BMS-09 — SOH Estimation

1. How Does the Battery Compare to When It Was New?

FACT — SOH (State of Health) expresses how much a battery has degraded relative to its new condition. It’s defined in two main ways: capacity-based and resistance-based.

Why is this needed? SOC answers “how full is it right now,” but a cell’s maximum capacity also shrinks over time. Without SOH, the BMS would always assume the cell’s day-one capacity, which can lead to wrong range estimates and potentially unsafe deep discharges in an aged pack.

2. Capacity-Based SOH

SOH_capacity = C_current / C_nominal × 100 (%)

FACT — Capacity loss (capacity fade) is a reduction in the total charge a cell can store. If a new cell is 100 Ah and it drops to 90 Ah, SOH is 90%.

EXAMPLE — If our hypothetical 112.5 Ah cell drops to 101 Ah with use: SOH_capacity = 101/112.5 × 100 ≈ 89.8%.

Where does this show up in the battery? Capacity loss comes from a reduction in usable lithium at the electrode surface — mainly through the thickening of the SEI (Solid Electrolyte Interphase) layer (irreversible lithium consumption at the anode) and active material loss.

3. Resistance-Based SOH

SOH_power = R_initial / R_current  (a power-capability indicator)

FACT — A rise in internal resistance (power fade) means a bigger voltage drop and more heating at the same current — which reduces power capability. Resistance-based SOH directly affects power limits (SOP).

EXAMPLE — If a new cell’s internal resistance is 1.5 mΩ and it’s now 2.4 mΩ: SOH_power = 1.5/2.4 ≈ 62.5%. Note that this ratio measures a different dimension of degradation than capacity-based SOH — a cell can largely retain its capacity while its resistance rises significantly (or vice versa).

4. Cycle Aging

FACT — Charge/discharge cycles accelerate degradation through stress factors like deep discharge, high C-rate, high temperature, and time spent at high SOC.

How it works: electrodes mechanically expand and contract with every charge/discharge cycle (lithium intercalation); this repeated mechanical stress leads to micro-cracks in the electrode structure and active material loss over time.

5. Calendar Aging

FACT — This is degradation that depends on time and temperature, independent of use. High temperature and time spent at high SOC accelerate calendar aging.

6. The Effect of Temperature

FACT — Temperature is the single most important external factor accelerating both cycle and calendar aging. That’s why thermal management is the key to preserving SOH (see BMS-12).

7. How Is SOH Estimated?

FACT — Capacity-based SOH can be estimated by comparing the capacity measured via Coulomb counting over a full charge/discharge cycle against nominal capacity — though full cycles happen infrequently. Resistance-based SOH can be continuously updated by calculating internal resistance from the voltage change (ΔV/ΔI) during short current pulses.

ASSUMPTION — Real production systems usually combine both methods (partial capacity estimation + pulse resistance) along with model-based approaches (observer structures similar to BMS-08’s); this course only covers the conceptual basics.

8. Realistic Lifetime Estimates: Cycles and Years

“How many years/km does an EV battery actually last?” There’s no single right answer, but you can at least get the order of magnitude. Below are a few references in both cycles and years; each is tagged with how reliable its source is (Officially disclosed / Reported / Academic / Inferred) — none of these are an OEM’s warranty commitment.

Cycle-based lifetime (cycle life)

Chemistry	Range commonly cited in the literature (down to 80% capacity)	Source type
NMC	~1,000–2,000 full cycles	Academic/Industry — general literature
LFP	~2,000–4,000+ cycles (higher in some formulations)	Academic/Industry — general literature

ASSUMPTION — These are general trends; only a manufacturer’s datasheet and independent testing can confirm a specific cell’s actual cycle life.

If we assume our hypothetical pack (96s2p, NMC) sees roughly one full cycle per day, a 1,500-cycle lifetime works out to roughly 4 years of heavy daily use. But in real life people rarely do full cycles — they do partial charge/discharge — so calendar life fills up much more slowly than cycle life. That’s exactly why the BMS tracks cycle and calendar aging separately (Sections 4–5).

Year-based and real-world examples

• Geotab’s analysis of fleet data across thousands of EVs shows most modern vehicles losing under 2% capacity per year on average — meaning well-thermally-managed packs can retain most of their capacity even after 8-10 years. (Reported — independent fleet data; varies by brand/model.)
• Idaho National Laboratory (INL)‘s long-term DOE field tests observed markedly faster capacity loss in some early-generation, air-cooled packs (without active thermal management) — especially in hot climates like Arizona. (Academic/Institutional.) This finding is specific to an older generation of design; the same magnitude of effect hasn’t been confirmed for modern liquid-cooled packs.
• Tesloop, a shuttle operator, reported that Tesla Model S/X vehicles it ran past 300,000 miles (~480,000 km) retained most of their battery capacity. (Reported — the operator’s own data, not independent certification.) This should be read as “reaching very high mileage with good thermal management is possible,” not as a claim about the average user’s experience.

Why isn’t there a single number? Because real-world lifetime is the combined result of cell chemistry (BMS-01), thermal management quality (BMS-12), charging habits (especially fast-charging frequency, BMS-10), and climate. Statements like “model X lasts Y years” don’t mean much unless they specify which of these variables they’re assuming. This is exactly the BMS’s SOH algorithm’s job: not to report a generic average, but to measure and track that specific pack’s real condition in the field.

9. The Difference Between SOC and SOH

FACT — SOC answers “how full is it right now?” (instantaneous, 0–100%); SOH answers “how does it compare to when it was new?” (long-term degradation). SOC changes quickly; SOH degrades slowly.

10. When Things Go Wrong

FACT — Typical things that corrupt SOH estimation: noisy or insufficient current-voltage data, missing temperature compensation, and rare full-cycle opportunities. A wrong (optimistic) SOH can lead the BMS to apply a power/energy limit that’s no longer actually safe — which is why SOH estimates are usually designed to be conservative.

11. How It’s Verified in Production

FACT — SOH models are calibrated through pre-production cell aging tests (long-term cycle and calendar tests, in a lab setting). Real-world data collected as vehicles operate in the field can then be used to monitor the model’s accuracy (post-production monitoring is OEM-specific and isn’t covered in detail in this lesson).

12. How It Connects to Other Systems

FACT — SOH directly affects the SOP calculation’s (BMS-10) power limit, updates usable capacity/range estimation (together with BMS-08), and reflects the long-term effectiveness of the thermal management strategy (BMS-12).

13. Summary

• SOH is tracked through capacity (Ah) and resistance (Ω), measuring two different dimensions of degradation.
• Cycle and calendar aging drive degradation; temperature accelerates both.
• Realistic lifetime estimates should be read in both cycles and years, with the source type noted — there’s no single “correct number.”
• SOH feeds SOP and usable-capacity limits.
• Because a falsely optimistic SOH carries safety risk, estimates are designed to be conservative.

Sources

• Gregory L. Plett, Battery Management Systems, Volume I & II (SOH/degradation modeling).
• Geotab — publicly shared EV battery degradation analyses (independent fleet data; Reported).
• Idaho National Laboratory / US DOE — Advanced Vehicle Testing Activity, long-term EV battery field tests (Academic/Institutional).
• Tesloop (publicly shared high-mileage fleet experience; Reported, not independently verified).

ASSUMPTION — The third-party sources above may be updated or become unavailable over time; this lesson reflects information verified as of its publication date.