Car alternators are 3 phase DC excited synchronous electric machines. They can be used as a motor by a proper 3 phase motor driver.

Let's think that we connected a 200Ah Lithium based (eg. LiFePO4) reservoir battery parallel to car's default 12V battery by a step down converter. As we are driving, step down converter is turned off, so our reservoir battery is disabled. When we need to apply brake, we simply press the brake pedal which now has a pressure sensor on it, seamlessly controls the step-down converter's output current (which is roughly equal to the output power, thereby the braking power), which in turns slows down the car without the need for the actual brakes.

The maximum breaking power with a 200Ah LFE battery will be roughly around 2kW according to my experiments with a 60Ah LFE battery connected parallel to my car's battery.

When we are driving normally, the motor driver connected to the generator slowly starts driving the generator in motor mode, which in turn make us feel like we are starting to drive downhill so we simply release the gas pedal accordingly. The motor driver will use the energy of reservoir battery as much as possible in order to open as much as space for the next braking event.

In this scenario, I guess the mechanic strength and aptitude of the car alternator that is required for the maximum allowable boost power in motor mode is equal to the maximum continuous output power of the alternator in generator mode. If the alternator is built to supply 100A continuously, then it generates 1380W continuously, so all construction would be appropriate to use it as a 1380W motor.

Is that accurate or can't we use the alternator in motor mode for some other reasons?

  • So you have come up with 2kW as the braking power through your "system" - how much energy do you think a car's brakes dissipate?
    – Solar Mike
    Jan 20, 2020 at 18:58
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    According to this publication, a "comfort braking" is usually around 3m/s². So the average braking power for a 2000kg car at 90km/h P = m·a·V/2 = 2k·3·25/2 = 75kW. 75kW? Huh. Okay.
    – ceremcem
    Jan 20, 2020 at 19:59
  • In addition to everything fred_dot_u mentioned, just to achieve 75kW with a 15VDC component (pushing upper limits I think) you'd be looking at momentary discharge of 5000 amps! Couple that with the fact that even if you could get the alternator to be capable of that, it has a very indirect interface with the rest of the drive train. You'd be looking at stopping 10s or 100s of thousands of kg of energy with a polyester tensile cord? Maybe beef it up to a steel chain? Not a chance it would be strong enough. Not to mention every other stress point through the entire drive train. Jan 22, 2020 at 21:11
  • Interesting idea though. :) Jan 22, 2020 at 21:11
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    @crenecem ah, gotchya. Just figured I'd add that for anyone that comes across this in the future. Cheers. :) Jan 23, 2020 at 20:35

1 Answer 1


What you describe, in somewhat inaccurate general terms is something that is in place today in hybrid vehicle designs.

The specifics you've presented have multiple technical problems, especially the aspect of using the vehicle's existing 12v battery.

The mechanical components of at least one hybrid vehicle uses a motor-generator component to perform the power assist, drawing power from a dedicated traction battery, while serving as a generator for slowing the vehicle and returning power to the battery as applicable.

This is accomplished through computer monitoring and control of the systems and would not necessarily be easily accomplished at a DIY level.

From a power-required reference, conventional alternators are not going to produce the necessary power, nor would something of that size serve as a suitable motor for an assist.

On production electric vehicles, regenerative braking is now common. One lifts the foot on the 'go-pedal' and the system converts kinetic energy to electrical energy. Even in the best systems, regenerative braking returns only about 20 percent of the energy used, making it a useful but not trip-extending feature.

The losses in the design you describe would reduce the 20 percent enough to consider it a severe loss.

  • I didn't say I would use the car's existing battery. On the contrary, I will leave the existing battery untouched, I'll use a 200Ah LFE battery as the reservoir battery.
    – ceremcem
    Jan 20, 2020 at 18:39
  • from your post: "reservoir battery parallel to car's default 12V battery by a step down converter. " This turns the conventional independent systems of today's hybrids into a joined system, with more complications than one could imagine.
    – fred_dot_u
    Jan 20, 2020 at 19:15
  • There may or may not be complications, but that doesn't necessarily mean that I meant to use the car's existing battery. While preparing a block diagram before this comment, I realized a joined system would require more layers (a step up converter) for a proper voltage conversion, so an independent system would perform better. Anyway, a car alternator seems to be able to convert only a 2.5% of required braking power.
    – ceremcem
    Jan 20, 2020 at 21:41
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    For more information, please check out this wiki: en.wikipedia.org/wiki/BAS_hybrid Jan 21, 2020 at 16:37

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