Maximizing NMC Battery Longevity: Why the ABC (Always Be Charging) Rule Isn’t Quite Correct

While the mantra “Always Be Charging” (ABC) might offer convenience and peace of mind by keeping your battery full, the underlying electro-chemistry of Lithium Nickel Manganese Cobalt (NMC) oxide batteries tells a more nuanced story. For optimal long-term battery health, time spent at a high State of Charge (SOC) is the most significant factor to minimize, even more so than the number of charge/discharge cycles.

Only Charge Below 40%

For electric vehicle owners looking to maximize battery longevity, a key strategy is to avoid unnecessarily frequent or shallow charging cycles. Instead of plugging in after driving just a few miles, it is better to wait until the State of Charge (SoC) drops to a lower, yet still healthy, level of about 40% before charging again.

This approach reduces the frequency of shallow charging events and keeps the battery chemistry (especially Nickel Manganese Cobalt, aka NMC) in its most stable and long-lasting state, typically the 20% to 80% mid-range.

BU-808: How to Prolong Lithium-based Batteries

“Cycling in mid-state-of-charge would have best longevity… Charging and discharging Li-ion only partially prolongs battery life but reduces utilization.” (The concept of charging from 40% to 60% fits this “partial charge” model).

Constantly topping up after minimal driving means the battery spends more time at a higher, more stressed voltage, so letting the charge drop significantly before initiating a charging session is a smarter, healthier habit for the vehicle’s battery pack.

The Science of NMC Battery Degradation

The capacity fade and increased internal resistance in NMC batteries are primarily due to two interrelated aging mechanisms, both significantly accelerated by a high State of Charge (SOC):

1. Calendar Aging: The High-SOC Stressor

Calendar aging refers to the degradation that occurs just by the passage of time, regardless of use. This process is drastically accelerated when the battery is held at a high voltage\State of Charge (especially above 80%) and at elevated temperatures.

• Solid-Electrolyte Interphase (SEI) Growth: Stalactites in your battery? Yup. The most critical factor is the continued parasitic reaction between the carbon-based anode and the electrolyte, which forms a film called the Solid-Electrolyte Interphase (SEI). SEI’s look and act like stalactites found in caves… and that is a very bad thing
  • Mechanism: At high SOC/high voltage, the lithium ions are tightly packed into the anode’s graphite structure. This high potential accelerates the reduction of the liquid electrolyte at the anode interface, causing the SEI layer to continuously thicken over time
  • Impact: The growth of the SEI layer consumes two things: active lithium ions (reducing the total available charge capacity) and electrolyte (increasing the battery’s internal resistance). This is a time-dependent, not cycle-dependent, degradation mechanism, making calendar aging worse when a vehicle is sitting unused at 100%

2. Increased Cathode Stress at High SOC

High SOC also increases mechanical and chemical stress on the cathode material (LiNixMnyCozO):

• Structural Instability: When the battery is full, the cathode is highly “delithiated” (most Li+ ions have left the structure). This can lead to mechanical stress and the structural collapse of the NMC lattice, particularly if combined with high temperatures, permanently diminishing the material’s ability to intercalate lithium ions.

Why the ABC Rule Is Misleading

Best Practices for Maximizing NMC Longevity

Based on the science, here are the core principles for maximizing the life of an NMC battery:

1. Daily Small Uses: Don’t plugin to charge is the vehicle has only drive a few miles; wait for it to drop at least 60% before charging again
2. Daily Charging Target: Set your charge limit to approximately 80% for everyday use. This keeps the battery out of the high-stress, accelerated calendar aging zone
3. Minimize Resting Time at Extremes: If you charge to 100% (which is fine for a road trip), start driving shortly after it completes. Do not let the vehicle sit for days at a full charge. Similarly, avoid letting it sit at SOC <20% for extended periods
4. Manage Temperature: High temperatures accelerate all chemical degradation processes. The Battery Management System (BMS) controls this, but avoiding high-power DC fast charging (which generates significant heat) when the ambient temperature is very high is prudent. The charging speed tapers significantly above 80% in most EVs as a direct protective measure against heat and high-voltage stress

Is Slow Charging Better than Fast Charging?

While fast charging generates more heat and poses a slight risk of lithium plating, which degrades batteries, the impact is largely managed by modern Battery Management Systems (BMS) and thermal management. However, for long-term battery health, slow charging is preferred as it allows lithium ions to embed more evenly, minimizing stress.

Ultimately, daily charging habits have the most significant impact on longevity, strongly advocating for the 20%-80% rule for routine use to minimize high-SoC stress and ensure the longest possible battery lifespan.

Why Different EV’s Have Slightly Different Charging Rules

While the general rule of thumb for Nickel Manganese Cobalt (NMC) batteries is to target a maximum of 80% charge for daily driving, Ford’s official recommendation for my 2022 Mustang Mach-E with the Extended Range (NMC) battery is to set the daily charge limit to 90% to reduce strain on the battery.

This slightly higher recommendation is attributed to the vehicle’s Battery Management System (BMS) which hiding a portion of the total battery capacity as a buffer, meaning the displayed 90% is often closer to the battery’s optimal 80-85% State of Charge (SoC).

Keep in mind that newer Standard Range Mach-E may have a Lithium Iron Phosphate (LFP) battery (depending on the model year), which is tolerant of frequent charging to 100%. Ford recommends charging LFP batteries to 100% at least once per month for calibration, but for daily use, keeping it at or below 90% still offers a good balance of range and longevity.

For Further Reading

While we used many sources for this article, we found the following most interesting:

• Understanding Cycle Life vs. Depth of Discharge: A Comprehensive Guide
  Provides a clear table illustrating that reducing the Depth of Discharge (DoD) from 100% to 50% can more than double the estimated cycle life of a battery, emphasizing that a deep discharge places significantly more stress on internal components.
• Real-world study for the optimal charging of Electric Vehicles
  Real-world study results suggest that continuous operation outside the 20%-80% SoC area is “very harmful and dangerous,” and that the fastest capacity fade occurs when batteries are cycled below 25% and above 75%
• Deep vs Shallow Discharge: How Battery Use Affects Lithium-Ion Lifespan
  This article explains that shallow discharges are better for preserving life by reducing wear and tear. Conversely, deep discharge cycles accelerate degradation by placing more strain and increasing the risk of over-discharge damage.
• Does frequently shallow charging speed the aging of Li-ion batteries?
  A discussion that cites the general rule: Shallow charging can actually help prolong the life of Li-ion batteries by reducing stress. The battery life is proportional to the overall charge transferred over its lifetime, not just the number of times you plug it in
• The Truth About Slow vs. Fast Charging and Your Battery’s Lifespan
  “The 20%-80% Rule: For lithium-ion batteries, the ideal charging range is typically between 20% and 80% battery capacity. Charging within this range is efficient and helps ensure your battery’s lifespan isn’t negatively affected.”