If the 12-volt battery has been flat more than once, do not let a second one be fitted before this is checked — on an electrified car the DC/DC converter is what charges it, and a converter that shuts down for temperature drains the battery it should be filling. The symptoms look exactly like a dying battery and the battery is usually innocent. Ask for the 12-volt charging voltage to be measured with the car running.
The temperature sensor watching the DC/DC converter has a circuit fault.
The DC/DC converter is the part of an electrified car that people are least aware of and most affected by, so it is worth naming plainly: it is the alternator's replacement. It steps the traction pack's several hundred volts down to the twelve or so that runs the lights, the computers, the pumps, the dashboard and the door locks. Everything that made a car work before hybrids existed still runs on 12 volts, and this is what keeps that supply alive.
Which gives its failures a signature that catches almost everybody out. When the converter derates or shuts down for temperature, the 12-volt battery stops being charged and starts being drained — and what follows is a car whose lights dim, whose modules start reporting nonsense, and which eventually will not power on. It presents as a flat 12-volt battery, so a 12-volt battery gets fitted, and the new one flattens too. A perfectly good traction pack sits underneath the whole time.
So the rule worth carrying: on an electrified vehicle, a 12-volt battery that goes flat repeatedly is a charging fault until proved otherwise, and the charger is this. The converter works hardest when the car is on and while it is charging, which is also when it has the least airflow, so temperature faults here are real rather than theoretical.
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An electric car still has a 12-volt battery, and it still runs the lights, the computers, the pumps and the door locks. What it does not have is an alternator. **The DC/DC converte...
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