Disabling the alternator on a car

Existential Angst · 2012-10-12T12:59:25+00:00

Awl -- Would seem like a no-brainer, but goddamm, it shore has me beat! Yeah, I know, pull the fuse!! Yeah, I know, the fuse labeled "alternator"..... :) :) But when I did that, the alternator light or battery lite would not come on. AND, iirc, I DID check the voltage at the battery, to see if it went down to 12, or was up at the ususal 13-14, and indeed, it stayed at 13-14, indicating a working alternator. And the engine compartment is so g-d crowded/complicated, I can hardly tell a wire from a hydraulic line, so it's not easy to even find the right wire. I figger at 100 A, the wire would have to be perty thick.... The reason for disconnecting the alternator is this: On an old mazda 929S, the alternator went -- and I could StoG that my mpg jumped by 15%.... would like to re-examine this phenom with my ScanGauge, to see if it is real. If this is indeed a true phenom, I could wangle myself a bit of a hybrid by just keeping a spare batt in the car, and switch in the alternator when the first batt dies. But basically try to do all the batt. charging at home. Altho, there proly is no free lunch: I'll proly use my gas savings to buy a new battery every year?? LOL But the actual tradeoff would be inneresting to see.

Oct 12, 2012 43 Replies

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Stanley Schaefer 13 years ago

Doesn't seem a likely source of engine drag to me. If you have a 30 amp alternator running flat out and it's putting out full output at

16v(before regulation), that's just over a half horsepower. Even if you've got a horribly inefficient alternator running into a dead short with bad bearings, it's unlikely to absorb more than 1 HP. If you start screwing with running down regular car batteries, you're going to have a lot of grief. There's a minimum voltage needed to just keep today's ECU and injection systems running, factory specs on my VW say 8v and that's 8v under load, not 8v with a VOM with the engine off. My experience with car batteries running down has been mostly bad. After one or two instances of leaving lights on all day, most will short out and won't charge anymore, the plates are thin and warp from the heat of charging.

Stan

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Bruce L. Bergman (munged human readable) 13 years ago

Stan's right - a regular car battery won't go a month with abuse like that and it'll crap out on you. You might be able to get it replaced once under warranty, maybe twice - Third time they'll see the pattern and start asking lots of questions.

You MIGHT be able to get 3 or 4 months out of a battery if you use a Deep Cycle rated Trolling Motor/RV battery, but they aren't very good at providing the big bursts of power (200 - 400+ Amps) needed to crank a starter motor over.

Trust me, the 1 - 2 HP that the alternator takes won't make that much difference in the long term.

Believe It Or Don't: Burning the gasoline to spin the alternator directly is actually far more efficient than using Utility Power to charge the battery at home.

Between the thermal losses incurred with a steam or fossil fueled generation system, step-up and step-down transformers and transmission line losses, you lose almost half the energy you started with through making the electricity and getting it from the power plant to your house.

Hydroelectric, Wind, Solar is a lot more efficient to make the power, true - but then they roll the cost of building and maintaining the powerplant (and the dam) into the power rates - and they aren't cheap.

Oh, and you want to kill the alternator, you have to pull the control wires - AND the Battery lead. Yes, there are some models that don't need any external excitement at all - If it's spinning and the big Output wire is connected to the battery, it's putting out ~13.8 Volts.

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Existential Angst 13 years ago

This all reminds me of the paraphrased Laws of Thermo:

You can't win
You can't break even
You can't even get out of the game.

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Cross-Slide 13 years ago

EA,

How you surviving the big storm?

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