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Old 7th December 2011, 10:03   #16
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Re: Ammeters, Voltmeters (and Ign Telltale)

Automotive ammeters were there in Ambassador, either -30 0 +30 or -50 0 +50 full scale. They were about 2" dia and I do think they weighed in more than 100g.

AC ammeters are mostly moving iron type and will not work on DC, as the DC meters can move in both positive and negative direction. Further most of the AC meters have a current transformer to prevent high voltage entering the metering circuit, which will not work on DC.
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Old 7th December 2011, 11:58   #17
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Re: Ammeters, Voltmeters (and Ign Telltale)

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Originally Posted by Sutripta View Post
Great that some discussion on! ...
Really! Wow! Takes us back years!!! :')

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Originally Posted by Sutripta View Post
... The nominal voltage of this power bus is 14.4 volts, not 12. All loads, except self starter (and I suppose glow plugs) are designed to operate at 14.4 volts. ...
If one considers it a 'bus', the nominal voltage is 12V, not 14.4V - as automotive electrical designers all over the world use as a convention. All loads are designed to 'operate' at 12V, and 'not fail' at >=9.6V or <=14.4V (i.e. +/-20% over nominal). Even if / specifically because the bus voltage varies continuously on the bus due to 2 sources of dissimilar nature. That includes inductive loads like window winder motors, wiper motors, etc., and resistive loads like headlamps, glow plugs etc. 'Not fail' as in continue to function, without being conformant to 'too much' / 'too little' / 'too fast' / 'too slow' type of questions. Were the nominal voltage to be 14.4V, car electricals would have been called '14.4V systems'!

Starter motors will crank the engine even at 9.6V if sufficient current can be supplied by the battery. Problem is, cell failure prevents that from taking place. The battery open circuit voltage could still be 12V, but may not be able to supply enough current on load. Why would one need a meter to diagnose that? Not cranking - replace the battery (after a simple shake-the-cable test to find a bad connection). Simple, no? Aaah, but then simplicity is not soul-satisfying!!!

The Ambys and the Padminis had the DC ammeter 'charging meter' - which was an instrument to look at system behavior (alternator AND battery AND loads together), not component behavior (alternator OR battery OR specific load individually). It was always possible to fit 90/100W bulbs (replacing the OE bulbs), and blame the alternator for being bad. Or blame the battery if the needle flicks momentarily to the left due to momentary heavy load. But, the long term general trend inferred from that meter (like 'needle stays more on the left side than on the right') is enough to trigger 'prevention is better than cure' thoughts - it was effective in it's function.

Pity there is no OBD on the Chassis Electronics modules - car manufacturers should at least give the 'charging meter' as OE.
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Old 7th December 2011, 13:04   #18
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Re: Ammeters, Voltmeters (and Ign Telltale)

^^^
Nominal vs actual voltage. 12 V too much part of our vocabulary. In working conditions, the bus spends most of its life at 14.4 V, not 12 V. Otherwise no quibbles. You have also highlighted a fact which I thought would come up much later:- trend is perhaps more important than spot readings.

Focus of thread laid out in post #1.

Sometime back, there was a proposal for '42 V' automotive electrical systems. Where did the value 42 come from?

************************************************** **********************
This section is slightly technical. And given my lack of command over languages, possibly confusing and disjointed.

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Originally Posted by Sutripta View Post
Immediate question should be how can one connect two sources in parallel, which is what we are doing when we are connecting them to the same bus. More in the next post.
Both the battery and the alternator are essentially voltage sources. ie the terminal voltage remains (essentially) steady independent of current drawn. So we need to know what happens and why, when we connect these two in parallel. And what voltages and currents do we get at various points if measured with a voltmeter and an ammeter.

Logically we cannot answer the question 'What is the terminal voltage if two voltage sources are paralleled together. It is much like asking 'What is the output if one shorts the output of two gates, one at 0, the other at 1. In the digital case, algebra and logic have no concept (operator) for 'shorting'. But of course we can do it in the physical world. And we can work out the terminal voltage (which might be an invalid value, neither 0 nor 1) from basic electrical equations.

Similarly too if we connect two voltage sources in parallel. The interconnections, and the devices themselves, have resistances. And other behavior which can be lumped as an impedence value for that instant of time. This prevents infinite currents, and in fact determines load sharing. The voltage sources are not perfect voltage sources.

The alternator has a bridge rectifier system built in. Its primary purpose is to generate a DC voltage from the raw AC output. This has another effect. As long as the diodes are OK, current cannot flow backwards through the alternator. This means that if the alternator, with a given terminal voltage, is connected to a bus with a higher voltage, the alternator will sit idle. There will be no current to or from the alternator. It is as if the alternator is not there. (NB. we are working with a simplified ideal model. Which does not take into account pesky details like current for field excitation etc.)

This is the case for the alternator not producing any output, not charging, with the battery powering the bus, and loads drawing power from the bus. We will have to work out what voltages and currents we will see in various parts of the circuit in this case. The bus voltage will essentially be a reflection of battery characteristics under discharge.

More later.

Regards
Sutripta
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Old 7th December 2011, 14:19   #19
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Re: Ammeters, Voltmeters (and Ign Telltale)

Quote:
Originally Posted by Sutripta View Post
...
Focus of thread laid out in post #1.

Sometime back, there was a proposal for '42 V' automotive electrical systems. Where did the value 42 come from?
From what I have heard in analog design forums it is usually called the 14V bus. Reason is obvious - it is at 14V (+/- tolerance) almost all the time.

on why 42 (and not 35 or 50 or so on) - 14=3*14 - you can put three 12V things in series

Seriously speaking - some smart guys in their ivory towers decided things and then that became the standard (I mean there is no conceivable reason why it can't be 40V for example)

Quote:
Originally Posted by Sutripta View Post
************************************************** **********************
This section is slightly technical. And given my lack of command over languages, possibly confusing and disjointed.



Both the battery and the alternator are essentially voltage sources. ie the terminal voltage remains (essentially) steady independent of current drawn. So we need to know what happens and why, when we connect these two in parallel. And what voltages and currents do we get at various points if measured with a voltmeter and an ammeter.
While an alternator + regulator approximates a voltage source quite well (unless you draw too much current ...), and so does the battery (if you are withing its limits) - once the applied voltage is above the Battery's voltage it will either charge up or not do anything.

I don't know about automotive batteries - for CE batteries once the charging is complete either the charging voltage is removed or it is kept at a slightly lower voltage (battery may still get a "trickle charge" and may supply current during large transients, but otherwise the charger - equivalent to alternator - provides the power).

for a fully charged up rechargeable battery (or close to fully charged) acting within its designed parameter, at voltages higher than open-circuit terminal voltage battery takes very little current - overall model of the battery then must be modified - it looks more like a voltage source in series with a diode (the diode may have some series resistance too)

if the battery is significantly drained up then if a voltage higher than its open-circuit terminal voltage is applied across terminals, it will act as a voltage source in series with a resistance - the resistance in the mode depicts any losses (thermal, IR, chemical ...) but for a good battery most of the R is "loss" of power that then gets stored in the battery for future. As battery gets charged up the R keeps increasing.

You can combine the two models above by taking the battery as a voltage source in series with an impedance, where the impedance in question is <the diode in parallel with the R> . For

Quote:
Originally Posted by Sutripta View Post

Logically we cannot answer the question 'What is the terminal voltage if two voltage sources are paralleled together. It is much like asking 'What is the output if one shorts the output of two gates, one at 0, the other at 1. In the digital case, algebra and logic have no concept (operator) for 'shorting'. But of course we can do it in the physical world. And we can work out the terminal voltage (which might be an invalid value, neither 0 nor 1) from basic electrical equations.

Similarly too if we connect two voltage sources in parallel. The interconnections, and the devices themselves, have resistances. And other behavior which can be lumped as an impedence value for that instant of time. This prevents infinite currents, and in fact determines load sharing. The voltage sources are not perfect voltage sources.
If you use the model of the battery as I described above (rather than a simple voltage source) - you can put it in parallel with any other load and expect correct behaviour


Quote:
Originally Posted by Sutripta View Post
The alternator has a bridge rectifier system built in. Its primary purpose is to generate a DC voltage from the raw AC output. This has another effect. As long as the diodes are OK, current cannot flow backwards through the alternator. This means that if the alternator, with a given terminal voltage, is connected to a bus with a higher voltage, the alternator will sit idle. There will be no current to or from the alternator. It is as if the alternator is not there. (NB. we are working with a simplified ideal model. Which does not take into account pesky details like current for field excitation etc.)

This is the case for the alternator not producing any output, not charging, with the battery powering the bus, and loads drawing power from the bus. We will have to work out what voltages and currents we will see in various parts of the circuit in this case. The bus voltage will essentially be a reflection of battery characteristics under discharge.

More later.
Waiting for more.
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Old 7th December 2011, 21:32   #20
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Re: Ammeters, Voltmeters (and Ign Telltale)

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From what I have heard in analog design forums it is usually called the 14V bus. Reason is obvious - it is at 14V (+/- tolerance) almost all the time.

on why 42 (and not 35 or 50 or so on) - 14=3*14 - you can put three 12V things in series
My point:- why not call it the 36 V system? In keeping with the 12 V system.


I don't know about automotive batteries -
Even amongst the Lead Acid cell, there are variations.
for CE batteries once the charging is complete either the charging voltage is removed or it is kept at a slightly lower voltage (battery may still get a "trickle charge"
Some battery chemistries do not like trickle charging.

for a fully charged up rechargeable battery (or close to fully charged) acting within its designed parameter, at voltages higher than open-circuit terminal voltage battery takes very little current - overall model of the battery then must be modified - it looks more like a voltage source in series with a diode (the diode may have some series resistance too)
Different models, each emphasising some particular aspect.


Quote:
if the battery is significantly drained up then if a voltage higher than its open-circuit terminal voltage is applied across terminals, it will act as a voltage source in series with a resistance - the resistance in the mode depicts any losses (thermal, IR, chemical ...) but for a good battery most of the R is "loss" of power that then gets stored in the battery for future. As battery gets charged up the R keeps increasing.

You can combine the two models above by taking the battery as a voltage source in series with an impedance, where the impedance in question is <the diode in parallel with the R> . For



If you use the model of the battery as I described above (rather than a simple voltage source) - you can put it in parallel with any other load and expect correct behaviour
As said before
Quote:
Originally Posted by Sutripta View Post
The interconnections, and the devices themselves, have resistances. And other behavior which can be lumped as an impedence value for that instant of time. This prevents infinite currents, and in fact determines load sharing.
Quote:
Waiting for more.
In good time! Or you or somebody else can take over!

Regards
Sutripta
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Old 8th December 2011, 03:04   #21
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Re: Ammeters, Voltmeters (and Ign Telltale)

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Originally Posted by Sutripta View Post
...


In good time! Or you or somebody else can take over!

Regards
Sutripta
That is generous, but I don't know much about cars and relatively little about specifics of car electricals.

By the way, someone mentioned "surface charging" if battery is charged at a relatively low rpm - what is this thing?
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Old 8th December 2011, 12:11   #22
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Re: Ammeters, Voltmeters (and Ign Telltale)

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I've seen more AVR failures than I can care to remember. And most were that the output voltage went uncontrolled!
Ditto with me.
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And why did the older car designers choose an ammeter?
Did any alternator-equipped car come with an ammeter at all? Incidentally, our Amby had both the ammeter and voltmeter since the times I can remember, and both were retained even after switching over from dynamo to alternator.
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Starter motors will crank the engine even at 9.6V if sufficient current can be supplied by the battery.
Is the cranking ability of the starter motor dependent on a specific voltage? Would the voltage determine the speed of the motor or its torque?
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..."surface charging" if battery is charged at a relatively low rpm - what is this thing?
I would like to know about this too.
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Old 8th December 2011, 14:45   #23
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Re: Ammeters, Voltmeters (and Ign Telltale)

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... Is the cranking ability of the starter motor dependent on a specific voltage? Would the voltage determine the speed of the motor or its torque? ...
Trick question - interplay of 2 variables / concepts. In a motor with no load, voltage will definitely govern the shaft speed. With load, it is not so simple.

The voltage at the terminals governs the starter's speed, yes. It also effectively governs the current drawn, which governs the torque produced. One has to assume the torque required is constant - the total friction and dead-weight didn't change since the last crank, nor during cranking. If the torque produced by the starter is insufficient to accelerate the cranked system, it will not manage to create the right firing conditions. Cranking 'inability', to borrow your term.

Should matter more in diesels than petrol. Especially in cold conditions.
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Old 8th December 2011, 19:08   #24
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Re: Ammeters, Voltmeters (and Ign Telltale)

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Trick question - interplay of 2 variables / concepts. In a motor with no load, voltage will definitely govern the shaft speed. With load, it is not so simple.

The voltage at the terminals governs the starter's speed, yes. It also effectively governs the current drawn, which governs the torque produced. One has to assume the torque required is constant - the total friction and dead-weight didn't change since the last crank, nor during cranking. If the torque produced by the starter is insufficient to accelerate the cranked system, it will not manage to create the right firing conditions. Cranking 'inability', to borrow your term.

Should matter more in diesels than petrol. Especially in cold conditions.
. Starting current in any motor is much more than the running current
. Getting engine to rotate initially will impose much higher load than when it has picked up speed
. At low temperatures the viscosity of engine oil is more so more load on the starter
. Higher compression of diesels will present more load than lower compression petrol engine
. As the current drawn by motor increase, the voltage drop across the starter cable will increase, which in turn may slow the motor down a bit.

In a nutshell the starter motor will never draw the same current, until and unless all the conditions remain same - engine temperature, battery health, cable connection. Even the position of the engine wrt the compression stroke. Those of us who had a go at starting Ambassador with the starting handle have experience all the above variable, and the trick was to slowly rotate the handle till it passed the compression stroke of one cylinder and then put full force to accelerate to get some speed.
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Old 8th December 2011, 22:01   #25
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Re: Ammeters, Voltmeters (and Ign Telltale)

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By the way, someone mentioned "surface charging" if battery is charged at a relatively low rpm - what is this thing?
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I would like to know about this too.
(Extremely simplified explanation) Not all parts of the cell undergo the same chemical reaction at the same time. When being charged fast, the material at the 'surface' reacts faster than the material inside. As if that portion of the battery has been charged faster. This leads to a slight elevation of terminal voltage. If the terminal voltage is used as a metric, there will be a problem.
Best let the cell rest for a few hrs to stabilise, or
discharge slightly, and then measure voltage.

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Those of us who had a go at starting Ambassador with the starting handle have experience all the above variable, and the trick was to slowly rotate the handle till it passed the compression stroke of one cylinder and then put full force to accelerate to get some speed.
DerAlte is an old Bulleteer!

When it comes to starting, we will have to visit the characteristics of the series wound DC motor (and yes, even though there are PM starters!), and battery characteristics under high discharge.
In the meantime, lets wait for other comments before proceeding.

One question: In the previously described scenario (Alternator not charging, battery supplying load, where should we connect the voltmeter and ammeter?

Regards
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Old 8th December 2011, 22:30   #26
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Re: Ammeters, Voltmeters (and Ign Telltale)

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Trick question - interplay of 2 variables / concepts. In a motor with no load, voltage will definitely govern the shaft speed. With load, it is not so simple.
Would it be right to say that the motor's demand for current would rise with a voltage drop? So, in attempting to start a 12V powered engine with a 10V battery (let's make one with 5 cells instead of 6!), the Ah-rating of such a battery would have to be higher (and higher by what percentage?) than a 12V battery, to do the same job? Conversely, would the demand on a 14V battery be less when wanting to crank the same engine?

Remembering those 8V batteries that were used decades ago to run the 6V system in our Morris-8 and a relative's Beetle. Why did they go the 8V way? To be able to crank the engine better and longer? How?
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Old 9th December 2011, 00:19   #27
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Originally Posted by SS-Traveller
Would it be right to say that the motor's demand for current would rise with a voltage drop? So, in attempting to start a 12V powered engine with a 10V battery (let's make one with 5 cells instead of 6!), the Ah-rating of such a battery would have to be higher (and higher by what percentage?) than a 12V battery, to do the same job? Conversely, would the demand on a 14V battery be less when wanting to crank the same engine?

Remembering those 8V batteries that were used decades ago to run the 6V system in our Morris-8 and a relative's Beetle. Why did they go the 8V way? To be able to crank the engine better and longer? How?
Voltage drop where? I would say, no the current drawn will reduce if voltage is less due a drop anywhere. That's because all the parts are passive components, and the is no active regulator trying to boost 'demand' to cater to load (like driver pressing the acc pedal more going uphill).

AH rating has less to in this situation than ability of the battery to dump current under short-circuit conditions (initial part of cranking is almost that). An 'incapable' battery will a. have higher internal voltage drop and b. temperature rise, both of which will throttle current output under such conditions. A 10V battery with twice the AH capacity of a 12V battery may still not be able to do the job. Yes, a 14V battery would be better than a 12V one even if it has half the AH but better SCCH capability.
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Old 9th December 2011, 10:33   #28
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Re: Ammeters, Voltmeters (and Ign Telltale)

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Voltage drop where? I would say, no the current drawn will reduce if voltage is less due a drop anywhere.
"Voltage drop" as in (i) drop in voltage across terminals for a 12v battery; or (ii) an intentionally reduced voltage from a 10v battery. What would happen in either case? Reduced current draw?
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A 10V battery with twice the AH capacity of a 12V battery may still not be able to do the job. Yes, a 14V battery would be better than a 12V one even if it has half the AH...
And a 10v battery at thrice the capacity? 5x the capacity?
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...better SCCH capability.
What's SCCH?
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Old 9th December 2011, 11:19   #29
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Re: Ammeters, Voltmeters (and Ign Telltale)

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"Voltage drop" as in (i) drop in voltage across terminals for a 12v battery; or (ii) an intentionally reduced voltage from a 10v battery. What would happen in either case? Reduced current draw? ...
Yes. And demise of the battery.

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... And a 10v battery at thrice the capacity? 5x the capacity? ...
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Originally Posted by SS-Traveller View Post
... What's SCCH?
Short Circuit Current Handling capability. AKA Flash Amps or Discharge Current Capability. Ability of the battery to dump current without committing suicide in the process. Couldn't find a treatise for Lead-Acid batteries (sorry, I am extremely lazy ) but here is one for Alkaline batteries - see Page 8. Also, please look up Peukert's Law (don't do it on an empty stomach ).

An 'incapable' battery *for cranking purpose* will be unable to discharge high currents in a short time, which is necessary to overcome the initial toque demand (see @aroy's post). It will either not crank at all, or, if it does manage to nudge the starter motor's shaft, there will be gradual loss of adhesion of the chemicals from the plates (same as what happens during overcharging).
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Old 9th December 2011, 21:37   #30
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Re: Ammeters, Voltmeters (and Ign Telltale)

Think time to move onto actual use, ie alternator working. To have a clear overview of this we should have some idea of alternator behaviour as power producer, and the behaviour of the battery under charge.

The power output of an alternator is a function of its design, and its rotational speed. We shall only consider the case with simple inbuilt AVRs. These AVRs measure, and control the voltage as seen by the alternator output terminal, by controlling field (rotor) current.

As long as the power required of the alternator is less than the maximum then producible by the alternator, the AVR will hold the terminal voltage steady. (Without the AVR, the voltage would rise). If however the load requirement is greater than what the alternator can produce at that point, the terminal voltage will reduce. (It is as if the AVR is not there).

For the purpose of this thread, we need to understand only the very basics of battery behaviour. Under light discharge, under very heavy discharge, and under charge. And for the purpose of this post, only behaviour under charge.

There is a huge amount of information on the Lead Acid cell available in any technical reference library, and on the net. (If from the net, do choose your source with care.) That literature will also show that there are various charging regimes, and endless debates about the merits and demerits of each. However with our simple AVR, we don't have much choice, Life is simpler!

Concisely, and very very simply, as the battery is being charged, the cell voltage shows a slight positive slope till the battery is almost fully charged when the cell voltage starts climbing more rapidly. When the battery is under charge, and fully charged, the terminal voltage will show a certain value (~14.4). If the battery is connected to a voltage source of this value, it will 'float' at this voltage. (Thus float chargers!) If the battery voltage reduces slightly, power will flow into the battery, charging it. The terminal voltage will rise, the charging current will stop, and the battery will be fully charged.

It is important to appreciate the importance of this 'float voltage'. If it is set too low, the battery will not charge fully. If set too high, the battery will never reach that voltage, current will always flow in, battery will be overcharged, and service life will be shortened, sometimes considerably.

Thus the AVR is set at this float voltage. This is a critical value, and thus needs to be measured and checked.

So what happens if one connects an alternator producing power to a battery which is completely discharged? What currents and voltages will we see at various points of the circuit? And why? (That after all is the focus of this thread. This is the scenario we will get if we have an almost dead battery, push start the vehicle, and then maintain something like ~2K rpm hoping to charge the battery.)

Regards
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