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GQ Dual Cab. TD42Ti with fruit.
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Discussion Starter #1
Looking to get a new alternator to replace the overpowered 60amp stocker.

Looks like my options are 90 or 120 amp. Obviously the 120 amp can take a greater load, but how do the charging rates differ?

If you have a flat auxilary battery will the 90 and 120 charge it at the same rate or will the 120 be quicker?
 

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GQ Dual Cab. TD42Ti with fruit.
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Yep, I found some info on the net about this too. Looks like a discharged battery will only take full charge for a few minutes before then only needing 15 amps or so while it recharges.

So it looks like the main factor to consider is general running of all components rather than charging a flat aux battery. Reckon a 90 amp unit will do that no worries. Powering winches would be a different matter though I reckon.
 

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BORDERTREK 4X4 & FABRICATION
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Up to a certain point you can pump about as much current into a battery as you can produce but once you reach a point in the charge cycle ( usually the last 20%) the charge rate has to drop dramatically to avoid over cooking the battery.
The regulator in the alternator limits the voltage going to the battery anyway so it doesn't 'cook'. As for charge rate the battery will take all available amps left over from other running items it needs to recharge.
 

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SNIP

The larger alt will always put more amps into the battery at lower revs than a smaller one.

SNIP
WRONG. It depends on the output curve of the alternator, i.e. how it has been designed to work.

If you are going to make a statement like that, I would suggest either checking your facts first, or using words like "generally".
 

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SNIP

Up to a certain point in the charge Cycle, it will also pump more amps in during highway driving until the battery reaches a certain threshold and then the alt will taper off the charge rate because it might only be say 15A to hold the battery at the correct voltage. Up to a certain point you can pump about as much current into a battery as you can produce but once you reach a point in the charge cycle ( usually the last 20%) the charge rate has to drop dramatically to avoid over cooking the battery.

SNIP
As far as I was aware, Volts are "pushed" and Amps are "pulled" i.e. the amount of volts is set by the regulator and will be as constant as possible, based on the efficiency of the alternator and the current being drawn, but the device will only draw the current (amps) that it requires up to the maximum that the alternator can supply based on engine speed and how many amps it has been made and designed to supply.

So your theory on the charge rates being set by the alternator is incorrect as far as I am aware. It is completely based on the charge characteristics of the battery itself and how the chemical reactions happen inside the battery.
 

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The nutty professor
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from memory , the 120 amp alt that people use to replace the 60 amp alts on their td42`s doesnt charge aswell at low rpms as the factory alt, the 120 amp alt charges better at a higher rpm where the 60 amp alt puts out more amps closer to idle

and the alt will only put out enough amps to bring the voltage up to 14.2 volts or so/ or whatever the regulater is set to
 

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Learned the hard way - RD28t out TD42 in
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This sort of misunderstanding gets to me - being an Electronic Engineer. The alt does not "pump" out amps ! Its a question of load, voltage differences and current draw. A load will draw a certain current depending upon its resistance and the available voltage, P=VxI or P=I2R or P=V2/R. This is where the often used analogy of water flow in and out of a hose pipe fails. Current (amps if you must) does not flow out of a wire, it needs a load to flow into, it is the load (the resistance, keeping it simple) that "pulls" the current and determines the power developed with the available voltage. Confused - well I know what I mean !

A 120 A alternator has the ability to delivery 120A at say 13.8V, draw more than that and the regulator in the alt will back off and the extra current will come from the battery, until it is flat or can no longer give sufficient current at a voltage required by the loads. The charge rate of a flat battery, with a VOLTAGE regulated current source (the alt) is determined by the battery not the alt.

As to whether you should buy a 120A or 90A alt, is dependent upon a nuimber of things. 1) your funds ! ; 2) the expected load, lots of extras (eg spotties) will demand the bigger alt; 3) quality of alt, I much prefer say a Bosch 90A unit to a ebay Chinese special 120 A one.

However, back to your original q, I very much doubt that the recharge time of a flat battery will differ much between the 2 alts. Chose the one that matches your loads.
 

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The nutty professor
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This sort of misunderstanding gets to me - being an Electronic Engineer. The alt does not "pump" out amps ! Its a question of load, voltage differences and current draw. A load will draw a certain current depending upon its resistance and the available voltage, P=VxI or P=I2R or P=V2/R. This is where the often used analogy of water flow in and out of a hose pipe fails. Current (amps if you must) does not flow out of a wire, it needs a load to flow into, it is the load (the resistance, keeping it simple) that "pulls" the current and determines the power developed with the available voltage. Confused - well I know what I mean !

A 120 A alternator has the ability to delivery 120A at say 13.8V, draw more than that and the regulator in the alt will back off and the extra current will come from the battery, until it is flat or can no longer give sufficient current at a voltage required by the loads. The charge rate of a flat battery, with a VOLTAGE regulated current source (the alt) is determined by the battery not the alt.

As to whether you should buy a 120A or 90A alt, is dependent upon a nuimber of things. 1) your funds ! ; 2) the expected load, lots of extras (eg spotties) will demand the bigger alt; 3) quality of alt, I much prefer say a Bosch 90A unit to a ebay Chinese special 120 A one.

However, back to your original q, I very much doubt that the recharge time of a flat battery will differ much between the 2 alts. Chose the one that matches your loads.
well i just tried to keep it simple and you go make it confusing again :p
 

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from memory , the 120 amp alt that people use to replace the 60 amp alts on their td42`s doesnt charge aswell at low rpms as the factory alt, the 120 amp alt charges better at a higher rpm where the 60 amp alt puts out more amps closer to idle

and the alt will only put out enough amps to bring the voltage up to 14.2 volts or so/ or whatever the regulater is set to

Correct. I have the 120amp unit and it doesn't produce that much charge at idle compared to a stock unit. I also got a performance graph with it when I got it and at about 2000rpm I have close to 80 amps and 120 plus at 3500
 

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Discussion Starter #11
The output at low revs was not something I had considered. Reckon the 90 is a goer.

Thanks for all input.
 

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T.T. and Marin are correct.

You should select your Alternator with respect to the load it has to supply and performance curve (RPM versus Amperage).

The Alternator Voltage is set by the Reg and the current flow varies as the apparent resistance of the load changes. The Reg has no idea of the State of Charge of the battery and as the Voltage is fixed the lower ressiatnce of a flat battery cause smore current to flow.

A heavily discharged SLI Wet battery will only initially accept up to about 40 amps maximum charging current no matter how big an Alternator is connected. In real terms this current will probaby be closer to 20 or 25 and only that much for a short period as it will quickly drop as the battery state of charge increases.

Any Alternator capacity past what the vehicle and accessories are consuming plus the battery charging current is 'wasted' capacity so a better performing smaller capacity alternator will be a much better choice and will charge an SLI Battery quicker than a bigger alternator.

Bottom line... choose the Alternator with the best output curve that is equal to or above the anticipated vehicle load. A bigger capacity alternator is not necessarily better in most situations.
 

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The nutty professor
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T.T. and Marin are correct.

You should select your Alternator with respect to the load it has to supply and performance curve (RPM versus Voltage).

The Alternator Voltage is set by the Reg and the current flow varies as the apparent resistance of the load changes. The Reg has no idea of the State of Charge of the battery and as the Voltage is fixed the lower ressiatnce of a flat battery cause smore current to flow.

A heavily discharged SLI Wet battery will only initially accept up to about 40 amps maximum charging current no matter how big an Alternator is connected. In real terms this current will probaby be closer to 20 or 25 and only that much for a short period as it will quickly drop as the battery state of charge increases.

Any Alternator capacity past what the vehicle and accessories are consuming plus the battery charging current is 'wasted' capacity so a better performing smaller capacity alternator will be a much better choice and will charge an SLI Battery quicker than a bigger alternator.

Bottom line... choose the Alternator with the best output curve that is equal to or above the anticipated vehicle load. A bigger capacity alternator is not necessarily better in most situations.

i would have said in your quote (RPM vs AMPS) not voltage , its the amp output that controls voltage, the alt will keep pumping out max amps (for its rpm) until the voltage reading at the alt (sense wire if fitted) reaches 14.2 ish (whatever the regulater is set too) if it takes 60 amps to bring the voltage up to spec then the alt will out put it , it just depends on how many amps it takes for the battery to reach 14.2 ish volts, plus if you have a 2nd battery as low as the main battery then you can double the amp out put needed to bring the battery voltage up to spec
 

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OK lots of confusion still I see.

A vehicle alternator is a voltage regulated current source. That means to say that its output voltage is constant over its operating range, the current varies depending upon load. A wet cell battery needs a constant voltage (in a vehicle any how), variable current source to charge. The current flow between the alternator and battery depends upon the voltage deference between the two and the internal resistance of the battery (and the alt's internal resistance too, but lets say that is limited to the max rating of the alt to keep it simple). As the battery charges up, its terminal volts increase and the volt difference between it and the alt reduces, therefore so does the current flow. It now comes down to the characteristics of the battery and how it behaves during its charge / discharge cycles and of course its age. The older the battery, the higher the internal resistance, this limits its ability to deliver high current outputs and its ability to draw a high current during charging. You could increase the voltage of the alt to get the battery to draw more current, but that would damage the connected ancillaries, not a good idea.

It's interesting to compare a lead acid charging system to that of NiCad battery. As already said, the lead acid works with a constant regulated voltage, the current reduces as the battery charges. A Nicad is the opposite, it as a regulated current source, the voltage varies to get the battery to draw the set current. With Nicads, it's easy to work out charge times, eg say a charge rate is 100mA and the battery is 1000mAH, it will take 10 hours to charge fully; its not so easy to work this out with lead acid batteries.

Another area of discussion this thread has raised is how the alt performs at different speeds, it would appear from some posts that the supposedly higher spec high o/p alts don't deliver the goods until they are spinning much faster. This is important because it may very well be that at low speed driving, all lights on etc, the lower spec alt would actually keep the battery topped up more than the supposedly higher spec unit !
 

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BORDERTREK 4X4 & FABRICATION
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Another area of discussion this thread has raised is how the alt performs at different speeds, it would appear from some posts that the supposedly higher spec high o/p alts don't deliver the goods until they are spinning much faster. This is important because it may very well be that at low speed driving, all lights on etc, the lower spec alt would actually keep the battery topped up more than the supposedly higher spec unit !
Thats true, an alernators output is directly related to it's rpm so it pays to know the output curves of the alternator your using.
eg: a factory TD42 60A alternator will not put out it's max amps until warmed up (which is called hot output current and can get slightly over 60A) until 5000rpm (alternator rpm that is! which is faster than engine rpm) and it's voltage is regulated at 14.4 - 15.0V.
Between 1300 and 2500rpm the hot output current should be between 26-58 amps, which is the rpm range where most driving is done in a 6cyl diesel.
 

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Hi GQshayne, it’s always better to have the largest alternator you are prepared to fit.

While, as glort pointed out, in city driving, the larger the alternator the higher the current production while in low rev situations, like driving in city traffic, BUT like most RV use, as it is more likely you are going to be on the open road with low batteries so even your 90 amp alternator will be heaps big enough to meet your needs.

Next, most batteries WILL NOT take everything your alternator will put out. While AGMs, is supplied with a high enough VOLTAGE, will take full inrush currents when they are low, there is a big, BIG problem with allowing this to happen.

Contrary to most beliefs that AGMs will take everything an alternator can produce, and while this IS TRUE of automotive grade AGMs like Optima, Odyssey and Orbital batteries, the vast majority of AGMs were never designed for automotive use and while they will take full inrush currents when they are low, they will be damage very quickly if allowed to do so.

If you want to use garden variety AGMs, as long as you fit them somewhere other than the engine bay, the long cable run between the alternator and these types of batteries will act as a quasi voltage/current regulator so the batteries will be fine.

If you fit standard flooded wet cell batteries as your auxiliary battery under bonnet, even if you fitted a 1,000 amp alternator, the battery, not the alternator, will decide how much current the battery will "PULL" from the alternator.

A rough example, at the start of your daily drive out on the open road, if you have a low 100 Ah flooded wet cell battery with a State of Charge ( SoC ) of around 40% and your alternator has an operating voltage of 14v, your auxiliary battery will start recharging at around 35+ amps and this will be the same if you have a 90, 120 or 1,000 amp alternator.

So a 90 amp alternator will easily handle you auxiliary battery’s needs and still leave adequate power to run your vehicle and power a heap of driving lights, BUT if you find you add just one more battery, say in a camper trailer or caravan, then as long as you only drive during day light, you will be fine but night driving will probable push your current requirements above a 90 amp alternators capacity.

This won’t harm the alternator, it will simply lower it’s output voltage while a higher current load is applied but the lower voltage will mean it takes longer to fully charge your batteries.

So GQshayne have a look at what you think your needs will be and pick the size of alternator that will do what you want.

BTW marin, I like your comment ‘Volts are "pushed" and Amps are "pulled"’, a simple but accurate statement.
 

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I think this all lost on glort and his lack of understanding of automotive electrical systems, or electricity in general.

drivesafe said:
SNIP

While, as glort pointed out, in city driving, the larger the alternator the higher the current production while in low rev situations,

SNIP
This is still untrue, the output of a alternator at a certain revs (the "low revs" you are talking about) depends entirely on the alternator and it's output curve.
 

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Hi marin and yes there are a number of factors that will effect the amount of current a given alternator will produce at low revs, such as pulley size and such, BUT the simplest way to increase output current at low revs is to fit a larger capacity alternator, which is exactly what many of the auto manufacturers are doing.

A perfect example is the Discovery 4, it comes standard with a 180 amp alternator ( generator ).

The D4 does not need 180 amps to function but it does, like many new vehicles, have a high power consumption, so the alternators large output capacity means a fair amount of power is going to be produced at low revs.
 

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Hi marin and yes there are a number of factors that will effect the amount of current a given alternator will produce at low revs, such as pulley size and such, BUT the simplest way to increase output current at low revs is to fit a larger capacity alternator, which is exactly what many of the auto manufacturers are doing.

A perfect example is the Discovery 4, it comes standard with a 180 amp alternator ( generator ).

The D4 does not need 180 amps to function but it does, like many new vehicles, have a high power consumption, so the alternators large output capacity means a fair amount of power is going to be produced at low revs.
Yes..... But in the end, it all depends COMPLETELY on the output curve of the alternator.
 

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The nutty professor
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lovely debate going on,,,,,,,,,,,,,,,, what you guys are saying is true more or less, but i think we are refering to the standard 60 amp alt and the 120 amp alt that commonly replaces the 60 amp unit, the spec sheet for both was posted up on here ages ago and i remember that the 120 amp unit produced less amps (given the same rpm) untill the revs reached around 2000 rpm , then the 120 amp alt started to produce more power, this has also been backed up by people who have the larger alt

fitting different alts will have its pros and cons as every motor has different sized pully on the crank shaft , this will affect the alts output
 
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