What Is SOH in Batteries and Differences with SOC | 2026 – AYAA

Core Metrics: State of Health (SoH) vs. SOC in Battery Management Systems

Among the critical smart BMS features deployed in modern industrial setups, real-time energy tracking (SOC) and aging diagnostics (SoH) form the bedrock of pack safety and operational longevity.

SOC: Real-Time Energy Tracking

SOC shows remaining usable charge (Q_rem) over maximum capacity (Q_max). Unlike State of Health, SOC changes constantly.

Current sensors drift over time. Standard BMS setups suffer from ~5% SOC estimation error. AYAA TECH fixes this issue. Our adaptive EKF algorithms keep SOC error below 3%. This accuracy prevents early system cutoffs under heavy loads.

State of Health: Irreversible Aging Metrics

State of Health tracks permanent capacity loss and resistance growth. It drops monotonically over the pack’s cycle life.

Three factors destroy energy density over time. First, Solid Electrolyte Interphase (SEI) layers grow. This consumes active lithium ions. Second, mechanical stress cracks cathode particles. Third, current collectors oxidize. Together, these degradation mechanisms reduce runtime and risk thermal runaway.

Mathematical Formulations for BMS Algorithms

BMS microcontrollers compute State of Health through two primary equations.

Internal resistance directly dictates terminal voltage under load:

The table below compares key operational traits of SOC and State of Health.

This comparison highlights why engineers must track both metrics simultaneously.

BMS Estimation Pitfalls and Real-World Degradation Drivers

Coulomb Counting Drifts and OCV Mapping Limitations

Shunt sensors suffer from thermal noise and offset drift. Small errors compound during partial charge cycles.

Lithium Iron Phosphate (LFP) cells feature a flat voltage plateau. Between 20% and 90% SOC, voltage barely moves. A tiny 2 mV measurement error creates a 15% SOC calculation error. Without long rest periods, Open-Circuit Voltage (OCV) tables fail.

Stress Factors Accelerating Capacity and Power Fade

Operating outside safe electro-thermal limits speeds up degradation.

• High C-Rates: Fast discharging cracks cathode structures. Fast charging below 0°C causes lithium plating, leading to micro-shorts.
• Thermal Strain: Temperatures above 45°C break down electrolyte solvents. This accelerates SEI expansion and cuts overall cycle life.
• High SOC Storage: Storing cells at 100% SOC accelerates calendar aging. It oxidizes cathode materials even without current flow.

Cell-to-Cell Imbalance and the “Weakest Link” Bottleneck

Series-connected packs age unevenly. Thermal gradients inside battery enclosures cause center cells to run hotter. Hotter cells age faster.

Under load, high-resistance cells suffer steep voltage sag. The BMS triggers under-voltage cutoffs based on the weakest cell. One degraded cell limits the entire pack.

Engineering Integration and Procurement SLA Guidelines

Hardware Integration: Designing for Voltage Sag and Thermal Overhead

Hardware engineers must size protection circuits for elevated R_internal at 80% State of Health. Otherwise, pulse loads trip low-voltage cutoffs.

Aging cells generate excessive I²R heat. AYAA TECH tackles thermal stress through active layout engineering. We position MOSFETs and sampling resistors symmetrically across the board. High-performance thermal pads, conductive gel, and aluminum or copper heat sinks pull heat away fast.

UAV avionics require clean telemetry integration. AYAA TECH Smart BMS designs support open-source flight controllers like PX4 and ArduPilot out of the box. Engineers avoid protocol debugging while streaming real-time SOC, cell voltages, and thermal telemetry over CAN bus.

Procurement Standards: Contractual SLA Terms and Verification Protocols

Vague vendor promises cause warranty disputes. Procurement managers need strict testing definitions.

Specify discharge C-rate, Depth of Discharge (DoD), and ambient temperature (25°C ± 2°C) in agreements. Require dual EOL limits. Demand minimum retained capacity (≥ 80%) and maximum internal resistance growth (≤ 1.5× initial R_DC).

End-of-Life (EOL) Strategy: Transitioning to Second-Life Applications

An 80% State of Health pack is not scrap metal. Heavy UAVs and EVs retire these packs due to weight and peak power limits.

However, stationary energy storage systems (ESS) can reuse them. Repurposed packs excel at grid-tied peak shaving and solar storage. Low discharge rates (<0.3C) mitigate internal resistance, maximizing economic asset value.

Frequently Asked Questions

What is the exact mathematical difference between State of Health and SOC?

SOC measures remaining charge versus current total capacity (Q_rem / Q_max). State of Health measures current capacity versus factory nominal capacity (C_current / C_nominal). SOC changes constantly during use. State of Health drops permanently over time.

Why does a battery pack with 85% State of Health suddenly shut down under high load?

As batteries age, internal DC resistance (R_internal) increases. High discharge current (I) causes severe voltage sag (V_sag = I · R_internal). The BMS cuts power to protect cells when voltage hits the minimum threshold.

How do BMS algorithms calculate State of Health without a full discharge cycle?

Modern BMS hardware uses Extended Kalman Filtering (EKF) and Recursive Least Squares (RLS). These algorithms evaluate partial charge cycles and rest-state OCV readings. Real-time pulse voltage drops also provide instant internal resistance feedback.

Why is 80% State of Health the standard End-of-Life threshold?

At 80% State of Health, cell degradation accelerates non-linearly. Internal resistance spikes, creating severe thermal risks and unpredictable runtimes. Beyond this point, risks of lithium plating and thermal runaway rise sharply.

How does LFP degradation differ from NMC chemistries?

LFP batteries yield longer cycle life (3,000–6,000 cycles) and higher thermal safety, but feature a flat OCV curve. NMC batteries offer higher energy density and simpler voltage tracking, but degrade faster (1,000–2,000 cycles).

What terms avoid warranty disputes in B2B supply contracts?

Contracts must state C-rate, DoD percentage, and testing temperature (25°C). Always include dual EOL targets: capacity retention (≥ 80%) and maximum DC resistance multiplier (≤ 1.5×).

Can elevated internal resistance be restored?

No. Chemical aging mechanisms—such as electrolyte breakdown and active lithium loss—are irreversible. BMS re-calibration adjusts software limits, but physical resistance remains high.