What is State of Health
State of Health (SOH) is the percentage of original capacity remaining in a battery, measuring long-term degradation accumulated over time and usage. A new battery has SOH of 100 percent. Over years of operation, SOH gradually drops due to chemical and mechanical aging mechanisms. End of life is typically defined as SOH reaching 80 percent of original capacity (some standards use 70 percent threshold).
SOH is distinct from SOC (State of Charge). SOC changes constantly during normal operation, while SOH changes slowly over years. SOH represents the underlying capacity available for use; SOC represents how much of that capacity is currently charged.
For solar storage, SOH tracking is critical for warranty enforcement, replacement planning and lifecycle cost calculation. Modern BMS estimate SOH from coulomb counting, internal resistance measurements and voltage characteristics. Customer-facing apps typically display SOH alongside SOC. See how long do solar batteries last in India for how Indian heat above 45°C accelerates the SOH decline that ends a battery’s useful life.
Why SOH matters
Warranty enforcement. Manufacturer warranties reference SOH thresholds.
Lifecycle planning. Predicts replacement timing.
Usable capacity. Determines actual energy available.
Customer expectations. Honest SOH reporting builds trust.
Diagnostics. Slower SOH decline indicates good operation.
SOH trajectories
| Chemistry | SOH at 10 years | End of life |
|---|---|---|
| LFP | 85-92% | 15+ years to 80% |
| NMC | 75-85% | 10-15 years to 80% |
| Lead acid (tubular) | 0% (replaced by 5-7 years) | 5-7 years |
| Sodium-ion | 80-90% | Comparable to LFP |
| Vanadium flow | 95+% | 20+ years |
LFP’s slower SOH decline is one of the main reasons it has displaced NMC in most new Indian residential storage; see LiFePO4 vs NMC solar battery in India for the full chemistry comparison.
Technical deep dive
SOH estimation methods
| Method | Accuracy | Description |
|---|---|---|
| Coulomb counting baseline | ±3-5% | Cumulative discharge vs original capacity |
| Internal resistance | ±5-10% | Resistance increases with aging |
| Open-circuit voltage curve | ±5% | OCV curve shifts with aging |
| Capacity test (direct) | ±1% | Full discharge measurement, disruptive |
| Kalman filter combination | ±2-3% | Combined methods, most accurate |
Factors affecting SOH
| Factor | Impact |
|---|---|
| Cycle count | Each cycle reduces SOH slightly |
| Depth of discharge | Deeper DOD per cycle = faster SOH decline |
| Temperature | High temperature accelerates aging |
| Charge rate (C-rate) | High C-rate accelerates aging |
| Calendar time | Aging occurs even without cycling |
| Manufacturing variation | Cell quality variation |
Typical Indian residential LFP SOH trajectory
| Year | SOH | Notes |
|---|---|---|
| 0 (new) | 100% | Commissioning |
| 1 | 99% | Mild break-in |
| 3 | 96% | Steady decline |
| 5 | 93% | |
| 7 | 89% | |
| 10 | 85% | Mid-life |
| 12 | 82% | |
| 15 | 78% | Approaching end of life |
Assumes proper operation within manufacturer’s recommended DOD, temperature and C-rate.
Warranty implications
Typical LFP residential warranty:
- 10 years OR 6,000 cycles
- Minimum SOH 70-80% at warranty end
- Replacement or refund if breach
Real-world applications
Warranty claim documentation.
Replacement planning.
Lifecycle cost calculation.
Insurance valuation.
Second-life battery sorting.
Best practices
Operate within manufacturer’s DOD and temperature recommendations.
Maintain BMS firmware updates.
Track SOH annually through BMS reports, factoring the results into your next battery sizing calculation for replacement or expansion.
Document SOH at warranty milestones.
Plan replacement budget for expected end of life.
Indian market context
The ALMM does not cover batteries, so it says nothing about SOH reporting. Check the battery pack’s own certificates and datasheet to see whether its BMS reports SOH and how the figure is measured.
Major Indian residential hybrid inverter storage brands (Goodwe, Sungrow, Solis, BYD, Pylon Tech) display SOH in apps.
Warranty enforcement increasingly references SOH measurement.
Second-life market emerging for retired EV batteries with SOH 70-80%.
Standards and certifications
| Standard | Scope |
|---|---|
| IEC 61960 | Performance of lithium cells |
| IS 16893 (Part 2) | Lithium-ion cells for electric road vehicle propulsion; not a stationary-storage standard |
| IEEE 1188 | VRLA battery maintenance |
| Manufacturer warranty documents | SOH thresholds |
Key takeaways
SOH (State of Health) measures long-term battery degradation as percentage of original capacity. Distinct from SOC (immediate charge level). Indian LFP residential systems: SOH 85-92% at 10 years, end of life (80% SOH) at 12-15+ years. Modern BMS estimate SOH; warranty references it. Operating within manufacturer’s recommendations preserves SOH and extends life.
Further reading
For how State of Health plays out in real projects, these guides go deeper:
- BMS in Hybrid Solar Inverters, Decoded
- Best Solar Battery Brands in India 2026
- Solar Battery Price Guide India 2026
- Satellite Roof Measurement for Solar: 2026 Guide
Sources
- IEC 61960, Performance of lithium cells.
- IS 16893 (Part 2), BIS (secondary lithium-ion cells for electric road vehicle propulsion).
- IEEE 1188, VRLA maintenance.
- Manufacturer BMS specifications from Pylon Tech, BYD, Goodwe.
- Plett G. L., Battery Management Systems Vol II, Artech House.
- NREL Battery Lifetime Analysis.
- BloombergNEF Battery Storage Outlook.
- IEA Energy Storage Outlook 2024.
- IRENA Battery Storage publications.
Frequently Asked Questions
What is SOH in simple words?
What does SOH stand for?
SOH vs SOC?
When is end of life?
How is SOH measured?
Does SOH drop linearly?
What affects SOH?
Can SOH recover?
What is the typical SOH after 10 years?
Does cold storage preserve SOH?
Is SOH covered in warranty?
Can I see SOH in the app?
Does SOH affect usable capacity?
Is SOH accurate?
Should I replace battery at 80% SOH?
From the Blog
Co-founder & CEO driving Qbits Energy's product strategy and growth across India.