I have an alternator that likely bad diodes - erratic output, drains the battery when off, etc. Automated in-vehicle test at an auto parts shop also reported bad diode as the problem. Since it does produce voltage above 12V when running, is it plausible to mitigate the draining temporarily and maintain some charging capability by splicing a suitably spec'd diode into the output cable? Presumably it should have low voltage drop, high current tolerance, and handle reverse voltage up to battery voltage.

  • Do what you want - whatever I pointed out does not sit well with you.
    – Solar Mike
    Aug 21, 2021 at 15:47
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    @SolarMike: I don't get why you're being hostile over it. I think you either misunderstood what I was asking about, or just want to say "it's a bad idea" but don't want to elaborate enough to establish why. Maybe it is, but if so I'd like to understand why rather than being treated like I'm being an annoying know-it-all for asking. Aug 21, 2021 at 15:49
  • Many alternatives to choose from; your trial and error idea no one's ever tried, diy repairs, buying a low cost replacement with or without a core charge/exchange, buying a new one with or without core/exchange. If your idea was sound, it would be well known around the world. My guess is when alternators were first invented in 1959, a shorted diode or two with a band aid repair idea like yours would have been tried in the last 70 years...........
    – F Dryer
    Aug 21, 2021 at 15:59
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    The only one who's exhibited "attitude" in this exchange is you. I've asked questions, asked legitimate followup questions to clarify assumptions and whether they're correct, etc. and acknowledged where I don't know things. The point of this site is to ask questions and learn things, not to belittle people who ask a question about something you think you're an expert on because they lack your expertise. Aug 21, 2021 at 17:56
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    What you told me you explicitly called a "first guess". Thr obvious reasonable response to that is asking followup questions to determine if the guess is based on valid assumptions Aug 21, 2021 at 19:40

2 Answers 2


A diode would likely stop the battery drain when the engine is off, since current would not be able to flow back to the alternator. You would however need a pretty big diode to carry the current capacity of the alternator.

You would then have a voltage drop across the diode which may cause the battery to not be fully charged.

My biggest concern would be that you are disguising a fault that could cause a potential fire. If the faulty diode is shorting 2 of the phases, you could have some serious heat build up within the alternator, which could lead to fire.

Overall, it doesn’t sound like something to experiment with.

You would likely find that the cost of a replacement diode pack for the alternator or a second hand alternator may be cheaper than a single diode that will suit your requirements.

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    These are some good concerns. The cost is something like $15 diode vs $200 plus a fairly difficult removal and installation, the latter of which is bothering me more in weather I really don't want to work on this in. Aug 22, 2021 at 13:43

Yes, this does work. After installing this rectifier:

image of part with schematic on label

I had the battery charging and no discharge, but it was getting moderately hot. After asking on the Electronics SE sister site, I got an answer confirming that connecting both AC inputs would be a good idea and now there's very little heat dissipation at the rectifier and battery voltage quickly returns to 13.8 or so after starting the engine.

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