Regenerative Braking: Where does the electricity go?

Today (6.16.22) Ray posted an interesting puzzlement about regenerative braking. If a regenerative-braking car is starting down a big hill, and the battery is fully charged, what happens to the electricity that is generated by the brakes during the descent? It can’t go into the battery, b/c the battery is already fully charged. Here’s two possibilities:

  • The regenerative braking doesn’t work, and the car uses the brake pad friction instead.
  • The regenerative braking still works, but instead of charging the battery it heats a big resistor, as is done in some trains that use a regenerative braking function. .

Then the regenerative braking doesn’t kick in. Only kicks in when needed. Braking down a hill with a fully charged battery will be all brakes.


I guarantee there’s no big resistor in my hybrid.

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My wife has been called a big resistor. Cept for spending habits.


@MikeInNH is correct… the conventional brakes do all the braking if the battery is fully charged. Hybrid cars try not to let the battery get full so that doesn’t happen.

The condition poses a problem for the brake system. It is not good to have a variable pedal force… low when regen is high and high when the batteries are charged. The brake system must be more complicated to solve that so the pedal feel is the same for every condition.


Letting off gas kicks in mild regen mode. My Prius had “B” position on shift lever for extra regen braking. Shifter was self centering. You push forward to drive and release and up it springs back to vert. Same with B mode.

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