Understanding the 40–80 Rule for Lithium-Ion Batteries
ZacharyWilliamWhy keeping most charges between ~40% and ~80% can slow degradation—plus peer-reviewed data, vendor guidance, and lab results you can trust.
What the “40–80 Rule” means
The rule is a practical guideline: on days you don’t need maximum runtime, charge to ~80% and try not to dip much below ~40%. It’s not a strict law—just a way to avoid the two high-stress extremes of very high voltage and deep discharge. Calendar-aging studies consistently show faster degradation at higher SOC and temperature.[1][2]
Data A — Cycle life vs. depth-of-discharge (DoD)
Aggregated lab data show shallower cycles dramatically extend life.
| Depth of Discharge | Approx. cycles (NMC) | Approx. cycles (LFP) |
|---|---|---|
| 100% DoD | ~300 | ~600 |
| 80% DoD | ~400 | ~900 |
| 60% DoD | ~600 | ~1,500 |
| 40% DoD | ~1,000 | ~3,000 |
| 20% DoD | ~2,000 | ~9,000 |
| 10% DoD | ~6,000 | ~15,000 |
Source: Battery University, BU-808, Table 2.
Data C — Lower charge ceiling, longer life
Reducing the per-cell charge limit sharply increases cycle life (with less per-cycle energy). Example estimates:
| Charge ceiling (V/cell) | Typical cycles to ~80% | Approx. stored energy |
|---|---|---|
| 4.20 | ~300–500 | 100% |
| 4.10 | ~600–1,000 | ~90% |
| 4.00 | ~850–1,500 | ~73% |
| 3.92 | ~1,200–2,000 | ~65% |
| 3.85 | ~2,400–4,000 | ~60% |
Data D — Vendors now ship 80–85% limits
- Apple iPhone 15: designed to retain ~80% after 1,000 full cycles; supports “Charge Limit” & “Optimized Battery Charging.”[3][4]
- Google Pixel: “Limit to 80%” and Adaptive Charging in Battery → Battery health → Charging optimization.[5]
- Samsung Galaxy: “Protect Battery / Maximum” stops at 80% (or 85% on some models); Galaxy Book also offers an 85% protection mode.[6][7][8]
Edge case — LFP vs. NMC/NCA
LFP (LiFePO₄) is generally more tolerant to high SOC than cobalt-rich chemistries, with long cycle life and strong safety—yet minimizing dwell time at 100% and temperature is still beneficial.[9][10]
Lab & field snapshots
Practical playbook
- Phones: enable 80–85% limits / optimized charging so it pauses around 80–85% or finishes near unplug time.[4][5][6][7]
- Laptops: use battery-conservation modes (60–80%) if plugged in often; avoid leaving it hot at 100%.
- Storage: park around 40–60% SOC in a cool room; heat + high SOC is the worst combo.[12]
References
- Werner, D. et al., Calendar Aging of Li-Ion Cells—Experimental Investigation and Empirical Correlation, MDPI Batteries (2021).
- Krupp, A. et al., Calendar aging model for lithium-ion batteries considering the influence of cell characterization, Journal of Energy Storage (2021/2022).
- Apple Support — iPhone battery and performance（iPhone 15: 80% after 1,000 cycles），link.
- Apple Support — About Charge Limit and Optimized Battery Charging on iPhone，link.
- Google Support — Get the most life from your Pixel phone battery（“Limit to 80%”），link.
- Samsung Support — ‘Protect battery’ feature in Galaxy phones（Limit to 85%），link.
- Samsung Support — Galaxy battery protection in One UI 6.1（Maximum=80%），link.
- Samsung Support — Galaxy Book Battery（Protect battery 85%），link.
- Battery University — BU-205: Types of Lithium-ion（LFP characteristics），link.
- Battery University — BU-216: Summary Table of Lithium-based Batteries，link.
- NASA Technical Memorandum — Performance and Comparison of Lithium-Ion Batteries Under Low-Earth-Orbit Mission Profiles（>12,000 cycles @ ~40% DoD），PDF.
- Battery University — BU-808: How to Prolong Lithium-based Batteries（DoD, temperature & SOC tables; voltage thresholds），link.
Note: Exact lifetimes vary by cell design, BMS, thermal management, and usage. Treat 40–80 as a useful default and adapt to your device chemistry and the manufacturer’s guidance.
