Lithium-ion Starting-Lighting-Ignition Batteries:
Examining the feasibility
Massimo Ceraolo, Tarun Huria
Giovanni Pede, Francesco Vellucci
Department of Energy and Systems Engineering,
University of Pisa, Italy
m.ceraolo@ing.unipi.it
ENEA - Agenzia nazionale per le nuove tecnologie, l’energia
e lo sviluppo economico sostenibile
S. Maria di Galeria, Italy
Abstract- Rapid developments in the lithium-ion battery
technology in the last decade have made it the overwhelming
choice over lead-acid batteries, especially for advanced vehicles
like hybrid and electric vehicles. However, for the traditional
starting-lighting-ignition (SLI) application, the lead-acid
technology continues to be dominant due to its low costs, despite
its shortcomings. This could change in the future as a
consequence of the introduction of newer, cheaper and safer
lithium technologies. This paper examines the feasibility of using
lithium-ion batteries for SLI application in conventional
vehicles, over the lifetime of the vehicle, along with their battery
management and thermal management systems and various
related issues.
Keywords- lithium SLI battery, lithium starter battery
I.
INTRODUCTION
Lead-acid batteries have been the power source of choice
for the automobile electric system since the very beginning of
widespread car usage in the early years of 20th century [1],
[2]. These automotive batteries are today often called ‘SLI’
batteries, named after the simple functions of starting (S),
lighting (L) and ignition (I) that they perform. Their operating
mode is characterised by ‘floating’ in a rather high state-ofcharge with shallow cycling, where full discharge is never
achieved [3], [4]. Presently, 12V lead-acid batteries are the
overwhelming choice for SLI batteries, primarily due to their
low cost.
Till about a decade ago, a number of researchers compared
the various options for storing electrical energy on-board
automobiles. All concluded that although the lithium ion
battery technology was superior to the existing lead-acid one,
for the near-term at least, the lead-acid battery would
continue to be used for SLI application, primarily due to its
unrivalled cost advantage [5]–[7]. However, recent
innovations in Li-ion chemistry has made it extremely
competitive in markets that are weight sensitive and
inconvenienced by lead acid battery’s need for frequent
maintenance. Companies like A123 Systems, Valence, K2
Battery, M2 Power etc. have also recently introduced lithiumbased batteries in the market as drop in replacements for 12-V
lead-acid batteries. These are interesting proposals that
suggest important advantages over conventional lead-acid
counterparts.
This paper attempts to make a balanced analysis of all the
pros and cons of lithium batteries over the conventional
alternative for the SLI application, thus providing unbiased
978-1-61284-246-9/11/$26.00 ©2011 IEEE
information to the potential purchaser of these new technical
proposals. More specifically, it evaluates the feasibility of
lithium-ion batteries for the SLI application.
The introduction of lithium-ion batteries into the SLI
market has many advantages as it shall lead to a reduction in
battery weight and size, rapidly decrease and stabilise
lithium-ion battery prices, potentially eliminate the need for
battery replacement over the life-time of the vehicle, and
reduce the potential for lead-based environmental pollution.
On the other hand, it would introduce elaborate battery
management and thermal management systems and higher
initial costs. The biggest inhibitor for the vehicle
manufacturers to switch from lead-acid based SLI batteries to
lithium-ion ones is their high cost. Porsche is today the only
vehicle maker to offer this option [8].
The paper is organised as follows: Section II provides an
overview of the SLI application, and highlights the basic
characteristics that must be considered when evaluating a new
battery for this kind of application. Then, in section III, the
different available lithium battery chemistries are evaluated
with respect to the performance requirements listed in section
II. Section IV provides a concluding discussion.
II.
THE SLI APPLICATION
The SLI battery in a vehicle with an internal combustion
engine has two main functions:
• starting function, i.e. starting up the internal combustion
engine; and
• service function, i.e. ensuring an electrical buffer
function between the production (alternator) and
consumption (lighting, air-conditioning, radio etc.) of
electrical energy in the vehicle.
The starting function is characterised by a high electrical
power requirement for a very short duration — typically, 1.55kW for a few seconds (i.e. currents of a few hundred amps
for a 12V battery). The service function, on the other hand,
requires low to medium energy levels for long periods with
repetition (i.e. currents from a few milliamps to several tens
of amps) [9]-[10]. The starting function was the main
function till the 1970s; thereafter the importance of the
service function increased drastically due to the very large
and fast increase in the electrification of existing vehicle
auxiliaries (steering, engine cooling fan, engine control etc.),
and the addition of new loads (more indicator lights, driving
assistance, anti-theft alarms etc), creatiing the need for
increasing the battery size. Some of thesee loads belong to a
new family, the so-called “key-off loads” (such as anti-theft
systems) that draw energy from the batteery even when the
vehicle is parked, and therefore require even
e
larger battery
sizes. Therefore, typical battery capacity has
h moved from 30
Ah to 60 Ah and more, in modern cars, and
a is predicted to
further increase [11]-[13]. The larger thee battery size, the
more competitive are the options to reducce the battery size
and mass, at equal performance.
A. The voltage window
The electrical system voltage, with the ICE operating, is
kept around 14 V rather than at 12 V; annd there is general
agreement that these systems be consideredd to have a nominal
voltage of 14 V [14]. At common ambiennt temperatures, the
lead-acid cell voltage varies between 2.13 and
a 1.65V, leading
to a battery voltage between 9.9 and 12.8 V. An absolute
minimum voltage of 6 to 8 V is consideered acceptable for
automotive applications, in severe conditioons such as starting
the engine at very cold temperatures [15].
Figure 1 shows the voltage variation with
w temperature in
voltage regulators by two different manufacturers.
m
The
voltage output from the regulator is deependent upon the
ambient temperature, as a consequence of the dependence of
the battery’s behaviour on temperature.
the new battery should be as neear as possible to the one used
in current vehicles, both when thhe ICE is running, and when it
is not. Therefore the system vooltage window operating with
any battery that is a candidatte for smooth substitution of
present-day lead-acid batteries must:
m
• deliver a voltage not lower than 9.9V with the ICE off;
• be charged safely over a voltage
v
range from 13 to 15V
with the ICE is in ON state, as per figure 1.
It must be added, however, that
t
slight deviations from the
values reported in figure 1 couldd be acceptable, since they are
compatible with all the car loadds, and should require only fine
tuning of the alternator’s regulation parameters, which is
already common practice [16].
An initial consideration too select a suitable lithium
chemistry can be by just evaluuating the voltage window at
room temperature, and verifyinng it to be between 10 and 14
V, which should ensure the full range of 8 to 15 V required to
be fulfilled at extreme temperaatures. It is also important to
verify that a battery at a volltage well below 15 V (say
between 14 and 14.5 V) at room
m temperature, when charged,
stays charged.
Table 1 provides the voltagee range of a battery with the
number of lithium cells in series, closest to the required range
of 10 to 14 V for SLI applicattions. Lithium iron phosphate
batteries appear most suitabble for SLI usage. Other
chemistries, such as LCO, LM
MO, NMC and NCA do not
appear to be adequate for the purpose. There is insufficient
data to judge LTO.
TABL
LE 1
LITHIUM CELLS MATCHING SYSTEM
M VOLTAGE OF 14 VOLTS FOR SLI
BATTEERIES
Fig. 1: Battery voltage window (V) vs. temperature (°C):
(
of the automobile
alternators by two different manufaccturers
An important limitation on any proposall for substitution of
the present-day lead acid SLI batteries witth a new battery is
that it should not require significant changes in other parts of
the vehicle electric system. Therefore, the voltage window of
No. of cells in
series
Chemistry Type
Material
Cell
Voltage
(Max-NomMin)
[Volts]
Lithium Cobalt
Oxide (LCO)
Lithium
Manganese Oxide
(LMO or spinel)
Lithium Nickel
Manganese Cobalt
Oxide (NMC)
Lithium Nickel
Cobalt Aluminium
Oxide (NCA)
Lithium Iron
Phosphate (LFP)
Lithium Titanate
(LTO)
LiCoO2
(60% Co)
4.2-3.7-2.7
4 cells in series
[16.8-14.8-10.8]
LiMn2O4
4.2-3.7-2.75
4 cells in series
[16.8-14.8-11]
LiNiMnCoO2
(10–20% Co)
4.2-3.6-3.0
4 cells in series
[16.8-14.4-10.8]
LiNiCoAlO2
9% Co)
4.2-3.6-3.0
4 cells in series
[16.8-14.4-10.8]
LiFePO4
3.65-3.2-2.5
Li4Ti5O12
NA-2.4-NA
Voltage (MaxNom-Min)
[Volts]
4 cells in series
[14.6-12.8-10]
5 cells in series
[NA-12-NA]
B. Cold-cranking
Cold cranking amperes (ccaa) refer to the heavy currents
needed for a few seconds, esp. in cold ambient temperatures,
for the ‘starting’ function. As mentioned,
m
the battery should
be capable of providing higgh currents at 6-8V at low
temperatures. Li-ion cells havee great cold-cranking and low
temperature performance [17].
C. Floating operation
The charging regime most often recommended by
manufacturers of lithium ion batteries and most commonly
used is the ‘constant current-constant voltage’ (CC/CV)
approach which foresees a first phase in which the current is
held constant (and the voltage rises), and a second one in
which the voltage is now kept constant and the current
reduces [18].
Lead acid batteries allow safe and reliable operation while
maintained, when charged, “float”: at a constant voltage,
around 2.33-2.5V/cell, where it is able to automatically
compensate for self discharge. This is also the case in SLI
application, where very often, the battery is charged and
subject to float voltage. Indeed, these float voltages are those
reported in fig. 1.
Individual lead-acid cells can absorb the excess current (if
it is not high) activating parasitic reactions: in recombination
batteries this leads to water decomposition (into hydrogen and
oxygen) and subsequent recombination. Thus, all the cells
have a mechanism to intrinsically balance themselves.
In case a Li-battery is subjected to an excessive “float”
voltage, it is not intrinsically able accept this and tends to
overheat and therefore cause safety problems. Its prevention
is required to be built into the battery management system
(BMS), as detailed in section IIIC later. With passive
equalisation, the Li battery should be as adaptable to float
operation as ordinary lead-acid batteries; while with active
equalisation, for safe operation, the float voltage value must
be carefully determined and coordinated through the BMS.
D. Calendar life
Since the SLI application only involves shallow discharges,
what is important for an automotive battery is its calendar life
or shelf life i.e. the time before a fully charged battery, when
not used, needs to be replaced. In geographical locations with
high ambient temperatures, such as South Europe, shelf life
could dominate over cycle life. Due to low ionic conductivity
of the electrolyte, shelf lives of lithium batteries are generally
the same or higher than those of flooded lead acid cells [6],
[13].
E. Environment
The vehicle is expected to operate in temperatures between
-30°C to +60°C [19]. Li-ion cells can safely operate in the
required temperature range [20]. The battery’s working
environment has a large bearing on its service life, and is
totally dependent upon factors like the ambient climate and
the battery location within the vehicle. The battery’s location
and mounting influences its working temperature and
exposure to vibration. Due to the vehicle vibrations, brazed
joints are susceptible to loosening, and eventually fatigue
failure. Proper battery mounting reduces vibration.
To improve the vehicle’s aerodynamics, the engine
compartments are smaller, and batteries are closer to the hot
components. Sometimes, the battery is shielded using thermal
insulations. These effectively protect the battery from direct
radiation and reduce the peak battery temperatures. However,
they hinder the heat dissipation from the battery. In winters,
the thermal shield prevents the battery from being warmed
from external heat sources. When no thermal shields are used,
the battery heat is dissipated by the air-flow. Batteries are
also located in the passenger compartment, where
temperatures are controlled. With respect to the risk of
leakage, this location is safer for lithium batteries as
compared to flooded lead acid batteries. In case the battery is
installed in the luggage compartment, it requires a longer
cable from the alternator and starter, which increase ohmic
losses. Most often, the hot exhaust pipes are mounted directly
below the trunk and close to the location of the battery, which
then experiences significant heating [4], [19].
F. Cost effectiveness
The passenger car market which forms the bulk of the road
vehicle market operates under conditions of heavy
competition, and is extremely price-sensitive. Manufacturers
are loath to introducing components with higher cost. The
high cost of lithium batteries is the biggest impediment to
their introduction.
Lead acid battery (€100/kWh) is far cheaper than the
cheapest 18650 lithium cells (€210/kWh) and the LFP cells
(€400/kWh). Additional costs are added towards the BMS for
lithium batteries.
Although lithium batteries have 4-6 times deep-cycle lives
compared to lead acid, to make a provisional cost comparison
between lead acid and LFP batteries, floating-charge battery
life has to be evaluated [20]. In the absence of reliable data, it
is reasonable to assume the life of the LFP battery to be at
least thrice that of the lead-acid one. This would match the 10
year life of a conventional vehicle. Hence, against three lead
acid batteries, a vehicle with a LFP battery should normally
not require battery replacements during its lifetime.
To provide a fair comparison, a partially mature market
(when the prices stabilise, after sufficient volumes are
achieved) can be considered for lithium batteries with the
following assumptions for cost estimation:
• the lithium cell’s cost would reduce by a third (from
today’s price of €400/kWh);
• cost of the BMS would be just its hardware cost, as the
high cost of its research and development, would have
been nearly recovered.
Hence, for LFP batteries, a cost (with BMS) of €300/kWh
is assumed. Lead-acid battery costs are heavily dependent on
the market price of lead. Considering the globally high prices
of lead, a cost of €100/kWh for lead acid battery is assumed.
The LFP batteries would not only weigh less, reducing the
propulsion energy by 0.1Wh/(kg.km) [21], and consequently
the fuel; but would also last the lifetime of the vehicle; and
take less space due to the higher energy density (kW/L).
Considering that the propulsion energy comes from an
internal combustion engine, and assuming an overall 27% gas
tank to wheels efficiency, and an annual mileage of 15000
km, at a null rate of interest for money, the weight advantage
of the lithium solution for a 10 year vehicle life works out to
€150-200 per kWh battery at current gas prices. Thus over the
lifetime of the vehicle, the battery costs are likely to be equal.
G. Failure and reliability issues
There are broadly three reasons for failures encountered by
current SLI batteries:
• the voltage regulator, which controls the alternator output
(and usually integral with the alternator), varies the
voltage to the battery, sensing the temperature of the
alternator and not that of the battery. Battery temperature
sensing involves an extra cost and is not carried out.
Since the alternator temperature rises much faster and
higher than that of the battery in short runs (urban
traffic), the regulator reduces the output voltage to the
battery, resulting in no charging in winters, and
overcharging in summers [22] and flat batteries in
vehicles which often make short runs.
• direct exposure to heat and consequent high battery
temperatures leads to low battery service life [23].
• Increased use of ‘key-off’ loads bleed the battery slowly,
leading to problems [2], [4].
Customer satisfaction is increasingly important in the
highly competitive automobile industry. Increasingly,
components are expected to conform to six-sigma (<12ppm
failures) reliability over the operational life of the vehicle,
assumed to be ten years or 240,000 km. Current SLI batteries
do not conform to this standard, and need to be regularly
replaced over the vehicle life. As battery performance is not
monitored, its failure is not predictable. This causes a serious
lack of reliability for the driver, and even a safety risk, if
critical components like brakes are electrified.
Most of these problems with lead acid batteries, would be
solved if the lithium battery is adopted. The battery
temperature is necessarily sensed as an input to the BMS;
hence voltage regulators would behave accordingly. Lithium
batteries have a larger capacity for deep discharges and a
longer life than current SLI batteries; and the lithium SLI
battery is more likely to last the lifetime of the vehicle.
III.
LITHIUM BATTERY OPTION
A. Brief summary of available chemistries
TABLE 2
SUMMARY OF IMPORTANT LITHIUM CHEMISTRIES
Specifications
Deep-cycle life
Operating temp
Specific energy
Specific power
Safety
Thermal
stability
Used since
Manufacturers
LCO
500
Avg
150190Wh/kg
1C
NMC
1000-2000
Good
140Wh/kg
Avg
Low
LMO
500-1000
Avg
100–
135Wh/kg
10-40C
pulse
Avg.
Good
10C
LFP
1000-2000
Good
90–
120Wh/kg
35C cont.
Good
Good
Very Good
Very Good
1994
Sony,
Sanyo
2002
NEC,
Samsung
2003
Kokam,
EIG
1999
A123,
Valence
Li-ion batteries encompass a number of battery chemistries,
with various combinations of anode and cathode materials,
each having their own advantages and disadvantages
regarding safety, voltage, capacity, cost etc. All these
chemistries require battery monitoring and/or management
systems to keep their operation under control, requiring
voltage and temperature monitoring as well as, in most cases,
balancing of the charge of individual cells. Table 2 briefly
summarizes the important characteristics of different lithium
chemistries, taken from manufacturer’s leaflets. More
comprehensive information is available in Appendix A.
B. SLI usage suitability
Although the 18650 cells are the cheapest, they are based
on the LCO chemistry, whose voltage window is not
compatible for SLI application (para IIA above). A 15 °C
increase in temperature and 0.1 V increase in charging
voltage could cut their cell life by half under floating charge
conditions [24]. They exhibit low thermal stability and safety
as compared to the newer LFP based cells. Moreover their
calendar life under float conditions is extremely poor [25],
[26].
The LFP chemistry is most suited for SLI applications, in
view of their matching the voltage window and high safety
and thermal stability. In fact, all the lithium 12V drop-in
replacement battery packs available in the market today are
based on the LFP technology.
C. Thermal and Battery management system
Traditionally the gassing mechanism is the natural method
for balancing a series of lead acid battery cells, without
permanent damage to the cells. Since lithium cells cannot be
overcharged, there is no natural mechanism for their cell
equalisation; they hence need an external battery management
system (BMS) [27], [28]. Moreover, they also need to be
protected against high temperatures (>60°C), and need some
thermal management as well.
Usually, the BMS manages cell-equalisation during the
end-of-charge phase, by selectively discharging the higher
voltage cells in resistors (passive equalisation), or transferring
their charge to lower voltage cells (active equalisation) to
bring all the cells to a common voltage. Under typical ‘float’
conditions, which are most common in SLI applications, the
battery is liable to be continuously (over)-charged; the most
robust operation is with passive equalisation, since active
equalisation might not be able to compensate for an excessive
float voltage. In case active equalisation is adopted, the float
voltage must be carefully matched with the BMS; or the BMS
must be also be able to control the voltage from the regulator.
IV.
CONCLUSIONS
Due to market pressures to reduce vehicle component
costs, the commonly used lead–acid battery, which enjoys
incredibly low costs and a well-established recycling process,
is able to maintain its strong position as the storage system of
choice for SLI applications. Weaknesses of lead–acid battery
performance, such as low specific energy and calendar and
cycle lives, are offset by the low costs. It provides a service
life of 4–6 years, if the voltage regulator of the charger works
correctly and the energy throughput is limited.
This paper investigated the feasibility of lithium batteries
for use SLI application in conventional vehicles, which
requires high current for extremely low durations, and is
mostly a stand-by application. Amongst the various lithium
chemistries, the LFP batteries offer the best match with the
technical requirements and cost, and they are widely
commercially available today.
LFP batteries would entail a higher initial cost, but would
save fuel and would require no battery change subsequently
during the lifetime of the vehicle.
The savings in fuel consumption due to the reduced battery
weight over the lifetime of the vehicle would compensate for
the higher initial cost of the LFP batteries. In the end, they are
expected to be more or less cost-neutral vis-a-vis existing
lead-acid SLI batteries.
They are environmentally friendly, and would cut leadbased environmental pollution. Since SLI batteries account
for approximately half the demand of secondary batteries
today [29], and conventional vehicles are expected to have a
market share of over 50% even in 2020 [30], an early
introduction of LFP-based SLI batteries shall catalyse a drop
in market prices of lithium batteries, as the development costs
would be offset over the large size of the market. Moreover,
they are more reliable, and would eliminate the ‘flat battery’
problem as battery temperatures are inherently sensed for the
BMS.
It must however be noted that the ‘float’ operation, that SLI
batteries experience for most of the time might create some
problems, in case the voltage regulator is not very accurate;
these can be solved by adequate BMS design.
In view of the above, Li batteries could be considered for
replacing lead-acid batteries.
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APPENDIX A: NUMERICAL DATA
TABLE A.1: SOME TYPICAL DATA OF DIFFERENT LITHIUM BATTERY CHEMISTRIES FROM DIFFERENT MANUFACTURERS.
Cell voltage (V)
Manufacturer
A123
K2
Hipower
EIG
Lishen
GAIA
Samsung
Valence
Thundersky
Ah
Max
Nom
Min
3.65
3.65
3.85
3.85
3.85
3.85
3.65
3.65
3.65
3.8
3.8
3.6
3.3
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
2.5
2.5
2.5
2.5
2.5
2.5
2.0
2.0
2.0
2.5
2.5
2.0
14.6
14.6
14.6
21.9
12.8
12.8
12.8
19.2
10
10
10
15
4.0
-
2.8
Cycle
Life
(deep)
Max
discharge
current
Specific
Energy
Wh/kg
Graphite / Lithium Iron Phosphate (LFP)
4.4
2.6
>1000
42°
30
2000
90°
83.5
60
2000
180°
94.1
100
2000
300°
100.9
200
2000
600°
100
14
3000
20C
120
7
3000
30C
95
10.5
18
>1000
396 A (22C)
62
38
>1000
570 A (15C)
84
1.1
Lithium Iron Magnesium Phosphate
40
2500
80A
79
110
2500
150A
89
138
2500
150A
91
69
2500
120A
89
Lithium Iron Yttrium Phosphate
60
>3000
4.2
>6000
Specific
Power
W/kg
417
471
404
500
2500
3200
Shelf Life
(months)
No information
available
2120
1630
No information
available
No information
available
≤3C
Lithium Titanate (LTO)
Toshiba
Kokam
EIG
Samsung
GAIA
LG
Samsung
Panasonic
Sanyo
Lishen
2.4
4.2
4.2
4.2
4.15
4.2
3.7
3.7
3.7
3.65
3.7
4.2
4.2
4.2
4.2
4.2
4.2
4.2
4.2
4.2
3.6
3.6
3.6
3.6
3.6
3.7
3.6
3.7
3.7
Graphite / Lithium Nickel Cobalt (NMC)
2.7
40
>800
200°
158
2.7
70
>800
350°
152
2.7
240
>800
480°
178
3.0
20
1000
10C
175
2.75 20
Graphite / Lithium Nickel Cobalt Aluminium (NCA)
3.0
7.5
>1000
150A (20C)
84
3.0
27
>1000
270A (10C)
99
3.0
45
>1000
450A (10C)
108
3.0
55
>1000
110A (2C)
132
3.0
2.4
500
2C
3.0
2.79
3.0
3.3
3.0
2.9
3.0
2.4
Note: Data taken from the respective manufacturer’s websites and datasheets
No information
available
8A
2300
2340
1910
2080
1460
No information
available
No information
available