Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Wiring the voltage regulator power to brake lights

I know that there’s a electric pigtail going to the voltage regulator on the alternator on my car, and the generator won’t run if this isn’t installed. I also figure that one of the inputs is 12VDC.

So, what would happen if, instead of supplying full-time 12VDC, you tapped into the wire to the brake lights? That way, the alternator would only work when you were trying to lose enegry anyway (although there would be parasitic losses). In effect, it would be a wimpy version of regenerative braking, as it would turn motion into electic energy.

A perusal of a “hypermiler” site mentions an alternator delete as saving “up to 10%,” though that is quite a bit too hardcore for me (plus eventually you have to recharge the battery).

The first two thoughts were: 1. it’ll fry the alt, and 2. it’ll fry the electronics. But, thinking further, the alternator has high recharge rates after every start, and every start involves going from battery voltage, to below battery, to alternator voltage…and nothing drastic happens.

Any ideas if this would yeild a net savings?

There are two kinds of power consumers on the alternator. There is the power that is consumed actuarially making the electrical power and there is the wasted energy used to move all the parts that create the power. Your idea would not reduce any of the energy that is used to just move the generator parts since they would be moving anyway and the added power needed to keep the alternator would not be reduced.

To do what you want would require a mechanical device to disconnect the alternator from the system allowing it to stop being turned when it is not need.

Yeah, I know an opitmized system would need a clutch or similar device to stop the alternator from spinning. I was just wondering how much of alternator load is parasitic in nature…if it’s smaller than the “work” load, then you could capture the lion’s share of efficiency just through a little wiring.

I thought of this from having worn brushes on my alternator and thus intermittant charging. I happened to be coasting once when the alt “cut back in,” and you could feel the resistance, similar to turning the a/c on while coasting.

(Altough I don’t suppose it’d be impossible to adapt an a/c clutch to this purpose?)

The “parasitic” energy required just to spin the hardware is nothing compared to what is required due to the back emf when it’s turned on full.

Your modification might work reasonably well in stop and go traffic but consider what will happen on long expressway trips without stops. The service battery in your car will be stone dead in no time if the alternator isn’t allowed to maintain charge. Just factor the run-time load required to support ignition and all of the electronics in a modern car.

Most of the time you’re braking or sitting at a light with the brake depressed, the engine is idling and so the alt isn’t putting out max capacity. In addition, lead-acid batteries fare best when kept fully charged.

The charging circuit is already pretty well optimized so that it only runs when the battery needs to be recharged. And it does it when the engine rpms are conducive to alternator efficiency.

Another misconception, is that you’re going to be harnessing lost energy like regenerative braking. But in a conventional car, the engine is idling soon after you apply the brakes in an automatic transmission equipped car. So you’re not converting motion to energy as you suspect. The vehicle speed has no effect on the charge cycle for the vast majority of cars that have automatic transmissions.

So in your case when you would be sitting at a light with the AC (and what ever else on)also with the brake on there would be no alternator output? I don’t like this idea.

You could save negative dollars by trying super goofed up engineering. Just don’t actually do anything without getting a wife and having her approve anything you want to do. She will be able to list your more important priorities. Later on, the children will help out too.

That electrical pigtail gets its 12 volts through the alternator idiot light and as soon as the alternator exites, it stays self exited as long as it is turning even with that wire diconnnected. The idiot light provides the “pump priming” current to initialize the exitation of the alternator.
Alternators will even self exite without that input if you really race the engine.

Even if it worked and your alternator was only charging during braking, your battery would slowly die because hypermilers almost never use their brakes.

An alternator will not work very hard at all if you keep every electrical power consuming item in your vehicle turned off while driving.

You could help your fuel mileage in a very minimal but positive way by installing a slightly larger pulley on the alternator.

In the bad old days, cars with generators would not charge at idle due to the belt drive ratio applied to the generator. If a generator at engine idle speed was geared to charge the battery, then the generator, due to the wound rotor design, would more likely be destroyed by centrifugal force at high speed and possibly commutator follower (brush) life would be compromised.

Alternator wound rotors are resistant to such destruction at speed and can therefore be run faster at idle but but you could regear one to begin charging just above idle as per the bad old days with generators and be OK. Alternator slip ring follower (brush) life is not a problem as slip rings are smooth compared to generator commutators which had segments with gaps between each.