1. A new constant-temperature constant-voltage (CT-CV) charging technique is proposed that adjusts the charging current in response to the battery's temperature to reduce charging time without increasing degradation. 2. Experimental results show that CT-CV charging can reduce charging time by 20% compared to constant-current constant-voltage charging while maintaining the same temperature rise. 3. CT-CV charging can also lower the temperature rise by 20% compared to constant-current constant-voltage charging during the same charging period.

1. This work was carried out at my previous affiliation:
STEER Group, Ontario Tech University, Oshawa, Canada
Constant-Temperature Constant-Voltage (CT-CV)
Charging Technique for Lithium-Ion Batteries
Dr. Lalit Patnaik
Senior Fellow, CERN, Geneva, Switzerland
(present affiliation)
Webinar on 15 July 2020
1
IEEE PELS & IES Bangalore Chapter
Can we charge batteries faster without killing them?

2. Outline
2
1. Why lithium-ion batteries?
2. Existing charging methods
3. Proposed CT-CV charging method
4. Experimental setup and results
5. Future work and summary

3. Image: [Loveridge 2019]
3
Why Lithium-Ion Batteries?
Pros
+ High energy density: 250-700 Wh/L
+ High specific energy: 100-300 Wh/kg
+ Good cycle life: 400-1200 cycles
+ Low self-discharge: <3% per month
+ Affordable cost: 150 $/kWh
+ Reasonably safe operation
+ Environment friendly: no Pb and Cd
Cons
- Underutilization
- Capacity fade: 10-20% fade in 500 cycles
- Limited temperature range: 15-35°C
- Thermal runaway

4. 4
Lithium-Ion Battery (LIB) Applications
Image: [Ding 2019]

5. 5
The Nobel Prize in Chemistry 2019
Image: Niklas Elmehed, Nobel Media

6. 6
Charging Methods for Li-ion Batteries
Charging technique Reference
Constant-current constant-voltage (CC-CV) [Cope 1999, Zhang 2006, Hoffart 2008]
Multistage constant current (MCC) [Liu 2005, Liu 2010, Liu 2011, Vo 2015,
Wang 2015, Min 2017, Kodali 2017]
4C-1C-CV [Fernandez 2016]
Pulse current [Chen 2007, Schalkwijk 2007, Savoye 2012]
Sinusoidal ripple current (SRC) [Chen 2013, Bessman 2018*]
Time [minutes]
Charging
current
1C
4C
Time [ms]
Charging
current
1C
2C Continue till
fully charged
Ripple frequency 900-1200 Hz

7. 7
Motivation for New Charging Scheme
Shortcomings of existing methods
• Open-loop method based on manufacturer datasheet
➢ Does not adjust for aging/temperature of cells
➢ Tends to be conservative
• Cell degradation due to uncontrolled heating in poor cooling conditions
Key research question
What charging profile ensures the fastest charging time without hampering cell life?

8. 8
CC-CV charging
• Lower temperature at beginning of CC mode
• Higher temperature towards end of CC mode
• Temperature rise, ΔT = 2-8 °C
CT-CV charging
• Cell is charged faster
• Same temperature rise as CC-CV
• Custom current profile for each cell
• Current profile adjusts for cell aging
Closed-Loop CT-CV Charging Concept
Premises:
1. Pumping energy into a battery: If it is not stored, it is dissipated!
2. Cell temperature is a key degradation metric [Amine 2005, Leng 2015, Kabir 2017]
3. Charging current ≤1C for SOC>70% to avoid Li plating [Zhang 2006]

9. 9
CT-CV Charging: Block Diagram
• Validation of charging technique
➢ First at cell level
➢ Then at pack level
• Large thermal time constant (∼minutes)
➢ Slow control loop permissible

10. 10
PLECS Simulation for Various Rint
• Parameters: Kp = 0.9, Ki = 0.0005, Kd = 0.05, Iff = 0, Tset = 28.5°C, Ci = 30 J/K,
Cs = 55 J/K, Ris = 10 K/W, Rsa = 18 K/W
• Higher Rint ⇒ Faster heating ⇒ Lower charging current
• Exponential approximation of charging current can serve as Iff
• PLECS Model of the Month – June 2018:
https://www.plexim.com/support/videos/mom_june_2018_ctcvcharge
[Gao 2002]
[Chen 2006]
[He 2011]
[Forgez 2010]
Caveat: 1st order model
does NOT work!
Rint
Ploss = Ich
2 Rint
Annotated Rint values are in Ω

11. USB
Keysight oscilloscope
Experimental Set-up for Charging
11
Samsung 18650 cell
LM335
Solid-state sensor
Rigol power supply
USB
Popular form factor:
From laptops to EVs
Scope in
the loop!

12. 12
Experimental Set-up for Charging
• Rigol DP831A programmable DC power supply serves as charger
• 3-channel data acquisition using Keysight DSOX3014T oscilloscope
• USB interface to PC for control and long-time (2 hours) data logging
• Control using MATLAB Instrument Control Toolbox and SCPI commands

13. 13
Experimental Set-up for Discharging
• Turnigy Reaktor balance charger/discharger
• Up to 20 A discharge current
• Appreciable current ripple: 0.5 A (pk-pk)
• Test discharge capacity in order to compare charging techniques
0.5 A

14. 14
Experimental Results: 20% Faster Charging
[Patnaik 2019]

15. 15
Experimental Results: 20% Faster Charging
In most EV applications, charging is done up to 70–80% SOC

16. 16
Experimental Results: 20% Lower ΔT
[Patnaik 2019]

17. 17
Experimental Results for Different Cooling Conditions
• Charging current dynamically adjusts to available cooling
• Better cooling ⇒ Higher charging current in CT mode
⇒ Shorter charging time
• Good cooling: Forced air
• Average cooling: Natural convection
• Poor cooling: Thermally insulated

18. 18
Towards “Sensorless” CT-CV Charging
1.4℃
• Replace temperature sensor with
estimator?
• 1.4℃ temperature error based on
➢ Constant Rint
➢ 2nd order thermal model
• More accurate temperature
estimator required

19. 19
No apparent capacity fade
[Marcis 2020]
(For NMC cells)

20. 20
Comparison with other charging methods
[Liu 2010]
[Chen 2007]
[Chen 2013]
*
*
*Not for the same cell type as other measurements

21. 21
Future Work
1. Testing for different types of Li-ion cells (and beyond?)
• Pouch, prismatic, built-in temperature sensors
• Cathode materials: LFP, LCO, LMO, LTO
2. Cycle-life and calendar-life testing
• Capacity fade / SOH implications
• Thousands of charge/discharge cycles
• Requires automated cell cycle tester
3. Sensorless CT-CV charging
• Detailed modeling: internal resistance, thermal
• Accurate temperature estimation

22. 22
4. CT-CV fast charging
• Requires elaborate cooling systems
➢ Collaboration with thermal experts
• Modeling and experiments on degradation mechanisms
➢ Collaboration with electrochemistry experts
5. Extension to battery pack level
• Integration with Battery Management System (BMS)
• 2S, 4S, … nS configurations
Future Work

23. Summary
1. Closed-loop charging scheme
• Adjusts charging current in response to battery condition
• Corrects for aging and thermal environment
2. Faster charging for given amount of degradation
• 20% lesser time with same ΔT as CC-CV technique
3. Lower temperature rise for given charge time
• 20% lower ΔT with same charge time as CC-CV technique
4. Enables controlled aging/degradation based on value of set
battery temperature
5. Simple, low band-width PID control with feedforward aid
23

24. 24
Thank You Very Much
Merci Beaucoup
Twitter: @lalitpatnaik
LinkedIn: lalitpatnaik

25. 25
Relevant Publications
• [Patnaik 2019] L. Patnaik, A.V.J.S. Praneeth, and S.S. Williamson, “A Closed-Loop
Constant-Temperature Constant-Voltage Charging Technique to Reduce Charge
Time of Lithium-Ion Batteries,” IEEE Transactions on Industrial Electronics, vol.
66, no. 2, pp.1059-1067, 2019.
https://ieeexplore.ieee.org/document/8353785
• [Marcis 2020] V.A. Marcis, A.V.J.S. Praneeth, L. Patnaik, S.S. Williamson, "Analysis
of CT-CV Charging Technique for Lithium-Ion and NMC 18650 Cells Over
Temperature Range," IEEE International Conference on Industrial Technology
(ICIT), 2020.
https://ieeexplore.ieee.org/abstract/document/9067186

26. 26
• [Loveridge 2019] M.J. Loveridge, C.C. Tan, F.M. Maddar, G. Remy, M. Abbott, S. Dixon, R. McMahon, O. Curnick, M. Ellis,
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