Glossary
EV battery terms, explained.
The words on a BattGrade certificate and on this site, in plain language. The definitions follow our methodology, which has the full rules.
The battery
- State of Health (SoH)
- The usable energy or capacity of the battery today, as a percentage of what the same battery variant offered when new. The reference is the exact variant fitted to the car, such as a specific pack size, not an average for the model. A BattGrade certificate shows it rounded to 0.1 and never above 100%.In the methodology
- Battery management system (BMS)
- The control unit that watches over the high voltage battery: it measures cell voltages, temperatures and current, and keeps its own figures for the charge level and the battery's health. The BattGrade app reads these values through the OBD2 port with read-only requests.
- Usable and nominal capacity
- Nominal capacity, also called gross, is the total energy a battery pack is built to hold. Usable capacity is the part the car lets you use; the rest is kept as a buffer that protects the cells. Pack sizes are quoted by either figure, so two numbers can describe the same pack. BattGrade measures the SoH against the usable energy or the capacity of the same variant when new.In the methodology
- State of charge (SoC)
- How full the battery is right now, as a percentage. It is not the same as the SoH: a worn battery can still be charged to 100%. The energy-based method is only used when the state of charge and the pack temperature allow a reliable estimate.
- Cell balance and cell voltage spread
- A high voltage battery is built from many cells, and in a balanced pack they sit at nearly the same voltage. The cell voltage spread is the gap between the highest and the lowest cell voltage. Above 30 mV it is marked for attention, above 50 mV as poor. LFP batteries have a flat voltage curve, so their spread is only judged at a very high or very low state of charge.In the methodology
- Temperature spread
- The gap between the highest and the lowest battery temperature reading. Above 5 °C it is a warning, above 8 °C it is marked as high. It can point to a cooling issue or a faulty sensor.In the methodology
- Insulation resistance
- The electrical resistance between the high voltage system and the rest of the car, relative to the system's voltage, in ohms per volt (Ω/V). It shows whether the high voltage parts are properly insulated. Below 500 Ω/V it is a warning, below 100 Ω/V a failure, following UN Regulation No. 100.In the methodology
- Signs of a BMS reset
- A reported SoH more than 5 points above the capacity-based value, or a near-new SoH on a car whose lifetime discharged energy is too low for its mileage. Both can follow a BMS reset or a pack replacement. The certificate shows the check, and the service history can explain it.Reading a certificate
Scores and grades
- SoH method
- How the SoH on a certificate was obtained. Depending on what the car exposes, BattGrade uses one of three methods, and the certificate names the one used. When more than one is available, energy-based is preferred, then capacity-based, then BMS-reported. If the BMS value and an energy-based or capacity-based value differ by more than 5 points, a flag is recorded, the grade is based on the lower of the two and the uncertainty band widens by 2 points.
- Energy-basedThe full-charge energy estimated from BMS data, divided by the usable energy of the same variant when new. Used only when the state of charge and the pack temperature allow a reliable estimate.
- Capacity-basedThe present capacity in amp-hours, divided by the capacity of the same variant when new.
- BMS-reportedThe SoH value reported by the battery management system, used as it is.
- Uncertainty band
- How precise the SoH is, in percentage points either side of the score (±). The band depends on the method and on the vehicle. Read the SoH together with its band: two cars whose bands overlap may be closer than their headline numbers suggest.Reading a certificate
- Grade
- A letter from A to F that comes strictly from the SoH rounded to 0.1, with the same bands for every make: A from 85%, B from 70%, C from 55%, D from 40%, F below 40%. It is never assigned by hand, and the checks do not change it by themselves.In the methodology
Adapters and protocols
- OBD2
- On-board diagnostics: the standard diagnostic socket of a car, with 16 pins, and the standard requests that go with it. On the cars the app reads today it sits under the steering wheel. Battery data is not part of the standard requests, so the app also uses manufacturer data reads through the same port.Adapters by car and phone
- ELM327 and STN
- ELM327 is the command set most Bluetooth OBD2 adapters understand, and many cheap adapters are clones of it. Clones often lack what battery reads need: 29-bit CAN on Volkswagen Group MEB cars and the Zoe Ph2, or custom flow control on the Leaf, the Zoe Ph1 and Stellantis cars. STN chips, used in OBDLink adapters such as the MX+ and the CX, understand the same commands and handle both.Adapters by car and phone
- CAN
- Controller Area Network: the network the control units of a car use to talk to each other, and the one diagnostic requests travel on. Each message carries an identifier of 11 bits on most cars, or of 29 bits on some, such as Volkswagen Group MEB cars and the Zoe Ph2. An adapter must support 29-bit identifiers to read those cars.Adapters by car and phone
- UDS
- Unified Diagnostic Services (ISO 14229): a standard set of diagnostic requests that runs over CAN. Manufacturers use it to expose data such as battery values under their own identifiers. The BattGrade app only uses read requests, such as reading data by identifier.
- VIN
- Vehicle identification number: the 17-character code that identifies a car. The app reads it from the car where the car reports it, and uses it to identify the make and model. Part of it is hidden on the public verification page, and if the car does not report it, the certificate says the inspector entered it.
- READY mode
- The state in which an electric car is switched on and ready to drive, usually shown by a READY light on the dashboard. Some models only answer battery requests in READY mode, not with the ignition merely on, and each model page says when a car needs it. A sleeping car returns no battery data.Cars the app reads
Certificates and verification
- Digital signature
- A cryptographic seal over a certificate's data. When a certificate is issued, BattGrade signs a hash of its public data with an ECDSA P-256 key. Anyone can check the signature with our public keys at /api/certificates/keys, and any change to the data breaks it. Customer details are not part of the signed data.How verification works
- Hash
- A short fingerprint computed from data, here with SHA-256. The same data always gives the same hash, and any change gives a different one. BattGrade computes it over a fixed form of the certificate's public data (the vehicle, the result, the readings, a hash of the raw data and the issuer), and the verification page computes it again each time a certificate is opened.In the methodology
See the terms on a certificate.
The sample certificate shows the SoH, the method, the uncertainty band, the grade, the checks and the signature.