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EV battery glossary: terms on a used-EV battery check explained
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Plain-English meanings of the terms you'll meet when checking a used electric car's battery: on certificates, in adverts and in our data. Each entry links to the source and to a fuller guide.
Jump to: State of health (SoH) · State of charge (SoC) · Battery management system (BMS) · kWh (kilowatt-hour) · Usable vs nominal (gross) capacity · Buffer (reserve capacity) · Battery degradation · Range retention · AVILOO FLASH and PREMIUM · OBD test / OBD app · Battery health certificate · Cell voltage imbalance (cell balance) · LFP and NMC · Battery thermal management (active cooling) · Capacity bars, LeafSpy SOH and Hx (Nissan Leaf) · Battery lease (battery hire) · Battery warranty threshold · Battery recall
- State of health (SoH)
How much energy a battery can store now, as a percentage of what it stored when new. 100% is a new battery; 90% means roughly a tenth of the usable capacity has gone.
It is the main number on a battery certificate. Different testers measure it in different ways, so two readings of the same car can differ. Full explanation: battery state of health explained.
- State of charge (SoC)
How full the battery is right now – the "battery %" on the dashboard. It is not a measure of battery health.
Some tests need a particular charge level; DEKRA's driving test, for example, needs about 40–80% (DEKRA).
- Battery management system (BMS)
The electronics and software in the car that monitor the cells, control charging and estimate the battery’s state of health.
The car's own SoH figure is the BMS estimate. AVILOO says the BMS value can be too high or too low (AVILOO GmbH), and a Technical University of Denmark study saw one Nissan Leaf's figure move up and down in steps about every 90 days (DTU – Technical University of Denmark).
- kWh (kilowatt-hour)
The unit of energy a battery stores. A bigger kWh figure means more energy and, all else equal, more range.
Spec sheets often give two kWh figures for the same car – see usable and nominal capacity below.
- Usable vs nominal (gross) capacity
Nominal or gross capacity is the whole battery; usable capacity is the part the car lets you use. Certificates normally state SoH against usable capacity.
Recurrent says usable capacity is usually 95–99% of the total (Recurrent). Example: the MG4 Long Range has 64.0 kWh nominal and 61.7 kWh usable (EV Database).
- Buffer (reserve capacity)
Capacity kept in reserve at the top and bottom of the charge window, so the dashboard’s 0% and 100% are not the cells’ physical limits.
Recurrent says some makers likely release reserve capacity over time, which can keep displayed range stable while the battery ages (Recurrent).
- Battery degradation
The gradual loss of capacity (and so range) as a battery ages and is used.
How fast it happens depends heavily on the model. Compare sourced figures by mileage on the battery degradation chart or get an estimate for one car with the battery health estimator.
- Range retention
How much of its original range a car still achieves. Related to SoH, but not the same thing.
Recurrent, which publishes range-based scores, says its data shows range retention, not battery health (Recurrent). Range also depends on temperature, speed and driving style.
- AVILOO FLASH and PREMIUM
Independent battery tests from AVILOO (Austria) that calculate SoH with AVILOO’s own algorithm from vehicle data, rather than copying the car’s BMS value.
FLASH is the quick test; PREMIUM uses a full discharge cycle. AVILOO says FLASH is within about ±3% of PREMIUM (AVILOO GmbH). How to read one: battery certificate guide.
- OBD test / OBD app
A test that reads data from the car through its diagnostic (OBD) socket, usually with a plug-in dongle and a phone app.
Generational's test, for example, takes about 2 minutes with an OBD dongle and reports the car's own BMS SoH (Generational). Test options and prices: how to check a used EV battery.
- Battery health certificate
A report from a battery test showing SoH and other details for one car on the day of the test.
Fields, tolerances and red flags are explained in how to read an EV battery health certificate. Selling? See how to prove battery health when you sell.
- Cell voltage imbalance (cell balance)
The difference between the highest and lowest cell (or module) voltage in the pack. A large gap can point to a weak cell even when headline SoH looks fine.
In its UK test data Generational found a median spread of 8 mV, with about 1 in 85 cars above 50 mV, its "requiring investigation" level (Generational).
- LFP and NMC
Two common battery chemistries: LFP (lithium iron phosphate) and NMC (nickel manganese cobalt). Many models were sold with either, depending on version.
In a Sandia lab study, LFP cells lasted 2,500–9,000 equivalent full cycles against 200–2,500 for NMC under the tested conditions (Journal of The Electrochemical Society 167 120532). Tesla's manual explains how to tell if a Model 3 has an LFP pack (Tesla, Inc.). More: MG4 LFP vs NMC.
- Battery thermal management (active cooling)
A system that heats or cools the battery with liquid or air to keep it in a healthy temperature range.
Not every EV has one: a Technical University of Denmark paper notes that the Nissan Leaf has no active cooling system and loses heat through the battery casing (DTU – Technical University of Denmark).
- Capacity bars, LeafSpy SOH and Hx (Nissan Leaf)
The Nissan Leaf shows battery capacity as bars next to the charge gauge; the LeafSpy app reads more detailed figures such as SOH and Hx.
LeafSpy's documentation says the first capacity bar is lost after SOH has been around 85% for several weeks, and that the meaning of Hx is "not fully understood" (LeafSpy). Step by step: checking a used Nissan Leaf battery.
- Battery lease (battery hire)
Some older Renault Zoes were sold with the battery hired separately for a monthly fee, so the car and battery can have different owners.
The Guardian reported a reader paying £50 a month for an 8-year-old Zoe's battery, with buy-out quotes rising during 2024 (The Guardian). What to check: used Zoe battery lease and health check.
- Battery warranty threshold
The capacity level below which the maker will repair or replace the battery within the warranty period.
Of the 49 battery-warranty terms we track, 36 pay out below 70% capacity; the rest use other thresholds or wording. Check time and miles left with the battery warranty checker.
- Battery recall
A manufacturer campaign to inspect or fix a known battery fault, usually free of charge.
In the UK you can check a car's outstanding safety recalls by registration number on GOV.UK (GOV.UK). Examples: Kona Electric recalls, Kia e-Niro.
Next steps: used-EV battery checklist and test prices · questions to ask the seller · model pages with sourced data · all models compared.
Sources used on this page
13 sources.
- Large test datasets (1 source): Generational
- Connected-car / telematics (1 source): Recurrent
- Academic research (2 sources): DTU – Technical University of Denmark, Journal of The Electrochemical Society 167 120532
- Manufacturer (1 source): Tesla, Inc.
- Other (specs, guides, news, surveys) (8 sources): DEKRA, AVILOO GmbH, Recurrent, EV Database, Generational, LeafSpy, The Guardian, GOV.UK
Owner-reported and listing values are single cars (n=1) read from the car, an app or an OBD tool – useful examples, not averages. Range-retention figures (Recurrent) measure range, not capacity. Full details: methodology & all sources.
We are not paid by the battery-test providers mentioned on this page. Links marked "paid link" in the "Useful products and services" box are affiliate links. See our disclosure.
Get this specific car tested
We are not paid by these test providers. How we make money.
Model averages can't tell you about the car in front of you. An independent battery test (typically a short OBD plug-in test, some remote) gives an SoH figure and often a certificate you can use when negotiating.
- AVILOO – Independent OBD-based battery test (FLASH / Premium) with certificate; available via partners in the UK and EU.
- Generational – UK OBD plug-in test with certificate (SoH, range, remaining warranty, cell balance); sold through dealers and independent mobile testers.
- Moba – Postal OBD kit: you run the test yourself and post the box back for a digital battery certificate (Europe – check vehicle support and delivery country).
Prices change – check the provider's site. Provider names are used for identification only.
Useful products and services
- Charging cable: check that the car comes with its charging cable(s) at the viewing. If one is missing, replacements are sold separately: Type 2 charging cables at EV King paid link.
- Tyres: check tread depth and wear at the viewing. To price replacements, search by the size printed on the tyre wall: car tyres at Tyres UK paid link.
- Parts and tools: wiper blades, bulbs, 12V batteries and tools for the car you buy: GSF Car Parts paid link (UK).
Some links are paid links: we may earn a small commission at no extra cost to you. This never changes the battery data we show (disclosure).
FAQ
What is the difference between SoH and SoC?
SoH (state of health) is how much the battery can store compared with when it was new. SoC (state of charge) is how full it is right now. A car can show 100% charge with a battery at 85% health.
Is the car’s own battery health reading reliable?
It is the battery management system’s estimate. It can be higher or lower than an independent test and can change after recalibration or software updates, so an independent test is a useful second opinion when buying.
What cell voltage difference is a problem?
Generational treats a spread above 50 mV as needing investigation and above 100 mV as a potential underlying fault, based on its UK test data.
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