49 votes
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Car battery at 10v, then shows 14,5v while car running after boost, then when car turns off, drops to 10v again within a minute

Nope. That pretty much sounds like a dead battery. It sounds as though one of the cells has a short in it, which is causing your rapid voltage decrease. There's no way to fix this other than battery ...
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27 votes
Accepted

Car won't start after sitting for 2 years even with a jump. Is it a starter or alternator issue?

If the battery is completely dead, especially after sitting for two years, it most likely won't take a charge. Without the battery getting charged, you most likely don't have enough juice coming ...
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23 votes

Alternator dying so junk car?

A rounded bolt is not a reason to junk a car. The numbers you're quoting sound fine for an alternator; if it was below 12V, I'd be worried. If you're still concerned, take the car to a spares store ...
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  • 8,582
21 votes
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Why do cars die after removing jumper cables?

It's because of a couple things: The battery is very dead and the alternator cannot put out enough power at idle RPMs to run the whole system and charge a battery. This leads to a lack of power which ...
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  • 2,849
19 votes

Can a weak battery destroy an alternator?

Your question is difficult to answer without you knowing the inner workings of an alternator. The basics. If you move a magnetic field near a coil of wire the electrons in the wire get excited and ...
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  • 15.9k
19 votes
Accepted

Why can't alternators provide full current at idle?

Cost is the main reason. An alternator that can provide full charging current would be far larger requiring bigger rotor and stator windings. This would make it more expensive and heavier. The ...
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  • 15.9k
18 votes
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Can a car AC unit be run on just the car battery?

I can't give you numbers or calculations without some work, but I can tell you than energy is never free. Cars have an A/C compressor that is mechanically driven by the engine because this is the ...
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  • 13.2k
16 votes

Will fitting a physically bigger battery reduce the working life of the alternator?

Great question! Taking a few minutes to think through this logically should come up with something. First of all, realise that a larger battery with a higher amperage (CCA) will not cause any ...
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  • 5,216
13 votes

Can a car AC unit be run on just the car battery?

Well, Teslas (and presumably other all-electric cars) have A/C so it's not impossible, but A/C takes a fair amount of power. On the other hand, Teslas store a lot of energy. The A/C is something ...
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13 votes

Car won't start after sitting for 2 years even with a jump. Is it a starter or alternator issue?

It's the battery Starting an engine takes a much larger energy impulse than jumper cables can deliver directly through their typically-thin wires and the small surface area of their alligator clamps. ...
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12 votes

Is any power from the alternator 'free'?

Given all the inefficiencies, is it "free" to plug in, say, a smartphone into a car to charge while I am driving? Short answer: no (but it's so hard to measure the impact that it be hard for you to ...
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  • 24.3k
11 votes

2 Volts on Car Battery

Lead sulfate crystals form on the battery plates as the battery discharges. As the battery becomes more discharged the crystals go from being soft and fluffy to much harder. Recharged in time, the ...
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11 votes

Can a weak battery destroy an alternator?

Absolutely yes. I sell alternators all day every day - and a poor battery is often the root cause of alternator failure. A battery with a short in it will cause the alternator to run at full output ...
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  • 551
11 votes

Technique to test an alternator - fact or fiction?

To directly answer this question, no it won't work and you run the risk of damaging your alternator if it is good while doing so. The test you are asking about was used quite successfully with ...
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11 votes

Will fitting a physically bigger battery reduce the working life of the alternator?

For most of the time, the battery doesn't provide any electrical power to the car. All the power is coming from the alternator. Think about it this way: the alternator probably has a maximum current ...
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  • 2,919
11 votes

Trying to use a car alternator for a power source (not for car)

Several features. Firstly, alternators have a magnetic field that is adjustable by adjusting the amount of current at the field. This causes the output voltage of the alternator to be adjusted. The ...
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  • 13.4k
10 votes
Accepted

Can I safely upgrade to an AGM battery?

Yes for the most part an AGM is a drop in replacement for your standard Lead Acid Battery. The charging voltages are almost identical. You are correct that they have a lower internal resistance and ...
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10 votes
Accepted

2 Volts on Car Battery

Since you have a battery which isn't that old (most batteries have about a five year life span), I'd suggest you put it on a charger and try to recharge it. This will allow the battery to come back to ...
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10 votes

Car won't start after sitting for 2 years even with a jump. Is it a starter or alternator issue?

Others have already told you the battery is a goner, I'll go into why: When a battery discharges lead sulfate builds up on the plates. It is deposited in very tiny crystals that will covert back to ...
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10 votes

Car battery at 10v, then shows 14,5v while car running after boost, then when car turns off, drops to 10v again within a minute

The answer from Paulster2 explains what happened. This answer deals with what will happen next: A battery with a dead cell can explode while being charged (while the car is run). In some cases, it ...
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  • 1,457
9 votes
Accepted

Is it safe to charge two batteries off of one alternator?

The most effective way to charge an additional battery from a single alternator is by use to a Split Charging setup. You can buy kits from various sources and they are common amongst both campers, ...
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9 votes
Accepted

Non-functional gas pedal, engine stalling, dead battery: broken alternator?

An insufficiently-charged battery would explain what you're seeing: the unresponsive throttle is because the throttle is electronically-actuated (at least that's what eBay reckons) Not sure about the ...
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  • 38.9k
9 votes

Can a car AC unit be run on just the car battery?

The short answer to your question is no. As @JPhi1618 noted, the compressor is mechanically driven. Without the compressor you don't have any cooling. The AC runs by changing the state and pressure of ...
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  • 9,276
9 votes
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I shorted the alternator for a second with a multimeter set to measure current

No, you most likely didn't mess anything on the car up, though you might have fried your multimeter. If the vehicle still runs and nothing seems messed up, more than likely nothing is. Something you ...
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8 votes
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DIY Alternator Belt Tensioner

So, bear with me on this and maybe I can help. Mind you, none of what I'm going to say is going to be easy to do, but that is because your issue is not an easy one to solve. The main problems I see ...
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8 votes
Accepted

What are slip rings with respect to a cars alternator?

In an alternator there are two major components; the stator and the rotor. The rotor has a coil wound on it. By applying a current to that coil a magnetic field is formed and then spinning the rotor ...
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  • 15.9k
8 votes

Can a weak battery destroy an alternator?

A battery that has an internal electrical short, usually one of the plates has come loose and is touching its neighboring plate will cause the alternator to work much harder than normal. This can ...
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  • 11.5k
8 votes

Can a car AC unit be run on just the car battery?

Yes, this is possible. Several companies (VAG, BMW) have switched to AC units that are driven by electric motors. For BMW, this is part of their Efficient Dynamics strategy, where they'll switch the ...
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  • 4,402
8 votes
Accepted

Why won't my car even crank?

That wire will have been the main power connection from the alternator to the battery it is used for powering the car and charging the battery while the engine is running. By forcing this onto the ...
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  • 20.1k
8 votes

Car battery dead for a while... now it won't charge?

I'm not going to tell you to try and utilize a battery charger to fix the issue as @MoveMoreCommentsLinkToTop has suggested in the comments, but it has been my experience when a battery has sat for an ...
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