If AC is rectified to make it to only positive AC, can it charge batteries without damaging them? If no, then how regenerative braking works in electric vehicles? Does every motor controller have big inductors to smooth DC?


2 Answers 2


Lithium batteries need careful attention to current, voltage and temperature to ensure safe charging. Same also applies to discharging - an EV that's thrashed hard will heat its battery and sometimes will have to engage 'limp-home' mode until its batteries cool down.

At any rate, electric vehicles include controllers for both grid charge and regenerative braking. These controllers are not 'big inductors', but rather very sophisticated two-way DC-AC converters that can channel energy to the battery from either the grid or motor braking, as well as energy from the battery to the motor.

Controller development is is a hot area that's seeing improvements with new SiC switch technology. Example: MacLaren (yes, that MacLaren): https://www.mclaren.com/applied/case-study/relentless-drive-power-density-and-efficiency/

  • \$\begingroup\$ I've read that some EVs use their motors as inductors for DC conversion, so I thought that regen also needs something like that. \$\endgroup\$
    – Ri Di
    Jan 11 at 8:23
  • \$\begingroup\$ What? No. The motors are AC machines. In regen mode they're outputting AC, like an alternator, which is then back-converted to DC in the controller and captured by the battery. \$\endgroup\$ Jan 11 at 8:27
  • \$\begingroup\$ @hacktastical What? Yes. RiDi is right. Some manufacturers are building their chargers to incorporate the motor inductance as a component. The article I read was on control strategies to sequence different motor phases such that it would generate near zero torque. The same trick is already used to make the motor inductance part of a boost converter for battery charging during regeneration (short the motor, current rises, then open and it kicks up in voltage to charge the battery, standard boost converter operation, albeit in three phases) \$\endgroup\$
    – Neil_UK
    Jan 11 at 11:43
  • \$\begingroup\$ @Neil_UK I’ve read about that. OP was asking about regen, but fair point. \$\endgroup\$ Jan 11 at 16:40
  • \$\begingroup\$ Here’s a paper reference about using zero-torque motor control to do grid power conversion. ieeexplore.ieee.org/document/7222668 \$\endgroup\$ Jan 11 at 18:59

While it is technically possible it would require careful design.

Regenerated AC from motors is invariably converted to DC and then smoothed to provide voltage suited to operating conventional LiIon chargers. In the case of highly variable voltages a boost or buck boost conversion scheme is liable to be used.

It may be hard to sensibly distinguish between whether AC-DC or DC-DC conversion is involved as the same (usually) 3 phase driver circuitry used to power the motors will almost invariably be used to implement both conversion and voltage conversion.

The AC frequency is such that smoothing is within "the usual range of hard to do" in power electronics applications. At very low wheel speeds frequencies may be low but regenerative energy is also low.

  • \$\begingroup\$ surely the frequency range is the same as that needed to convert the power from the DC battery to drive the motor? ie \$\endgroup\$
    – Solar Mike
    Jan 11 at 8:15
  • \$\begingroup\$ @SolarMike Arguably regeneration frequency is no higher than the drive frequency unless special action is taken (see below). Motor basic output is wheel RPS X pole pairs in alternator (assuming unity gearing). Drive frequency may if desired use multiple drive cycles per pole transition. // I have met systems (not in vehicles) that PWM the alternator output load and use the alternator inductors as part of a boost converter to produce higher voltages from the alternator proper (!). \$\endgroup\$
    Jan 11 at 8:27

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