, a 1 C-rate discharge is equivalent to a discharge current that will discharge the entire battery in one hour.)
The silent mobility power requirement
has been normalized for a hybrid-electric
combat vehicle platform weight (3.9
kW/t) and the battery pack is proportionally sized (0.6 kWh/t). Thus, the discharge
rate for the silent mobility capability is
constant across different platform sizes.
This discharge rate can be met using
existing HEV ESS solutions. However, as
the platform size decreases, the ESS discharge rates for DE capabilities increase
significantly beyond standard HEV ESS
solutions. In these pulse power applications, the high-power pulse duty cycles
can have discharge rates that are significantly higher (>10 C) than commercial
HEV ESS systems, resulting in increased
thermal and electrical stress.

Li+

LiS
e-

Graphite

Previous Work
There has been limited published experimental work on high-rate discharge.
However, Wong et al. tested LiNixCoyAl1x-yO2 (NCA) and LiFePO4 (LFP) for pulse
at high rate. For the LFP cells tested at 15
C discharge rate, the rapid cell capacity
decay was attributed to the increase in
cell resistance.
Cell degradation theory and prediction
is critically important to multiple commercial applications and is an active area
of research. Models have been proposed
based on empirical and physics-based
aging mechanisms.
Based on previous work, the cell degrades due to the consumption of active
Li material via solid electrolyte interphase (SEI) growth. As shown in Figure 2,

Figure 2. The desired electrochemical reaction is the lithium intercalation in graphite but lithium can also react with components of the electrolyte to form a solid-electrolyte interphase.
(US Army CCDC Ground Vehicle Systems Center)

the electrolyte reacts and consumes lithium to form an insoluble interface that
decreases cell capacity.

ronmental control at 10 °C for automated
lifetime testing.

Results
Experimental
Based on the use of LFP cells in commercial pulse power applications, such as
power tools with a long lifetime, a 26650
LFP cell was selected.
The cells were attached to an A&D/
BITRODE electronic load with thermocouples affixed to the cell negative tab
and cell skin surface. The cells were then
placed into a thermal chamber for envi-

Battery Technology, May 2021

Figure 3 shows the cell's voltage and
current response to a load profile, as
shown in Table 2, with a 120-A pulse for
two seconds. It can be seen that the cell
can sustain the pulse for six minutes
before it reaches the 2V discharge limit.
The initial capacity with this profile was
1.84 Ah. The cell under a 120-A pulse for
3s shows a similar profile, with an initial
capacity of 1.95 Ah.
9

Cov

ToC

http://www.abpi.net/ntbpdfclicks/l.php?202105TBNAV

Battery Technology - May 2021

Table of Contents for the Digital Edition of Battery Technology - May 2021