Download Capacity Decrease vs Impedance Increase of Lithium

Capacity Decrease vs. Impedance
Increase of Lithium Batteries.
A comparative study.
Hartmut Popp; Markus Einhorn; Fiorentino Valerio Conte
[email protected]
AIT Austrian Institute of Technology, Mobility Department, Vienna, Austria.
I. OUTLINE
Batteries are subjected to permanent aging during their lifetime. Literature states that an increase in impedance and a decrease in capacity is
observed during cycling and storage [1]. A method using this phenomenon to improve battery management routines for lead acid batteries was
introduced [2]. In this study the capabilities of the method for lithium ion batteries are investigated.
II. EXPERIMENTAL SETUP
• Two manganese spinel based chemistries
LMO
NMC
SOC range
100 – 0 %
95 – 20 %
Profile
Full cycles
• Three cells per chemistries
• Two different cycling profiles
• Two different temperature ranges
LMO
NMC
Nominal capacity
5.2 Ah
37.8 Ah
PHEV profile [3]
Min. voltage
2.8 V
2.7 V
2C
5C
Max. voltage
4.2 V
4.1 V
23°C
45°C
Case
Pouch
Cylindrical
• Capacity checkup with coulomb counting
• Resistance checkup with impedance
spectroscopy or current pulses respectively
Allows comparable results
Max. current
Test temp.
IIIa. LMO CYCLING RESULTS
• About 1000 cycles before capacity is lower
than 70% of initial capacity
• High increase in impedance compared
to decrease in capacity
• Dentrite growth observed at final stages
of cycling
IIIb. NMC CYCLING RESULTS
• About 1100 cycles before capacity is
lower than 70% of initial capacity
• Low increase in impedance compared to
decrease in capacity
• Performance recovery after 6 weeks of
pause in cycling
IV. DISCUSSION
• Dependency between increase in
impedance and decrease in capacity
clearly visible.
• Method can be applied for both cases
even if different chemistries, profiles and
temperatures were used. Aging
progression not relevant.
• Higher accuracy (deviation <5%) achieved
for the NMC cell; Most likely this can be
assigned to more stable measurement
conditions.
• Method can be used for onboard battery
management systems to improve their
accuracy regarding the SOH prediction.
MAIN REFERENCES
[1] J. Vetter et. al., Ageing mechanisms in lithium-ion batteries, Journal of Power Sources, 147(1-2):269 – 281, 2005
[2] K. Muramatsu, Battery condition monitor and monitoring method, 1985
[3] H. Mettlach et. al., Initial cycling and calendar aging test procedures and check ups for high energy li-ion battery cells, Helios FP7 project, 2011