# Check Your Car Battery and Alternator with OBD2 > Odo reads your car’s 12 V battery voltage through an OBD2 adapter, keeps its lowest and highest readings, and tells you whether the charging system reaches a healthy voltage with the engine running. App: Odo: Speedometer, Drive & Ride (iPhone, iPad, Mac with Apple silicon, Apple CarPlay, Apple Watch). Free to download: https://apps.apple.com/app/id6759611784 Page: https://speedometerproapp.com/features/car-battery-check Last updated: 2026-10-28 Odo reads the 12 V system voltage through your OBD2 adapter and checks the peak it reaches with the engine running, the clearest sign of whether the alternator is keeping the battery charged. ## Why it matters A weak charging system usually shows up as a car that won’t start. Watching the voltage while you drive catches it earlier: a healthy 12 V system reaches about 13.5 to 14.5 V with the engine running. ## A plain verdict In Garage, each vehicle’s Battery section shows the latest, lowest and highest voltage Odo has read, and a verdict on the charging system. - Charging normally, with the peak voltage reached - A warning when charging peaks below 13 V, with advice to have it checked - Latest, lifetime minimum and lifetime maximum - Judged only on readings taken with the engine running ## Live voltage while you drive Battery voltage is one of the live readings on the OBD screen, on iPhone and CarPlay. CarPlay’s session panel also shows the lowest voltage since the adapter connected. - Battery voltage in the Electrical readings - Lowest battery voltage this session on CarPlay - Start, end, lowest and highest voltage for each trip in the OBD data export - The charging peak of each trip in the export too ## Look after the battery An adapter left plugged in draws a little power. Odo’s OBD guide explains when to unplug it, so a car that sits for days still starts. - Unplug the adapter if the car will sit unused for more than a few days - Guidance in the app under About OBD II ## How to set it up 1. **Plug in your adapter**: Fit a Bluetooth LE or Wi-Fi ELM327 adapter and connect it from Garage → OBD II. 2. **Drive with it connected**: The verdict needs readings with the engine running, so take a short drive. 3. **Check the verdict**: Open Garage, choose your vehicle and look at the Battery section. Note: Odo checks the charging voltage, not the battery’s capacity. A battery can charge normally and still be worn, so a load test at a garage is the definitive check. ## Questions and answers ### What voltage is normal for a car battery while the engine is running? A healthy 12 V system typically reaches about 13.5 to 14.5 V with the engine running. Odo warns you if charging peaks below 13 V, a sign the battery and charging system are worth having checked. ### What voltage should a car battery have with the engine off? About 12.6 V when it has rested, and below about 12.4 V usually means it's discharged. Readings at the diagnostic port run slightly lower than at the battery itself. ### Will an OBD2 adapter drain my battery if I leave it plugged in? It can. Some adapters keep drawing power with the ignition off and can discharge a 12 V battery within one to two weeks, so unplug the adapter if the car will sit unused for more than a few days. Adapters with a sleep or low-power mode draw far less. ### Can OBD2 tell me if my car battery is bad? Not completely. Odo checks whether the charging system reaches a healthy voltage, not the battery's capacity, so a worn battery can still charge normally, and a load test at a garage is the definitive check. ### Will OBD2 work with my car? Compatibility depends on your car's market and year, its communication protocol, the adapter, and how the car's computer is set up. As a guide, most US vehicles from the 1996 model year, European petrol cars from 2001 and diesels from 2004, and most cars worldwide from around 2008 use standard OBD2. Honda, Toyota, Lexus, Subaru, Mazda, Ford, GM, Nissan, Hyundai, Kia, Volvo and BMW F- and G-series work well with standard data. Some makes limit access: Volkswagen Group cars from 2020 and Mercedes-Benz from 2018 provide standard engine data but block enhanced diagnostics, Jeep, RAM, Dodge, Chrysler and Fiat from 2018 use a security gateway that blocks third-party scanners unless a bypass module is fitted, and Tesla doesn't provide standard OBD2 vehicle data. The full guide is at /features/obd2-compatibility. If you're unsure, try connecting first: the app shows exactly which readings your car reports. ### Which OBD2 adapters work with Odo? Odo works with ELM327-compatible OBD2 adapters that connect over Bluetooth Low Energy (BLE 4.0 or newer) or Wi-Fi. Adapters differ in quality and in how fully they implement the ELM327 command set, so results can vary: very cheap clones may drop the connection, read the VIN only partly, or miss some data. An adapter from an established seller is the safest choice, and if you already own one, try it first. Adapters that only work with their maker's own app, and classic Bluetooth adapters made for Android, won't connect. Setup steps and troubleshooting tips are in the app under Garage → OBD II → About & Compatibility, and on our compatibility page at /features/obd2-compatibility. ## Related - [OBD2 live data explained](https://speedometerproapp.com/features/obd2-live-data): What each engine reading means, what normal looks like, and why it matters. - [Maintenance reminders](https://speedometerproapp.com/features/maintenance-reminders): Service reminders by date or mileage that reschedule themselves. - [OBD2 trip recording](https://speedometerproapp.com/features/obd2-trip-recording): Engine data saved with every trip, with a synchronized replay.