EC Power
200 Innovation Blvd.
State College, PA, 16803, USA
services@ecpowergroup.com
Ph: +1-814-861-6233
System Architecture Design for 48V Li-ion/Lead-Acid Battery for
Micro-Hybrid Application
Introduction
48V micro-hybrid systems have gained much interest
recently as a cost-efficient way to boost fuel
efficiency of a standard vehicle by ~ 10% [1], with a
growing market in Europe and a projected large
market in the US in coming years. In this case study
we use USCAR performance goals for a 48V microhybrid battery pack [2] to design a dual battery (Liion and lead-acid) 48V micro-hybrid pack. In this
case study, we focus on a baseline design and
illustrate the design challenges of the dual battery
architecture. In additional case studies, we will
demonstrate how AutoLionTM can be used to rapidly
asses design modifications to enhance the design of
the dual battery system for size, efficiency, and cost.



Technology Used
 AutoLion-STTM for Li-ion battery
 A beta version of AutoPbA-ST™ for the Leadacid battery

Setup
 A 48V Li-ion battery pack is designed in
AutoLion-ST™, and a 12V Lead-acid (PbA)
battery pack is designed using a beta version of
AutoPbA-ST™. The two packs are connected
using a DC/DC converter in the Simulink
workspace. Final design of the two battery packs
to meet performance standards are as follows:
o Li-ion pack: 1p/12s configuration (nominal
pack voltage ~ 44V); NMC/graphite cell
chemistry; 16 Ah (~ 190 Wh/kg cell-level)
1C/25oC capacity/specific energy, or 8 Ah (~
95
Wh/kg
cell-level)
1C/25oC
capacity/specific energy between the SOC
operating window of 70% and 20% (SOC =
50%) (useable capacity/energy for this
application).
o Lead-acid battery pack: 12V, 170 Ah; ~ 1ft3
= 28 L volume; ~ 100 lb = 45 kg; SOC =
70% (floating).
 For the 48V bus (Li-ion battery pack), we impose
the voltage limits as given by USCAR [2]:
o Upper voltage limit: 52V
© 2014 EC Power, LLC. All Rights Reserved.


o Lower voltage limit during standard
operation: 38V
o Minimum voltage limit during cold crank:
26V.
For the 12V bus (lead-acid battery), we impose
the following limits:
o Upper voltage limit: 15V
o Lower voltage limit: 7.2V
We assume that all loads as given in the USCAR
micro-hybrid goals sheet can be shared between
the PbA and Li-ion batteries. A DC/DC converter
is developed to connect the 12 and 48V buses; we
assume a DC/DC converter efficiency of 100% or
simplicity.
In this case study, we focus on the four primary
performance tests: cold crank, 1s discharge, 5s
regen, and 10s discharge. The 1s discharge and 5s
regen tests have requirements from 25oC down to
-30oC.
For the given Li-ion battery design (cell internal
structure, material loading, etc.), we chose the cell
capacity (cell size), based on the UCAR energy
requirement (375 Wh see table below) and an
operating window of SOC = 50%. By using the
highly energy dense cell sized to meet the energy
requirements of the 48V system, our goal was to
minimize the size of the Li-ion battery pack (most
expensive component of the hybrid battery pack).
However, a higher power cell with large SOC
operating window may prove more beneficial and
may facilitate a smaller Li-ion battery pack size.
This will be explored in future case studies.
A controller was designed to maximize the load
on the Li-ion battery pack while maintaining the
required performance (voltage) limits. In all
simulations carried out in this case study, our
goal was to modify the controller logic to
determine the maximum load that could be
sourced by the Li-ion battery pack, thereby
leading to the smallest PbA battery pack
required.
A summary of all performance requirements used
in this study are given below. Given that the
USCAR requirements are end of life (EOL)
requirements, we have assumed a 30% drop in
power and 20% drop in energy over the life of the
battery; we therefore increased by 30% and 20%
the goals listed on the USCAR spec sheet to get
the standards below:
1
Results
System Load
20
10
1
2
3
4
5
15
13
40
11
38
9
Pack
(V)
42
Controller &
DC/DC converter
36
0
12V PbA Pack
1
2
3
4
VPbA (V)
0
0
44
VLiB
EOL
BOL*
o
Temperature
(
C)
Test
Power (kW) Power (kW)
10s Discharge
25
9
12.86
1s Discharge
25
11
15.71
-30
1.1
1.57
5s Regen
25
11
15.71
-30
1.1
1.57
6 kW for 0.5s, 8.6 kW for 0.5s,
Cold Crank
-30
followed by followed by
4 kw for 4s 5.7 kw for 4s
*We have assumed 30% power fade
System Load (kW)
EC Power
200 Innovation Blvd.
State College, PA, 16803, USA
services@ecpowergroup.com
Ph: +1-814-861-6233
7
5
time (s)
48V Li-Ion Pack
Pack
10
5
0
0
0
0
44
5
10
15
44
15
42
13
40
11
38
9
3
4
5
15
13
40
11
38
9
5
10
1
2
3
4
7
5
time (s)
VPbA (V)
(V)
VLiB
Pack
2
42
36
0
36
0
1
(V)
15
1
VLiB
System Load (kW)
Figure 1: Setup of dual battery system in
Simulink, highlighting AutoLion-ST™, AutoPbAST™, DC/DC converter and controller.
2
VPbA (V)
System Load (kW)
Figure 3: 48V hybrid battery 1s discharge
performance at 25oC; 1s discharge test is
performed at minimum SOC of 0.2 (Li-ion) and
floating SOC of 0.7 (PbA).
Figure 4: 48V hybrid battery 1s discharge
performance at -30oC; 1s discharge test is
performed at minimum SOC of 0.2 (Li-ion) and
floating SOC of 0.7 (PbA).
7
15
time (s)
Figure 2: 48V hybrid battery pack 10s discharge
performance (at 25oC); 10s discharge test is
performed at minimum SOC of 0.2 (Li-ion) and
floating SOC of 0.7 (PbA).
© 2014 EC Power, LLC. All Rights Reserved.
2
20
System Load (kW)
System Load (kW)
EC Power
200 Innovation Blvd.
State College, PA, 16803, USA
services@ecpowergroup.com
Ph: +1-814-861-6233
10
0
0
56
2
4
6
8
10
15
10
5
0
0
10
20
30
40
50
15
44
9
44
0
2
4
6
8
(V)
Pack
40
11
36
9
32
7
10
28
0
VPbA (V)
48
VLiB
Pack
VLiB
11
13
VPbA (V)
(V)
13
52
10
20
30
7
50
40
time (s)
Figure 5: 48V hybrid battery pack regen
performance at 25oC; regen test is performed at
maximum SOC of 0.7 (Li-ion) and floating SOC of
0.7 (PbA).
Figure 7: 48V hybrid battery pack cold crank
performance (at -30oC); cold crank test is
performed at minimum SOC of 0.2 (Li-ion) and
floating SOC of 0.7 (PbA).
System Load (kW)
time (s)
1
Table 1: Summary of maximum percent of system
load that can successfully be carried by Li-ion
battery pack.
Temperature (oC) Max % LiB Load
Test
0
0
Cold Crank
Regen
2
2
4
6
8
10
56
15
1s Discharge
11
10s Discharge
VPbA (V)
VLiB
Pack
(V)
13
52
-30
-30
25
-30
25
25
7%
70%
68%
30%
45%
38%
Analysis, Conclusions, and Benefits
48
9
44
0
2
4
6
8
7
10
time (s)
Figure 6: 48V hybrid battery pack regen
performance at -30oC; regen test is performed at
maximum SOC of 0.7 (Li-ion) and floating SOC of
0.7 (PbA).
© 2014 EC Power, LLC. All Rights Reserved.
 The figures above show that the conditions under
which the Li-ion battery pack shares the
maximum possible load for each test, as
determined by the voltage limits. Note in figure 7
that by the goals sheet, it is acceptable for the Liion battery voltage to drop to 26V. However,
from our analysis, > 7% load on the Li-ion battery
immediately leads to the Li-ion battery pack’s
inability to source the load and a nearly singular
drop in voltage (due to the anode graphite
material poor rate capability at -30oC).
3
EC Power
200 Innovation Blvd.
State College, PA, 16803, USA
services@ecpowergroup.com
Ph: +1-814-861-6233
 Clearly the condition which leads to the worst
utilization of the Li-ion battery is the cold crank
requirement. The poor Li-ion performance
under this condition stems from its inability to
generate high power at -30oC. The Li-ion battery
pack is only able to provide 7% of the required
power during cold crank at -30oC. While this test
was carried out at the minimum operating SOC of
20% (LiB), performing the same cold crank test at
SOC up to 50% yielded similar results (Li-ion
could only source < 10% of required load). The
inability of the Li-ion battery to assist during cold
crank directly leads to the requirement for such a
large and heavy lead acid battery, which as
highlighted in table 1 and the figures above, is
clearly
oversized
for
all
performance
requirements other than cold crank.
 Future case studies will focus on design
approaches to improve the Li-ion battery’s ability
to help source the load during cold crank (-30oC),
which should lead to great minimization of the
micro-hybrid pack size and weight.
 Battery design from an energy requirement
standpoint is fairly straightforward. However,
being able to assess the power capability of the
battery under the wide-ranging temperature and
load conditions required by a given application
such as that for an automotive micro-hybrid
battery pack, requires a thermally-coupled,
physics-based design tool.
 To design from scratch and run all tests given
above for the Li-ion battery pack, the PbA battery
pack, the DC/DC converter, and the entire 48V
micro-hybrid battery pack took approximately 2
hours. Total simulation time for all simulations
above took less than 10 minutes.
References
[1] U. Wang, “Meet The Microhybrid: A New Class
Of Green Cars,” Forbes, January 17, 2012.
[2] USCAR publication, “USABC Goals for
Advanced Batteries for 48V Hybrid Electric Vehicle
Applications,”
http://www.uscar.org/guest/publications.php
© 2014 EC Power, LLC. All Rights Reserved.
4