Hybrid Auxiliary Battery Keeps Going Flat
Vehicle
- ModelTraining Hybrid Crossover
- Engine1.6 hybrid
- Mileage77,000 km
Customer complaint
The hybrid will not start after standing for a few days. The auxiliary (12 V) battery is repeatedly flat though the traction battery is fine.
History
The auxiliary battery was replaced twice with no lasting improvement.
Fault codes
Live data
- 12 V battery voltage at restLow
- DC/DC converter outputNot rising when expected
- HV traction battery state of chargeHealthy
Measurements
- Parasitic drain, vehicle asleepHigh
- DC/DC enable signalPresent but no output
Diagnostic case
Next step — Reasoning.
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Confirmed cause
The DC/DC converter is not maintaining the 12 V network (and a module may not be sleeping), draining the auxiliary battery.
Reasoning
On a hybrid the 12 V network is maintained by the DC/DC converter rather than an alternator. A healthy traction battery with a repeatedly flat auxiliary battery and a DC/DC output that does not rise when enabled localises the fault to the DC/DC path. Measuring sleep current confirms whether the drain is the converter or a module that fails to sleep. High-voltage work requires qualification.
Recommended repair
- Diagnose the DC/DC converter and its enable/communication circuit.
- Check for modules that fail to sleep and contribute to the drain.
- Repair, then verify 12 V maintenance and sleep current. High-voltage components must only be handled by qualified technicians.