Lithium-ion battery life

Lithium-ion battery life
Solar photovoltaic (PV) — Energy Storage Systems (ESS)
What does battery life really mean?
There are two distinct, but inter-related ways that are used to
measure the life of lithium-ion (Li-ion) batteries deployed in
ESS applications for solar photovoltaic (PV) installations –
calendar life and cycle life. Both are important.
Cycle life is expressed in terms of the number of charge and
discharge cycles that can be achieved depending to what level
the battery is discharged – its ‘depth of discharge’ or DOD.
It is influenced by both the DOD and the charging rate.
Cycle life at + 25°C/+ 77°F
Calendar life is simply how long the battery might be expected
to last in terms of calendar years. It is independent of how
much the battery is charged and discharged. But it is influenced
by the state of charge (SOC), which reflects how much charge is
kept in the battery, and its operating temperature.
Expected life for VL Li-ion cells according to temperature
(EOL for capacity loss of 20%)
Number of cycles
10 000 000
1 000 000
100 000
10 000
Expected life (years)
20
1 000
18
16
0
10
20
30
40
50
60
70
80
90
100
Depth of discharge (%)
Energy applications 70% capacity at EOL
14
12
10
8
6
When does a Li-ion battery reach
its end of life (EOL)?
4
2
0
20
30
SOC 100%
40
50
SOC 75%
60
70
Temperature (°C)
SOC 50%
In contrast to other battery technologies, such as lead-acid
batteries, Li-ion batteries do not suffer from ‘sudden-death’
failure. Instead they exhibit a gradual decrease in performance
over their service life. So their end of life (EOL) is defined either
by a reduction in initial capacity (typically 20 to 30%) or increase
in impedance – which is important for power applications. It is
important to remember that a Li-ion battery is not ‘dead’ at its
EOL, it has simply reached a pre-defined measure of ageing.
Saft Li-ion batteries – lifetime tests
in real-world PV applications
Saft VL Li-ion batteries outperform
the competition
The Sol-ion project saw Li-ion batteries deployed for energy
storage in PV systems on the largest scale in Europe to date.
This provided the opportunity for Saft’s Li-ion battery to be
tested by independent research institutes(1) in different charge
and discharge cycle profiles.
A key test focused on the realistic representation of the very
complex and random cycling experienced in a PV application, with
micro-cycles due to PV intermittency superimposed on macrocycles due to energy demand shifting between day and evening.
Tests carried out against other types of Li-ion battery show
that Saft’s VL Li-ion battery technology is superior in both
calendar life and cycle life.
Calendar life time
Residual capacity
110%
100%
90%
Cycle profile defined for testing
80%
SOC (%)
Combined cycles
100
End of life criteria
Further usable area
70%
90
80
60%
200
0
400
600
800
60
1.000
Times / Days
50
Saft Li-ion : 55°C, 85% SOC
Li-ion type 1 : 50°C, 50% SOC
40
Li-ion type 2 : 50°C, 50% SOC
Li-ion type 3 : 50°C, 50% SOC
30
Source: ISEA/RWTH/Bosch
20
10
0
0
60
120
180
240
300
360
420
420
420
420
420
720
Time (min)
Test Conditions:
Temperature = 40 ° C
Charging current: 0.5 C (20.5 A)
Discharge current: 1 C (41 A)
Calendar life testing is usually carried out at higher temperatures
in order to achieve meaningful results in a shorter time frame.
In this case, one year calendar life time at 50°C corresponds
to approximately 5.6 years at 25°C.
Cycling life time
Influence of the cycle depth on Saft’s battery system
Relative capacity
Combined cycles
Macrocycles
Residual capacity
110%
100%
1,05
90%
1,00
0,95
80%
0,90
70%
End of life criteria
Further usable area
0,85
60%
0
0,80
2.000
4.000
6.000
Equivalent full cycles
Saft Li-ion : 35°C, 60% DOD
0,75
0
1000
2000
3000
4000
5000
6000
7000
Equivalent full cycle (#)
Li-ion type 1 : 40°C, 100% DOD
Li-ion type 2 : 35°C, 100% DOD
Source: ISEA/RWTH/Bosch
The tests showed that Saft’s battery had a capacity loss of only 20% after the equivalent
of 6000 full cycles (1 full cycle = 100% of battery energy charged and discharged)
(1) All tests conducted by Institut für Stromrichtertechnik und Elektrische Antriebe
(ISEA) at RWTH Aachen, Germany.
There are considerable differences in the cycle life offered by
the various types of Li-ion cell commercially available for PV
applications. The Saft battery maintains its high capacity,
even after more than 6,000 charge/discharge cycles.
Battery life time predictions – based on 20
years of practical and experimental evidence
Saft
12, rue Sadi Carnot
93170 Bagnolet - France
Tel. : +33 1 49 93 19 18
Fax : +33 1 49 93 19 64
www.saftbatteries.com
The battery life time information presented here draws on Saft’s
20 years broad experience in R&D, technology development
and industrialization of Li-ion chemistry. Combined with Saft’s
capabilities in long term lab-testing, characterization and
modeling, this ensures the complete accuracy and reliability
of the system life time and performance projections.
© Saft – Document N° 21893-2-0514 – Edition: May 2014 – Data in this document is subject to change without notice and becomes contractual only after written confirmation. – Photo credits: Fotolia – R299/A
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