Calculate battery health percentage from current and original capacity, or find the missing value by entering any two fields in mAh or Ah.
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Battery Health Formula
The following formula is used to calculate battery health percentage, also known as State of Health (SOH).
- H = battery health percentage (SOH)
- C_c = current maximum capacity (mAh or Ah)
- C_o = original rated capacity when new (mAh or Ah)
This is the capacity-based SOH method. Two other methods exist for advanced applications: internal resistance-based SOH (requires impedance measurement equipment) and electrochemical impedance spectroscopy (laboratory use only).
What is Battery Health?
Battery health (State of Health / SOH) is the ratio of a battery’s current maximum charge capacity to its original rated capacity. A new battery starts at 100%. Every charge cycle, high temperature exposure, and deep discharge permanently reduces this number through electrochemical degradation. Unlike State of Charge (SOC), which measures how full the battery is at a given moment, SOH measures permanent, cumulative capacity loss that cannot be recovered by charging.
SOH Replacement Thresholds by Device Type
| Device Type | Replacement Threshold | Notes |
|---|---|---|
| iPhone / iPad | 80% | Apple’s official threshold; iOS may throttle CPU performance below this |
| Android smartphones | 75-80% | Varies by OEM; most display health warnings below 80% |
| Laptop batteries | 70-75% | Runtime is typically halved relative to original by this point |
| Electric vehicles | 70-80% | Industry EOL standard; impacts range and regenerative braking efficiency |
| Grid / solar storage | 60-70% | Economics-driven threshold; capacity contracts commonly warranty to 80% |
Why 80% Is the Standard Threshold
Battery degradation is not linear. Most lithium-ion cells follow a curve with a gradual, nearly linear capacity fade from 100% to roughly 80% SOH, followed by a sharp inflection called the “knee point” where degradation accelerates significantly per cycle. The 80% replacement standard exists because it approximates this knee point, not because it is arbitrary. Operating a battery below 70% SOH means running through the steepest portion of the degradation curve, where each additional cycle causes disproportionately more capacity loss.
Battery Chemistry and Cycle Life to 80% SOH
The number of charge cycles before a battery reaches 80% SOH varies dramatically by chemistry:
| Chemistry | Cycles to 80% SOH | Common Applications | Degradation Notes |
|---|---|---|---|
| LFP (LiFePO4) | 3,000-4,000 | BYD EVs, Tesla Standard Range, solar storage | Slowest degradation; most thermally stable |
| NMC (Li-NiMnCoO2) | 1,000-2,000 | Consumer electronics, premium EVs | ~2x faster degradation per cycle than LFP |
| NCA (Li-NiCoAlO2) | 500-1,000 | Older Tesla models, power tools | Fast fade rate; trades longevity for energy density |
| NiMH | 300-500 | Hybrid vehicles, rechargeable AA | Less temperature sensitive than Li-ion chemistries |
U.S. Department of Energy research found that after reaching 80% initial capacity, NMC, NCA, and LFP cells retained mean capacities of 63%, 60%, and 74% respectively, confirming LFP’s superior long-tail durability. Across all lithium-ion chemistries, published studies show a median degradation rate of approximately 0.04% per cycle, with operating temperature and charge cutoff voltage as the two dominant influencing factors.
Factors Affecting Battery Health Loss
| Factor | Impact Level | Quantified Effect |
|---|---|---|
| Storage temperature | Highest | 40°C storage: ~35% capacity loss per year vs. ~4% at 25°C |
| Depth of Discharge | High | 100% DoD: ~500 cycles; 50% DoD: ~1,500 cycles (same cell) |
| Charge rate (C-rate) | Moderate | Fast charging above 0.5C increases per-cycle capacity loss measurably |
| Storage State of Charge | Moderate | Long-term storage at 100% SOC accelerates calendar aging; optimal: 40-60% SOC |
| Sub-zero charging | Severe | Charging below 0°C causes lithium plating on anodes, causing immediate permanent capacity loss |
Silicon-anode batteries (common in flagship smartphones) degrade over 2% per cycle below 0°C due to silicon’s volumetric expansion during lithiation. Standard graphite-anode cells typically lose 0.025-0.048% of capacity per cycle under normal operating conditions, consistent with the 2003-era baseline data and confirmed by more recent meta-analyses.