I’ve bought my share of used cars, and with EVs the stakes feel different: the battery and the onboard charger are components you can’t easily swap like a faulty alternator. During a test drive, you won’t fully stress the powertrain or the charger, but with a few targeted checks and some intentional charging steps you can reveal early warning signs of an onboard charger (OBC) that’s about to fail. Here’s the hands-on approach I use when evaluating a used EV — practical, practical, and focused on spotting problems before you sign anything.

Why the onboard charger matters

The onboard charger converts AC power (from Level 1 and Level 2 chargers) into DC to charge the battery. If it’s failing, charging will be slow, inconsistent, or impossible on AC chargers — forcing expensive DC fast charging or leaving you stranded. A partially failing OBC might work intermittently, show overheating symptoms, trip faults, or derate charging current. You want to catch that early.

What I bring on a test-drive

Pack a small kit: things that help you test charging and capture evidence.

  • Phone with a camera and note app
  • Portable charger cable (if you have one) or an EVSE that can limit current
  • Access to a Level 2 public charger and, if possible, a destination/third-party app for a nearby charger
  • OBD2 dongle (for advanced checks) and an app like Torque Pro, EVNotify or manufacturer-specific tools
  • Infrared thermometer or point-and-shoot temp gun (optional but useful)

    • Quick visual & documentation checks

    Before you press the accelerator, do a few simple checks that often reveal past electrical problems.

  • Look at the charging port: corrosion, melted plastic, or discolouration around pins often means overheating or poor connections.
  • Check service records: any repeated charging-related repairs or unexplained faults are red flags.
  • Ask about recent firmware updates and if the dealership or seller has applied them — sometimes charging bugs are fixed by software.

    • Start the basic cabin/cluster checks

    Turn the car on and watch for warning lights. Many EVs show the charging system and battery warnings on startup. If any persistent charging or thermal warnings appear before you start driving, walk away or investigate further.

    On the road — sounds, smells and behavior

    During the drive pay attention to unusual electrical noises (buzzing, whining that change with vehicle speed or HVAC use), burning smells, or an oddly hot center console area. Some onboard chargers are under seats or trunks; heat can migrate and become noticeable. I once test-drove a vehicle with faint burning plastic smell during a long stop — turned out to be a partially failed OBC overheating under load.

    Real test: AC charging check (Level 2)

    This is where you can most reliably stress-test the onboard charger in a short time.

  • Find a nearby Level 2 charger (Type 2/Mennekes in Europe, J1772 in the US). Plug in with the car at around 20–50% SoC — not 90% where the car will taper charging.
  • Record the initial charge current (kW or amps) reported on the car’s screen and the charger. Typical OBC limits vary: many cars accept 3.3 kW, 6.6 kW, 7.2 kW, 11 kW, or even 22 kW depending on the model and variant. Compare reported vs. expected.
  • Let the car charge for 5–10 minutes while watching the current. A healthy OBC will maintain a steady current appropriate to its rating and the charger capability. Look for sudden drops, erratic swings, or repeated stopping and restarting (cycling).
  • Use an IR thermometer on the charging port, nearby fuses/contacts, and the area where the OBC sits (if access exists). Temperatures rising rapidly or >60–70°C at contacts is a sign of overheating.

    • Symptoms that suggest OBC trouble

    During the Level 2 test, keep an eye out for these specific issues:

  • Reduced charging current: If the car draws far less than its known onboard charger capacity (e.g., 6.6 kW car drawing 1–2 kW) that’s suspicious unless temperature or state-of-charge explains it.
  • Charge cycling or interruptions: If charging repeatedly stops and restarts or shows errors on the dash or charger, that indicates instability.
  • Fault lights or error messages: Any charging system, inverter, or thermal warnings that appear during charging are immediate red flags.
  • Excessive heat: Hot connectors, plugs, or nearby panels that warm quickly suggests electrical resistance or partial failure.

    • AC vs DC behaviour — what to try

    Onboard chargers only affect AC charging. If you can access a DC fast charger, see how the vehicle behaves on DC charging as well:

  • If DC charging works fine but AC charging is reduced or erratic, suspect the onboard charger specifically (since DC bypasses the OBC and feeds DC directly to the battery).
  • If both DC and AC are problematic, the issue might be upstream: battery thermal management, battery pack, or BMS. That’s a different and more serious issue.

    • Using OBD2 and software to dig deeper

    If you have the seller’s permission and an OBD2 dongle, look for stored trouble codes and live data:

  • Check for P0xxx or manufacturer-specific codes related to charging, inverter, or thermal sensors.
  • Monitor charging current, OBC temperature, and BMS voltages during the Level 2 charge. Sudden cuts or thermal-driven derates show up here.
  • Some apps allow live logging — save the logs. If the buyer refuses or the seller hides OBD access, treat that as suspicious.

    • Questions to ask the seller/dealer

    I always ask direct questions that reveal ownership history and prior issues:

  • Have you experienced any charging faults, intermittent AC charging, or warning lights related to the charging system?
  • Has the car had any repairs or replacements to the onboard charger, charging port, or charging cables?
  • Has the car had any thermal issues, overheating warnings, or battery-related repairs?
  • Are there any outstanding software updates or recalls related to charging?

    • Quick reference checklist

    StepWhat to watch forPass/Fail
    Visual port inspection Corrosion, melted plastic, discoloured pins
    Startup warnings Charging or battery warnings on dash
    Level 2 charge current Steady expected kW/amps for 5–10 min
    Temperature check Rapid heat rise at port or OBC area
    OBD2 codes Charging, inverter, or thermal codes
    DC fast charge Works normally (if available)

    Red flags that should stop the deal

    If you see any of these, I’d be extremely cautious:

  • Consistent AC charging below the car’s published onboard charger capacity without reasonable explanation.
  • Repeated charging error messages or warning lights during a short test session.
  • Signs of heat damage or melted connector housing.
  • Seller refuses to allow an AC charge test, OBD2 check, or to let you try a public charger.

    • If you suspect a problem

    Ask for a professional inspection at an EV-specialist shop. Many shops can do a more thorough diagnostic: measure insulation resistance, test the OBC under load, and read manufacturer-level codes. Compare the repair estimate to your negotiating leverage. Sometimes an OBC replacement is costly; other times it’s covered under warranty if the car is relatively new.

    Testing a used EV’s onboard charger during a test drive isn’t foolproof, but it’s the best practical defense most buyers have. With a little prep — a Level 2 charger, an OBD2 tool if possible, and a focused checklist — you can spot the most common signs of impending failure and save yourself from an expensive surprise.