Why temperature and SOC of lithium cells are important

The 600 Ah Winston battery bank on my boat has lost nearly 15% of it’s capacity over the last 5 years. I spent three years in the tropics with temperatures around 30°C and an average SOC estimated at 70% or above. Is that loss of capacity normal?
To answer this question I spent a few days reading all the research papers I could find on the aging process of LiFePo4 lithium cells. I am pleased to share my findings.

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What I got from the research:

Although each research is based on different assumptions, testing procedures and data points, they all show similar results. This is a summary in a simplified form…

Two main aging processes are additive and contribute to capacity loss. The Calendar Aging that takes place whether the battery is used or not, and the Cycle Capacity Loss when the battery is charged and discharged.

Calendar Aging:

Calendar aging is more important at higher temperature and higher SOC

• after 200 days at 100% SOC, the battery loses 60% more capacity at 35°C than at 15°C
  • @ 15°C, capacity loss = 3.5%
  • @ 25°C, capacity loss = 4.4%
  • @ 35°C, capacity loss = 5.6%
    
• Compared to the capacity loss at 100% SOC (test at 45°C):
  • @ 75% SOC, the loss is 25% less
  • @ 50% SOC, the loss is 40% less
  • @ 25% SOC, the loss is half
    
• capacity loss is proportional to the square root of time (if temperature and SOC are constant)
  • if in the first 200 days the capacity loss is 2.5%…
  • after one year capacity loss = 3.5%
  • after 5 years capacity loss = 8%
  • after 15 years capacity loss = 13%

Cycle Capacity Loss:

Cycle Capacity Loss is most important at low temperature and when current and SOC are high

• Cycle capacity loss is lowest at 35°C (excluding calendar aging)
  
• at lower temperature the capacity loss increases sharply:
  • @ 25°C, cycle capacity loss is 10% more than at 35°C
  • @ 10°C, cycle capacity loss is 200% more than at 35°C
  • @ 0°C, cycle capacity loss is > 400% more than at 35°C
    
• at low temperature (close to 0°C):
  • capacity loss increases sharply with charge current
  • charging at high SOC leads to a strong increase in capacity loss
    
• at higher temperature the capacity loss increases only slightly:
  • at 45°C, cycle capacity loss is about 20% more than at 35°C
  • state of charge and current have less influence on cycle aging
    
• capacity loss is proportional to the square root of the number of cycles

In summary

By combining data for the two aging processes we can estimate the capacity loss over time. It is a common practice to consider that the battery has reached its end of life when the capacity loss is above 20% (numbers in red). The following table is based on the hypothesis of 200 full equivalent cycles per year.

SOC	5 years	5 years	5 years	10 years	10 years	10 years	15 years	15 years	15 years
	15°C	25°C	35°C	15°C	25°C	35°C	15°C	25°C	35°C
25%	-10%	-10%	-11%	-14%	-14%	-16%	-18%	-18%	-20%
50%	-12%	-12%	-13%	-17%	-17%	-19%	-22%	-21%	-24%
75%	-15%	-14%	-16%	-22%	-21%	-24%	-27%	-26%	-30%
100%	-21%	-19%	-22%	-29%	-28%	-32%	-36%	-35%	-40%

These numbers happen to correlate quite well with the capacity loss I observed on my battery bank!

It would therefore appear that the optimal temperature to get maximum battery life is…

25°C if the battery is cycled regularly

15°C (or less) if the battery is in storage or used lightly

Loss of capacity increases sharply with battery state of charge

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Guidelines to maximize battery life

It depends on the environment and the usage pattern of the battery:

Hot climate (above 30°C)

• in a boat place the battery in a low position close to the hull where sea water keeps it cooler
• in a house, RV or truck place the battery in the living area where there is an air conditionner or ventilation
• in a vehicle do not place the battery just above the undercarriage where the heat of the road radiates
• add ventilation to the battery compartment – can be controlled (on/off) by TAO BMS
• if that is not enough, place the battery in an insulated compartment and add a small Peltier system to cool it by a few degrees – can be controlled (on/off) by TAO BMS
• do not place chargers and inverters in the same compartment as the battery (they can generate a lot of heat)

Cold climate (below 5°C)

• in a boat place the battery in a low position close to the hull as sea water is often warmer that the air
• in a house, boat, RV or truck place the battery in the living area where there is a heater
• if that is not possible, place the battery in an insulated compartment and add a small heater with a fan – can be controlled (on/off) by TAO BMS
• Do not charge most lithium batteries at temperature below 0°C (some batteries can support it, so check the specs)
• Below 5°C, reduce the charge current significantly (check battery specs) – TAO BMS can control the current supplied by chargers and regulators via CANbus

Light use or storage (cycles once a week or less)

• keep the SOC below 50% – TAO BMS manages the charge cycle to do just that
• keep the battery as cool as possible
• if no power is needed, charge the battery at 50% SOC and disconnect all leads from the battery (including the BMS)

TAO BMS monitors the temperature of each cell, warns you of any abnormal condition
and can control heating / cooling equipment

Other points to watch

At high current the heat can be generated in the battery itself or in the BMS if:

• a cable or cell link connector is not tight enough (but do not over-tighten as you could damage the battery)
• a cell has a high internal resistance
• the BMS has a passive balancing that dissipates heat (most critical for “drop-in” batteries with passive balancer just above the cells)
• the relays you use are solid state (SSR) with high “ON resistance” / high voltage drop that dissipate enormous amount of heat (most critical for “drop-in” batteries with FET’s just above the cells)

All these situations will lead to accelerated aging of one or more battery cells

TAO BMS warns you if there is a difference of temperature between cells

In all situations

Considering that high battery SOC contributes to faster aging:

• keep the average battery state of charge as low as possible
• the battery does not need to be filled up at 100% at every charge
• avoid float charge once the battery is full

TAO BMS has a charge cycle management feature to control the chargers
and keep the SOC within a defined range

Feedback and personal experience is welcome in the comments section or in the discussion on that topic

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References:

1. Comprehensive Modeling of Temperature-Dependent Degradation Mechanisms in Lithium Iron Phosphate Batteries. M. Schimpe, M. E. von Kuepach, M. Naumann, H. C. Hesse, K. Smith,2, A. Jossen
   Journal of The Electrochemical Society, 165 (2) A181-A193 (2018)
   
2. Analysis of Degradation Mechanism of Lithium Iron Phosphate Battery. Genki KANEKO, Soichiro INOUE, Koichiro TANIGUCHI, Toshio HIROTA, Yushi KAMIYA1, Yasuhiro DAISHO1, Shoichi INAMI2 – World Electric Vehicle Journal Vol. 6 – ISSN 2032-6653 (2013)
   
3. The Degradation Behavior of LiFePO4/C Batteries during Long-Term Calendar Aging. Xin Sui, Maciej Swierczynski, Remus Teodorescu, Daniel-Ioan Stroe – Energies 2021, 14, 1732.