Regenerative braking can be AC or DC depending on the type of motor and power electronics used, but in modern electric vehicles it is BOTH at different stages of the process.

Here’s the full, clear explanation:

Inside the motor: Regenerative braking produces AC electricity (because the motor is an AC machine).
Inside the car’s power electronics: This AC is then converted to DC using the inverter.
Into the battery: The battery receives DC, because lithium-ion batteries can only charge with DC.

So:

Regenerative braking starts as AC → ends as DC.

1. Why regenerative braking produces AC

Most EVs use three-phase AC motors, typically:

Permanent magnet synchronous motors (PMSM)
Induction motors
Switched reluctance motors

During regen braking:

The wheels spin the motor.
The motor acts like a generator.
A three-phase AC voltage is produced at the stator windings.

📌 This is the same physics as any AC generator.

2. Why the battery must receive DC

Lithium-ion batteries can only store DC energy.
They do not accept AC.

Therefore, the AC from the motor must be converted.

3. What converts AC to DC? → The Inverter

The same power electronics module that normally converts battery DC → AC to drive the motor does the opposite during regen:

During acceleration:

Inverter converts DC → AC to power the motor.

During regenerative braking:

Inverter converts AC → DC to charge the battery.

This bidirectional capability is what makes regenerative braking possible.

4. Energy Path During Regenerative Braking

Step-by-step:

Vehicle slows → motor spins as a generator
Motor outputs 3-phase AC
Inverter converts AC → DC
DC flows into:
- Battery
- DC bus
- Or auxiliary systems (DC/DC converter)
Excess heat is managed through cooling systems

5. Why this process limits regen braking

Regen braking is limited by:

How much DC current the battery can accept
Battery temperature
State of charge (full battery = little/no regen)
Inverter and motor thermal limits

This is why regen may feel weaker in:

Cold weather
At 100% charge
Hard braking situations

6. Special Case: DC Motors (rare today)

Older EVs (pre-2000s golf carts, early prototypes) used DC motors, which produce DC during regen.
Modern EVs use AC motors for efficiency, torque control, and reliability, so regen is now almost always AC→DC conversion.