science

State of health

State of health is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand State of health rather than just read about it. In short: State of health (SoH) is a figure of merit of the condition of a battery (or a cell, or a battery pack), compared to its ideal conditions. The unit of SoH is percent (100% = the battery's conditions match the battery's specifications).

Key takeaways

State of health belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect State of health to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of State of health from memory before moving on to harder problems.

Reference excerpt

State of health (SoH) is a figure of merit of the condition of a battery (or a cell, or a battery pack), compared to its ideal conditions. The unit of SoH is percent (100% = the battery's conditions match the battery's specifications). For example, when the capacity of a new battery is same as the nominal capacity as per the battery specification, it is said to be in optimal health (SoH = 100%). As the battery is further utilized in a device, its health as in its capacity and other useful parameters deteriorate till it reaches the end of life (SoH = ~70-80%). Consequently, such batteries are replaced from regular usage pertaining to their unstable and unreliable performance. Typically, a battery's SoH will be 100% at the time of manufacture and will decrease over time and use. However, a battery's performance at the time of manufacture may not meet its specifications, in which case its initial SoH will be less than 100%. In the domain of electric vehicles, the biggest factors that contribute to battery degradation are driver patterns, driver aggression, climate, cabin thermal dynamics, and infrastructure, with driver patterns and climate being the biggest.

SoH evaluation First, a battery management system evaluates the SoH of the battery under its management and reports it. Then, the SoH is compared to a threshold (typically done by the application in which the battery is used), to determine the suitability of the battery to a given application. Knowing the SoH of a given battery and the SoH threshold of a given application:

a determination can be made whether the present battery conditions make it suitable for that application an estimate can be made of the battery's useful lifetime in that application

Parameters As SoH does not correspond to a particular physical quality, there is no consensus in the industry on how SoH should be determined. The designer of a battery management system may use any of the following parameters (singly or in combination) to derive an arbitrary value for the SoH.

Internal resistance / impedance / conductance Capacity Voltage Self-discharge Ability to accept a charge Number of charge–discharge cycles Age of the battery Temperature of battery during its previous uses Total energy charged and discharged In addition, the designer of the battery management system defines an arbitrary weight for each of the parameter's contribution to the SoH value. The definition of how SoH is evaluated can be a trade secret.

SoH threshold As stated before, the method by which the battery management system evaluates the SoH of a battery is arbitrary. Similarly, the SoH threshold below which an application deems a particular battery unsuitable is also arbitrary; a given application may accept a battery with a SoH of 50% and above, while a more critical application may only accept batteries with a SoH of 90% and above. In general, the first usage of batteries is limited to the SoH values of ~70-80% for electric vehicles and electronic equipment. Typically this relates to instantaneous drops in the supplied voltage, and subsequent inability for the connected power electronics to operate normally.

See also Battery balancer Battery charger Battery fade Battery monitoring Depth of discharge Recovery effect State of charge

References

External links State of Health (SOH) Determination Fuzzy logic estimation of SOH of 125Ah VRLA batteries Impedance Data and State of Health Archived 2012-12-12 at the Wayback Machine

Worked examples

Example 1 — a first encounter with State of health

Start with the simplest possible case. Write down what State of health claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to State of health before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about State of health ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of State of health

In research: State of health appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses State of health in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: State of health is common in secondary-school and first-year university syllabi. It links to neighbouring topics Automotive technologies, Battery charging, Electric vehicle technologies, so understanding it makes those chapters shorter.
In everyday life: Look for State of health outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study State of health in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what State of health means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain State of health out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is State of health in simple terms?

Why does State of health matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study State of health?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on State of health.