SoC and SoH Explained: Understanding State of Charge and Battery Health

Anyone who understands the difference between SoC and SoH charges intelligently, plans their range safely and protects the most expensive component in their vehicle. This guide explains both values clearly and shows you how to use them in everyday driving.

SoC and SoH: your battery's two key metrics at a glance

First, a brief summary of both terms. Picture your traction battery as a glass of water. The SoC (State of Charge) describes how full the glass is right now. The SoH (State of Health), by contrast, describes how large the glass still is at all after years of use. SoC and SoH therefore measure two completely different aspects of the same battery.

The SoC is a snapshot and changes with every journey and every charge. The SoH stays almost constant over weeks and only declines slowly across the years. Both values come together in the battery management system (BMS), which monitors and protects the cells.

What is the State of Charge (SoC) in detail?

The State of Charge indicates the battery's current charge level as a percentage, meaning how much energy is stored relative to the capacity usable at that moment. A high SoC means more range, a low SoC correspondingly less. This makes the SoC the value you keep an eye on constantly in the cockpit.

The SoC is not calculated from a single measurement but through a combination of several methods. The BMS records the flow of current into and out of the battery and additionally evaluates the cell voltage. From this data, the system estimates the charge level as precisely as possible.

Worth knowing: although the range display relies on the SoC, it also takes further factors into account, such as your driving style, the topography, the outside temperature and the consumption of the heating or air conditioning. That is why the predicted remaining range varies even at an identical charge level.

What exactly is the State of Health (SoH)?

The State of Health describes the battery's condition and expresses how much of the original capacity is still available compared with its as-new state. A factory-new battery has, by definition, an SoH of 100%. When the value falls, specialists speak of degradation.

This ageing takes place along two paths at once. Calendar ageing progresses simply through the passing of time, even when the vehicle is standing still. Cyclic ageing arises from repeated charging and discharging. High temperatures and a permanently very high charge level accelerate both processes.

Unlike the SoC, the SoH is rarely shown openly on the display. It sits deeper in the BMS and is not displayed as standard by most manufacturers. That is exactly what makes it so decisive when buying a used vehicle, because a weakening battery gives no sign of itself through sound or through a glance from the outside.

As a rough guide, and following an analysis of around 28,500 measurements on plug-in hybrids, the ADAC quotes the following SoH benchmarks: after about 50,000 km at least 92%, after 100,000 km at least 88%, after 150,000 km at least 84% and after 200,000 km at least 80%. If a vehicle falls well below these, it may indicate an excessively aged battery. Thanks to larger batteries and fewer charging cycles, pure electric cars actually age more slowly on average.

SoC and SoH compared: the difference at a glance

Here is all the information on the difference between the two terms, briefly listed:

Characteristic

SoC (State of Charge)

SoH (State of Health)

Meaning

Current charge level

Battery's state of health

Core question

How full is the battery right now?

How much capacity is left?

Time horizon

Short-term, changes continuously

Long-term, declines over the years

Display

Visible in the cockpit

Usually hidden in the BMS

Reference value

Percentage of current capacity

Percentage of original capacity

Benefit

Estimate range

Assess value and ageing

Why the SoC matters so much when rapid charging

The charge level largely determines how quickly your vehicle takes on power. At a low SoC, the battery charges especially rapidly. From a charge level of around 80%, however, the BMS noticeably throttles the power in order to protect the cells. This progression is known as the charging curve, and anyone who understands it saves valuable time on long journeys.

You therefore charge most time-efficiently when you head for a rapid charger at the lowest possible SoC, ideally between 10 and 30%, and end the session at around 80%. The remaining 20% takes disproportionately long because of the flattening curve. We explain how this curve works technically in our guide to the EV charging curve.

Checking the SoH: how to read out your battery's condition

Because the SoH is rarely displayed directly, you need the right approach to obtain the value. You have these options:

• Manufacturer app or on-board menu: some brands show the battery condition directly, for example through a brand-specific app or a service menu in the vehicle.
• OBD2 adapter with a diagnostic app: via the diagnostic interface, the internally calculated value can be read out with suitable apps.
• Independent battery test: neutral bodies such as the ADAC, TÜV, DEKRA or GTÜ determine the SoH using standardised procedures and issue a reliable battery certificate.

An important note for context: as yet, there is no uniformly standardised measurement procedure. Different providers use different reference values, which is why results cannot always be compared one to one. More transparency is on the horizon, because under the Euro 7 emissions standard newly type-approved vehicles are set to display their battery condition directly in the vehicle in future, and the digital battery passport will later make the data accessible via QR code. If you want to check the SoH specifically before a purchase, read our guide Buying a used EV: how to check the battery.

Protecting SoC and SoH with the right charging behaviour

You can actively influence the SoH through your charging behaviour. Today's SoC affects tomorrow's over the long term. These rules of thumb help your battery:

• Stay between 20 and 80% day to day: this charging range is considered especially gentle, because it avoids the critical voltage peaks in the cells.
• Use full charges deliberately: only charge to 100% when needed, for instance before a long journey, and then do not leave the car standing brim-full for hours.
• Mind the temperature: lithium-ion cells are happiest between roughly 20 and 40 degrees Celsius. Pre-conditioning brings the battery up to operating temperature before rapid charging.
• Ration rapid charging: frequent DC charging places somewhat greater strain on the cells. Modern battery management systems actively regulate this, however, so occasional rapid charging remains unproblematic.

Charge ultra-fast with JOLT!

Anyone who has understood the relationship between SoC and SoH knows that fast yet gentle charging is no contradiction. That is exactly why JOLT set out as a Charge Point Operator (CPO). We bring ultra-fast charging to where you need it in everyday life, right in the heart of the city.

Our charging solutions deliver up to 320 kW of charging power and add around 200 km of range in about 10 minutes. This is made possible by integrated battery storage, which enables ultra-fast charging without costly grid expansion. And because our sites are available around the clock, you top up precisely within the optimal SoC window, whenever it fits into your day.

This is interesting for businesses too: as a JOLT site partner, you enhance your property and attract customers with spending power, entirely without investing in the charging infrastructure yourself.

Conclusion: SoC and SoH belong together

SoC and SoH are two sides of the same coin. The State of Charge shows you how far you will get today, the State of Health how fit your battery is overall. Anyone who keeps both metrics in view charges more efficiently, plans more safely and preserves the value of their vehicle for longer. And to keep your battery fit for as long as possible, charge gently and ultra-fast with JOLT from now on.

Find a JOLT charging station near you now and charge ultra-fast!

Sources

1. ADAC: Electric car battery, service life and warranty. https://www.adac.de/rund-ums-fahrzeug/elektromobilitaet/laden/elektroauto-batterie/
2. ADAC (press): Study on plug-in hybrids, batteries age very differently (SoH benchmarks, around 28,500 measurements). https://presse.adac.de/meldungen/adac-ev/technik/adac-studie-zu-plug-in-hybriden-batterien-altern-sehr-unterschiedlich.html
3. ADAC: Electric car, tips on rapid charging (charging curve, 80% rule of thumb). https://www.adac.de/rund-ums-fahrzeug/elektromobilitaet/laden/elektroauto-tipps-zum-schnellladen/
4. Amperfied: What is the State of Charge (SoC)? (calculation via current and voltage, distinction from the SoH). https://www.amperfied.de/wissen/was-ist-state-of-charge/
5. Autolexicon: SoH (State of Health), nominal capacity and distinction from the SoC. https://www.autolexicon.net/de/articles/soh-state-of-health/
6. Over the Air: Understanding the SoH battery, degradation, calendar and cyclic ageing. https://over-the-air.de/beitrage/soh/soh-batterie-verstehen-was-der-state-of-health-ueber-den-akku-eines-elektroautos-verraet/
7. AUTO BILD: Used electric cars, how to check the battery (reading out the SoH, independent tests). https://www.autobild.de/artikel/gebrauchte-elektroautos-so-checkt-man-den-akku-vor-dem-kauf-21495483.html
8. EnBW: Charging power of electric cars simply explained (SoC 20 to 80%, rapid charging at a low charge level). https://www.enbw.com/blog/elektromobilitaet/laden/die-ladeleistung-eines-elektroautos/
9. JOLT Energy: The EV charging curve, why charging speed fluctuates. https://jolt.energy/en-gb/ev-charging-curve-why-charging-speed-fluctuates/