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Old 27th April 2011, 18:46   #31
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Default Re: Can a dead battery damage an alternator?

For starters, mine did. Even if not originally, it was fitted as we had fried the alternator quite a few times. The ignition light was the only clue of a defect in the charging system.

I am aware of the cutout function, and if any of the coils opened the ignition light would come on. after staring at it for a few years my father decided on the ammeter and using of manual control. This was in the 70s, when few mechs in Calcutta knew much about the electrical system.

Quote:
The main fuse does all this?
Lack of overcurrent protection has the risk of any of this, or all of this, or any combination thereof.

In any closed-loop system an overcurrent condition will fry the weakest component. Hopefully a fuse or a breaker. Mechanical malfunctions aside, a break in the field will stop current from flowing through each component and this protects the alternator as well as connected components.

Quote:
You 'trickle' charge a Lead Acid Accumulator to a terminal voltage above its norm
As long as the overvoltage is within norms (14.7 max for 6-cell 12V LA, IIRC) it should be fine for the battery, and will simply demand less current. Fault conditions excepted, a normal car electrical system with engine running will not generate more than that. Given this terminal voltage a car battery cannot be damaged or overcharged.

Any CV charging system depends on a certain overvoltage to properly charge the battery. In case of small deficit the current flowing can be calculated by the potential difference between the battery and the source, divided by the impedance of the total system. As the battery rises to charge its internal impedance decreases (which increases risk of overcharging for large potential differences) but cell voltage rises much faster, reducing the potential difference to reduce the system current.

Battery in deep discharge state cannot be handled the same way as a partially charged battery. Nor are the calculations valid because the currents involved are very high and system non-linearity comes into play.
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Old 27th April 2011, 22:02   #32
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Default Re: Can a dead battery damage an alternator?

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Originally Posted by cranky View Post
For starters, mine did. Even if not originally, it was fitted as we had fried the alternator quite a few times. The ignition light was the only clue of a defect in the charging system.
Alternator or dynamo?
...
In any closed-loop system an overcurrent condition will fry the weakest component. Hopefully a fuse or a breaker.
What exactly is the loop, and where is it closed?

Mechanical malfunctions aside, a break in the field will stop current from flowing through each component and this protects the alternator as well as connected components.
Who breaks the field? When and why?
Actually, rather than getting sidetracked, would like to know how the alternator is protected from excess currents. The main fuse is on the battery side, not alternator side. Blowing of that will disconnect the battery from the rest of the circuit, not the alternator.

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Sutripta
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Old 28th April 2011, 00:12   #33
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Default Re: Can a dead battery damage an alternator?

In any electrical system there is only one path for current. A single source component may be in two paths at the same time, or follow a master-slave relationship where it either supersedes or submits to a higher voltage source.

All voltage sources also act as a load when subjected to a voltage higher than their output.

So let's take a typical car electrical system (not all systems will follow this or similar or standard designs) with an alternator, and two loads - one of the battery and one of the rest of the electrical system (lights, horn, ECU, etc). This is two complete loops with the alternator at the center of both loops.

Ideally one connects the main fuse at the junction of the two loops in the positive line, and then to the alternator. Any overcurrent here cuts out the alternator completely. However the battery stays connected to the loop so the car can continue to function, hopefully there is a warning light that indicates alternator cutout. Obviously each car follows a different system, but the alternator is always slaved to the battery for optimum operation.

There may be other fuses or protection for battery overcurrent. If all the loops blow the car is left without functioning electrical system, results will vary by car. Sometimes the main fuse in the fuse box will be in the battery line only, which means the alternator is the only source of current when and if it blows.

In start mode the battery is the active voltage source and after that the alternator takes over. When it does that the battery changes role from a source to a load.

Now let us assume a flat battery has been used in the car. The car won't even start. In a situation where an alternator is connected to a flat battery the initial charging current will be very high. For even one second I can't think of many situations where this could practically happen - if someone is trying jump leads to start a car with a terminally flat battery they have to be very brave. The moment the installed battery is connected to chassis there's going to be fireworks.

To specifically answer your question, the only two things that protect an alternator are the main fuse connecting it to the battery, and the fuse box connecting the mian loop to everything else. Some alts have two output cables/terminals, one for the battery with inline fuse and another for the main box. There are probably as many configurations as there are cars.
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Old 28th April 2011, 15:52   #34
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Default Re: Can a dead battery damage an alternator?

^^^^
Hi,
Once again too many things to discuss and debate (and disagree on!). Resisting the temptation of discussing 'closed loop' and 'master - slave'.

Maruti shop manuals are easily available. Check out the electrical diagram. Esp position of fuses.

Please consider these (real world) scenarios.

Dynamos: Any implementation where some form of output current monitoring (and subsequent control) is not done?
Any in which the protection is only by means of a fuse in the dynamo output circuit.

Alternators: It would be fairly simple to monitor the output current, and to introduce this into the AVR circuit which controls the field. Is this done?
In how many cars does the alternator output have a fuse, as opposed to the battery having a fuse (fusible link as Maruti call it).

All these questions are interrelated. Something to mull over.

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Sutripta
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Old 28th April 2011, 19:26   #35
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Default Re: Can a dead battery damage an alternator?

Sutripta, at the risk of sounding snobbish or pedantic, I am not going to accept Maruti wiring diagrams as a reference for any kind of electrical (or other) standards that should be followed by the automotive industry or indeed in this discussion.

I would consider the German high-end marques the gold standard for electrical system design. I am aware that not all will follow those procedures (specially not the Japs and specially not a cheap Jap car).

I think the discussion has veered between theory and practice. To clarify, I'm talking in pure theory terms as the question was a theoretical one. In practical situations much depends on the designer's implementation and as with all else, idealism usually takes a back seat to practicality and cost - specially in budget marques.

Have a pleasant day
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Old 28th April 2011, 21:05   #36
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Default Re: Can a dead battery damage an alternator?

Quote:
Originally Posted by cranky View Post
Sutripta, at the risk of sounding snobbish or pedantic, I am not going to accept Maruti wiring diagrams as a reference for any kind of electrical (or other) standards that should be followed by the automotive industry or indeed in this discussion.
Though I'm no wordsmith, I can think of some other adjectives.

Discussion closed, I guess.

Sutripta
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