Introduzione
State of Health (SOH). Two simple words, yet in the battery world, they might as well be a secret code. SOH tells you how “fit” your battery is—how close it is to that fresh-from-the-factory condition. Sounds simple, right? But don’t be fooled. SOH is the metric that makes or breaks decisions—from EV resale pricing to second-life energy storage repurposing. After 25+ years working with batteries—from field deployments to 100 kwh battery commercial energy storage systems—here’s a blunt truth: misunderstanding SOH causes 90% of battery-related headaches, from premature failures to overvalued assets.
This post cuts through the jargon and industry buzz. It’s not just another “what SOH means” explanation. We’ll reveal the messy realities, evolving measurement methods, the wild west of SOH data you might be trusting, and yes, even the common myths. By the end, you’ll not only understand SOH—you’ll rethink how you use it.
Batteria da 100 kWh
What Is Battery SOH (State of Health)?
What Does SOH Mean?
At its core, SOH is a snapshot—a percentage showing how much of a battery’s original capability remains. Imagine a brand-new battery is 100% healthy. Over time, that number shrinks as capacity fades and internal resistance rises. SOH isn’t just about capacity loss; voltage response and internal impedance matter too. Think of SOH like your car’s health score—it’s about how well it performs compared to when it rolled off the assembly line.
A quick note—SOH is non the same as SOC (State of Charge), which tells you how full the battery is in questo momento, nor SOE (State of Energy), a related term. Mixing these up is like confusing your car’s fuel gauge with its engine health—they’re completely different stories.
Where SOH Matters Most
SOH isn’t just a technical number for engineers. In the EV market, it drives resale value and warranty terms. Drop below about 70%, and suddenly your battery becomes a liability instead of an asset. The same holds true for sistemi commerciali di accumulo di energia (ESS)—a low SOH could mean safety risks or reduced reliability. And here’s a key point: for second-life batteries, SOH is the gatekeeper. It determines if a retired EV battery pack gets a second life powering homes or if it’s headed straight for recycling. But is SOH always the reliable gatekeeper it’s made out to be? We’ll get to that.
How Is Battery SOH Calculated?
Method 1 – Capacity-Based Estimation
The most intuitive approach: measure how much charge the battery in realtà holds versus its rated capacity. If a battery was rated for 100Ah but now only holds 80Ah, SOH is roughly 80%. This method is widely accepted because it directly reflects usable energy. However, it’s slow and tricky to perform under partial or irregular cycling conditions. It’s also less practical when you need quick field assessments.
Method 2 – Impedance/Resistance-Based Estimation
Tracking changes in internal resistance is common, especially in Battery Management Systems (BMS). As batteries age, internal resistance rises, restricting current flow. This method is fast and can provide real-time insights, which makes it appealing. But temperature swings and load variations can skew results significantly. I’ve seen fleets show “healthy” SOH one day, then plummet the next—ambient temperature was the culprit. Impedance methods are powerful, but results must be interpreted in context.
Hybrid or AI-Based SOH Estimation
Welcome to the future—or the hype zone, depending on who you ask. Modern BMS systems combine voltage curves, temperature data, current profiles, and resistance measurements into AI algorithms that predict SOH dynamically. It’s complex and promising. But these systems aren’t perfect. AI models trained on limited data can misjudge battery life by 20%, sometimes missing hidden faults altogether. It’s an exciting area with huge potential, but don’t blindly trust the black box.
Coulomb Counting Across Charge Cycles
Coulomb counting tracks charge in and out to estimate capacity over time. Most commercial BMS rely on this. It’s elegant in theory but sensitive to sensor drift—errors accumulate if recalibration is skipped. I recall operators believing their batteries were at 95% SOH, only to find real-world capacity closer to 75%. This kind of gap can be catastrophic for planning and operations.
Impedance Spectroscopy and Pulse Testing
Electrochemical impedance spectroscopy (EIS) and pulse testing offer nuanced insights by identifying degradation modes and faults under simulated loads. While these methods are gold standards in controlled environments, they aren’t practical for routine field checks.
Fleet EV Battery with 84% SOH but High Heat Signature
Here’s a real-world example. A fleet EV’s BMS reported 84% SOH—looking solid. Yet thermal imaging revealed hotspots during operation. Deeper analysis showed the SOH metric lagged behind chemical degradation, especially internal short circuits. This mismatch is a ticking time bomb for thermal runaway. SOH gave a false sense of security, proving no single metric tells the full story.
Common SOH Misinterpretations and Risks
SOH Is High, But Battery Still Fails Phenomenon
I call this the “False Hope Syndrome.” Batteries can have decent SOH numbers but still fail due to thermal stress, dendrite growth, or cell imbalance invisible to basic SOH metrics. I’ve witnessed high-SOH batteries suddenly die mid-cycle—frustrating, costly, and dangerous.
Trusting the BMS Blindly
The industry loves the convenience of BMS-calculated SOH. But here’s the dirty secret: these readings can be misleading or outright wrong if not independently cross-checked. In second-life battery markets, where risk tolerance is low, buyers often regret skipping independent diagnostics. Trust but verify.
SOH in the Battery Lifecycle: From Sale to Second Life
SOH in Warranty, Leasing, and Resale Decisions
Battery SOH often underpins resale and warranty policies. OEMs typically set benchmarks around 70%—below which warranties expire or leasing terms change. Insurers use similar thresholds. But these are blunt tools that rarely capture nuanced real-world use or abuse.
How SOH Affects Battery Repurposing (EV to ESS)
Repurposing batteries requires rigorous SOH screening. I remember a project reusing EV batteries at 65% SOH for commercial solar ESS. Initial tests looked promising, but unexpected cycling caused accelerated degradation, reminding us second-life use isn’t just about SOH—application matters.
Conclusione
SOH is the heartbeat of battery health—a critical metric for safety, performance, and value. But don’t take the number at face value. Always ask: Come was SOH measured? Under what conditions? In my experience, a dashboard SOH readout is just the starting point. Dive deeper. Verify. Because batteries don’t lie—but people interpreting them sometimes do.
FAQ
Q1. What’s the difference between SOH and SOC?
SOC tells you how much charge remains in questo momento—like a gas gauge. SOH tells you how healthy the battery is—like engine condition.
Q2. What is considered a “good” SOH value?
Above 80% usually means the battery is healthy. Below 70% signals aging or suitability mostly for second-life uses.
Q3. Can SOH be reset or faked?
Absolutely. Firmware hacks or calibration tricks can inflate SOH readings. Independent testing is the best safeguard.
Q4. How does temperature affect SOH?
High temperatures accelerate chemical degradation and increase internal resistance, skewing SOH unless compensated for.
Q5. Is SOH different across lithium-ion chemistries (e.g., LFP vs. NMC)?
Yes. LFP batteries degrade more slowly but differently than NMC or LCO chemistries, affecting SOH calculations and interpretation.
Q6. Can I rely on SOH alone to determine battery safety?
No. SOH is just one piece of the puzzle. You must also consider cycle count, temperature history, and detailed diagnostics.