How does a hybrid work when the power battery is fully charged ?

I recently bought a 2013 Yaris Hybrid and I am delighted with it. Is the 12v battery charged by the engine or from the power battery ? As I understand it, the power battery is charged by the engine and by braking. On a long journey, can I assume that once the power battery is fully charged, all of the output from the petrol engine goes to the driving wheels? If the power battery is fully charged on a long journey and the car is driven downhill for several miles, does the hybrid braking effect still function? I would have thought that a fully charged power battery would have no braking effect because the power battery is already fully charged.

Asked on 4 December 2025 by John Hughes

Answered by David Ross

In Toyota hybrids, the 12v battery is charged by the drive battery, via a converter which steps down the high voltage of the drive battery to the 12v battery. Once the drive battery is fully charged, all the engine output will be used to drive the wheels and the engine braking effect will still function, but it is worth noting that in situations where the drive battery is fully charged and you are travelling downhill where energy would be fed into the battery, the engine may automatically switch on - this is to prevent the system from overheating.

Tags: hybrid cars

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