Battery Life Explained - How to Prolong the Life of Lithium Batteries

As home energy storage systems grow in popularity and electricity prices continue to increase, more households are installing lithium batteries to reduce energy costs and provide backup power. These batteries are a significant investment, often costing upwards of $10k for a typical 10kWh system, so it is vital to understand how to make the most of this asset. Most home solar battery systems sold today use lithium iron phosphate or LFP cells due to the longer lifespan and very low risk of thermal runaway (fire). There are other lithium cell chemistries available, such as NCA and NMC, which are used in some electric vehicles, but these are rarely used for home storage batteries. For this reason, this article is primarily focused on how to prolong the life of lithium LFP batteries.

While most solar battery manufacturers offer a 10-year warranty, there is confusion over the capacity loss over time and how to ensure the battery lasts up to and beyond the warranty period. To prolong battery life, it’s crucial to know how to maintain and operate lithium battery systems in ways that protect and extend their lifespan. This article explains good battery management practices and delves into the technical considerations behind battery depth of discharge (DOD) and its effect on battery degradation, reliability and lifespan.

Battery Lifespan and Capacity

Common Lithium (LFP) batteries used in most on-grid and off-grid solar systems hold a specific amount of energy (measured in kWh). The battery lifespan is based on the number of charge and discharge cycles until a certain amount of energy is lost. Based on accelerated testing and real-world results, battery lifespan is typically 8 to 15 years, after which 20 to 30% of the original capacity is lost. The rate of capacity loss is influenced by factors like cycling frequency, temperature, and depth of discharge (DOD). In general, less intensive cycling with a shallower discharge leads to a longer lifespan. However, there are some alternative views and controversies about this, which I will discuss.

State of Charge SOC & Depth of Discharge DOD

The state of charge (SOC) is a percentage of how much a battery is charged at any moment, while the depth of discharge (DOD) indicates how much of the battery’s capacity is used in a cycle. For instance, if a 10 kWh battery discharges down to 3 kWh (or 70% of its total capacity), the battery SOC is 30%, and the DOD is 70%. In general, most lithium battery systems are not discharged below 20% SOC to ensure some capacity is left for emergency situations and, in some instances, to ensure the battery is operated within the manufacturer’s warranty specifications.

Battery State of Health (SOH)

State of health (SOH) is a percentage of how much battery capacity is remaining. Battery capacity typically decreases by 1-4% annually, influenced by various factors, such as temperature, charge and discharge rates, and the frequency and depth of discharge. This natural degradation process is often referred to as capacity fade. For example, a 10 kWh battery with 85% SOH after 7 years of use will have 8.5 kWh of usable capacity. Monitoring and measuring SOH is essential in determining a battery’s aging process and assessing whether the battery is within the manufacturer’s specifications. Most batteries with managed BMS units will report the battery SOH to the inverter or battery controller.

Battery End of Life (EOL)

Battery end-of-life (EOF) is when its capacity has declined to a certain percentage of the original rated capacity. The EOL capacity is defined by the battery manufacturer and generally ranges from 60% to 80% SOH after 10 years, depending on the warranty. The warranty document should state that the battery will retain a specific percentage of the original capacity after a certain number of years or after a specific amount of energy throughput (kWh). Once the battery SOH has reached the EOL, the battery is not considered unusable and should still function for several years, but at a reduced capacity. However, some manufacturers may request that the battery be returned after it has reached it’s EOL.

Four Rules to Prolong Lithium Battery Life

All modern lithium batteries contain a battery management system or BMS that monitors the internal battery cell voltages, temperature and charge rates. The BMS also disconnects the battery if it detects a problem or voltage spike. However, the BMS can only do so much, so these four tips will help users extend battery life, improve system reliability and reduce the risk of premature failure.

1. Avoid discharging below 20% SOC

   In general, for daily use, no more than 80% of the total battery capacity should be used, and ideally, the SOC should not be discharged below 20% unless in an emergency. This is to ensure some battery capacity remains in the event of a blackout. Another issue is that deeply discharging an LFP battery can cause the inverter to shut down due to low voltage, especially when under high or surging loads.

2. Avoid Extreme Temperatures

   Prolonged high temperatures above 45°C (113°F) accelerate degradation and possible thermal expansion, while cold temperatures below 0°C (zero) reduce performance and charge rates. Battery systems should be protected from temperature extremes, and charging should be carefully regulated if operated outside the recommended temperature range. While cold temperatures can damage the battery if it is (force) charged too quickly, prolonged high temperatures above 45°C will cause accelerated degradation, no matter the charge rate.

3. Fully charge regularly

   Charge LFP batteries to 100% every 7 to 10 days to top balance the cells and ensure all battery modules are at a similar SOC. However, if the battery is not used regularly, such as in an off-grid vacation home, the battery should not be held at 100% SOC for a prolonged amount of time. In this instance, battery SOC should be reduced to 50 to 60% for LFP batteries. Also, refer to the manufacturer’s guidelines.

4. Manage Charge Rates

   Charging too quickly adds internal stress and can heat the cells, increasing degradation. Generally, the BMS will manage the charge rates to avoid these issues, but the inverter should also be configured correctly so as not to overcharge the battery. Ensure the battery charge rate settings match the manufacturer’s specifications (maximum C rate). To reduce thermal stress, LFP batteries should be charged at a rate of 0.5C or C/2. In basic terms, this means it should take approximately two hours to charge a low battery. For example, a flat battery with 10kWh capacity should be charged at a maximum rate of 5kW for 2 hours.

Controversy - Depth of Discharge Vs Energy Throughput

Most battery manufacturers specify a certain amount of energy throughput is covered under the warranty. The energy throughput is the total amount of energy that can be charged and discharged over the (warranted) life of the battery, and it is not affected by the depth of discharge (DOD). When calculated, this often equates to approximately one full charge-discharge cycle per day over the warranty period. Based on this, some have argued that DOD is less relevant if it aims to use a battery to its full potential. The idea here is that maximising the battery’s energy throughput justifies deeper cycling as long as you can fully utilise the battery, despite the potential impacts on longevity. Below is an example of battery warranty details and expected lifespan according to depth of Discharge (DOD)

Battery warranty details from PowerPlus Energy show the estimated cycle life at various DOD levels. Note the battery warranty is void (N/A) if the battery is drained to 100% DOD.

In addition to some manufacturers’ warranty limits regarding DOD, research shows that high DOD cycling lithium iron phosphate (LFP) batteries, such as discharging down to 5 or 10% SOC daily, accelerate battery wear significantly compared to discharging down to 20 or 25% SOC. In other words, while the total energy throughput might be higher, deeper discharges introduce more degradation and shorten the number of viable cycles. This trade-off becomes far more important when battery life is required to ensure a return on investment, as the payback period for some battery systems can be longer than the batteries’s warranty period. Additionally, deeper discharge cycles result in a greater risk that you will void the manufacturer’s warranty conditions. As explained in more detail later, a further potential problem with deep discharges is the risk that the battery BMS will shut off at low voltage, causing the inverter to shut down and a system failure.

Of course, how the battery is used depends on the user’s goals: prioritising energy delivery may support high DOD, whereas focusing on increased lifespan and reliability benefits from lower DOD. Understanding the right balance is needed to ensure the system performs as expected.

Risks When Deep-discharging LFP Batteries

In addition to reduced lifespan, deep discharging lithium iron phosphate (LFP) batteries pose several risks due to the nature of their voltage curves and the sensitivity of inverters and battery management systems (BMS) to low voltage conditions. Here are the main issues encountered when discharging lithium batteries to very low levels:

Graph highlighting the steep discharge voltage curve towards the end of the LFP battery discharging.

1. Rapid Voltage Drop

   The LFP voltage curve remains relatively flat for a significant portion of the discharge cycle but begins to decline sharply once the battery drops below approximately 20% state of charge (SoC). During this phase, a small drop in capacity translates to a substantial voltage reduction. This can cause efficiency losses in inverters, as they may operate less efficiently near their minimum voltage input range.

2. Risk of BMS Shutdown at Low Voltage

   To protect the battery cells, the BMS monitors voltage levels and will shut down the battery if it detects a dangerously low voltage (often around 2.7 to 2.9V per cell for LFP) or below 44V for 48V battery systems. This protective measure prevents over-discharge and potential cell damage. However, when this happens, the entire battery pack becomes unavailable, which can lead to a complete system shutdown. If the BMS shuts down while under load from the inverter, it will cause a system failure and may be very difficult to restart without an acceptable battery reference voltage.

3. Risk of Inverter Shutdown at Low Battery Voltage

   Inverters have a minimum voltage threshold, often slightly above the BMS’s cutoff voltage, to prevent damage and preserve the inverter’s efficiency. When the battery voltage falls close to this threshold, the inverter may shut down to protect itself, leading to an interruption in power supply to any connected loads. This risk is particularly high during deep discharges, where voltage can fluctuate and momentarily dip below the minimum operating range of the inverter.

4. Risk of a Complete System Blackout

   If both the BMS and inverter shut down, the system may experience a complete blackout, as neither the BMS nor the inverter can initiate a restart without an external power source. In many cases, the BMS will not allow the battery to restart without sufficient charge recovery to bring cell voltages back to a normal level. This inability to restart can be particularly problematic in off-grid systems, where an external charge source, like an AC-coupled solar inverter, may be required to reboot the system and bring the battery voltage to an operational level.

5. Inability to Restart the System After BMS Shutdown

   In deep discharge events where the BMS has completely shut down the battery and is left in a discharged state for a prolonged time, the cell voltage may not recover. If no external power source is available, such as grid power or a backup generator, it may be impossible to initiate a charge, leaving the system inoperable and permanently damaging the battery.

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Technical Analysis - Battery Life Vs Depth of Discharge

Research shows that high DOD cycles cause greater degradation in lithium iron phosphate (LFP) batteries, leading to capacity loss and decreased SOH. The battery's internal structure responds dynamically to each cycle: at higher DOD, materials within the cells experience more stress, leading to cumulative degradation effects. Over many cycles, this results in more pronounced capacity fade compared to lower discharge levels.

As detailed below, there are several well-studied degradation mechanisms that shorten battery life in stationary storage applications, including electrode degradation, where lithium plating on the anode and graphite structure breakdown occur under low state of charge (SoC) conditions. Additional electrolyte decomposition at low SOC is a process that thickens the solid-electrolyte interphase (SEI) layer on the anode surface, leading to increased internal resistance and heat generation. Furthermore, deep discharge cycles induce structural strain within the cell’s crystal lattice that can cause micro-cracks in the active material, reducing its ability to store charge.

Battery Degradation Mechanisms Explained

1. Electrode Degradation

• Lithium Plating and Dendrite Formation: At low charge levels, the lithium ions in the cell’s electrolyte can migrate and plate onto the anode. This lithium plating increases internal resistance and can lead to dendrite formation, which may eventually cause short circuits. This effect is more common at low states of charge (SoC) during deep discharge.

• Graphite Anode Degradation: The anode material, typically graphite in lithium-ion cells, undergoes structural changes with deep discharges. The increased strain on the graphite structure from deeper cycling promotes structural breakdown and results in irreversible capacity loss over time.

• When batteries are discharged deeply, active lithium ions are more likely to become irreversibly trapped in the electrodes. The increased cycling range increases the chance that some lithium ions will not return to the electrolyte, resulting in a gradual loss of capacity (often called capacity fade). The loss of active lithium ions reduces the overall energy that the battery can store, leading to a shorter lifespan and lower performance.

2. SEI thickening and Decomposition

• SEI Layer Growth: The solid-electrolyte interphase (SEI) layer on the anode surface is essential for battery stability, as it protects the anode and regulates ion flow. However, deeper discharges can stress this layer, accelerating its growth and causing it to thicken. As the SEI layer thickens, it consumes electrolyte material and increases internal resistance, leading to more heat generation and accelerated aging of the battery.

• Electrolyte Oxidation and Gas Generation: Lower SoC levels are linked to more aggressive electrolyte decomposition, which results in gas formation within the cell. Gas generation can increase cell pressure, reduce performance, and lead to cell swelling over time.

3. Thermal Mand Heat Generation

• High discharge depths contribute to heat generation within the cell due to increased internal resistance and energy losses. Over time, this sustained heat generation can degrade the cell materials, impacting the battery’s longevity and its safe operation range. Studies indicate that LFP cells, although more thermally stable than other chemistries, still suffer from elevated temperatures under deep discharge cycles, which accelerates aging and capacity fade .

4. Phase Transitions & Strain

• LFP cells undergo slight structural changes in their crystal lattice during charge and discharge cycles. Deep discharges, especially when repeated frequently, exert significant mechanical strain on the lattice structure, leading to micro-cracks and fractures within the active material. These structural changes eventually reduce the cell’s ability to hold and transfer charge efficiently, further decreasing its capacity and energy retention capability.

Further Research and References

Research on lithium iron phosphate (LFP) battery degradation consistently shows that greater depth of discharge (DOD) contributes to accelerated aging, even when total energy throughput is controlled. Below are several peer-reviewed sources that delve into this topic and outline how deep cycling affects LFP cell longevity:

1. Rumpf et al. (2015) - Journal of Power Sources: This study examined aging in LFP cells and found that increased DODs led to faster capacity fade, especially when cycling occurred beyond 80% DOD. It identified that deep discharges stress the cathode structure and electrolyte, contributing to increased impedance and faster aging.

   • Reference: Rumpf, K., et al. (2015). "Cycle Life Analysis of Lithium Iron Phosphate (LFP) Cells." Journal of Power Sources, 282, 296-306.

2. Peterson et al. (2010) - Journal of the Electrochemical Society: Research here focused on LFP batteries' cycling at various DOD levels and confirmed that high DOD accelerates degradation, particularly due to higher structural stress on the active materials and increased likelihood of side reactions at low SoC.

   • Reference: Peterson, S. B., Apt, J., & Whitacre, J. F. (2010). "Lithium-Ion Battery Cell Degradation Resulting from Realistic Vehicle and Grid Duty Cycles." Journal of the Electrochemical Society, 157(10), A1419-A1431.

3. Yang et al. (2019) - Energy Storage Materials: This article studied aging behaviour in LFP cells across varied DODs and concluded that deeper discharge cycles increase mechanical stress in the active materials. The findings underscore how extreme cycling conditions, especially at low SoC, accelerate capacity loss.

   • Reference: Yang, D., et al. (2019). "Degradation Mechanisms of LiFePO4 Batteries Under Different Depths of Discharge." Energy Storage Materials, 23, 566-575.

4. Schmalstieg et al. (2018) - Journal of Energy Storage: This research analyzed long-term cycling data on LFP cells and confirmed that deep discharges contribute to increased capacity fade, specifically highlighting the role of increased internal resistance and active material degradation.

   • Reference: Schmalstieg, J., et al. (2018). "Post-mortem Analysis of Aged LiFePO4 Automotive Cells." Journal of Energy Storage, 17, 365-375.

5. Keil & Jossen (2017) - Journal of Energy Storage: In this study, Keil and Jossen found that limiting DOD, especially by avoiding low SoC cycling, extends LFP battery lifespan. Their work quantified how deeper DOD correlates with greater capacity fade over time, driven by electrode degradation and electrolyte wear.

   • Reference: Keil, P., & Jossen, A. (2017). "Aging of Lithium-Ion Batteries in Electric Vehicles under Different Operating Conditions." Journal of Energy Storage, 6, 125-141.

6. Jae-Hun Kim, Sang Cheol Woo, (2013) - Science direct: Capacity fading mechanism of LiFePO4-based lithium secondary batteries for stationary energy storage