Is there any difference between a Automotive battery charger and a alternator?

steppinthrax

Diamond Member
Jul 17, 2006
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Just curious?

I know the automotive battery chargers have a desulficator and battery maintainer (trickle charge). Other than that, is there any real difference?
 

Scarpozzi

Lifer
Jun 13, 2000
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Outputs are going to vary all around. You can get alternators and battery chargers that are all rated for different outputs.

Alternators are rated to provide 12 volts + a little extra to maintain the charge.... If you ever throw a volt meter on your battery when the engine is running, you'll see about 13.8-14 volts...and anywhere from 50-120 amps. It all depends on what the manufacturer spec'ed out. It also depends on how efficient the alternator operates within the RPMs it's currently running at.

My battery charger has 3 settings, 2 amp, 25 amp, and 50 amp settings. (trickle, battery charger, starter charger) At 50amps, it will start a car, but may require a battery to be present to crank. The application always makes a difference on the best selection to choose.
 

Zenmervolt

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Oct 22, 2000
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Just curious?

I know the automotive battery chargers have a desulficator and battery maintainer (trickle charge). Other than that, is there any real difference?
An alternator is a device that creates alternating current from rotational energy.

A battery charger takes mains current, converts it to DC, and supplies it to a battery in a manner that is designed specifically to charge the battery in the best way possible.

They are two very different devices used for two very different applications and are not really comparable.

The alternator in a car gets its rotational energy from a drive belt attached to the engine's crank pulley. The alternating current that it generates passes through a rectifier which converts the AC to DC. In automotive applications the voltage regulator is typically set for around 13.8 volts DC, but non-automotive alternators may generate any voltage, depending on their design (the AC power for your home comes from giant alternators, usually driven by some form of turbine, for example). In an automotive application, both the rectifier and the voltage regulator are typically built into the alternator housing and all three are sold together as an "alternator" by a parts store.

The alternator is effectively a "dumb" device. As it functions in an automotive application it simply tries to maintain a set voltage level.

So why doesn't the alternator overcharge the crap out of your battery?

Because the battery is wired in parallel with the rest of the car's electrical system and, as a result, the car's electronics can place a load on the battery. When the battery is undercharged, this load will cause the voltage level to drop, at which point the alternator will start providing more current. To borrow an explanation from another site:

Amperage or current is regulated by the state of charge of the battery. When the battery is weak, the electromotive force (voltage) is not strong enough to hold back the current from the alternator trying to recharge the battery. As the battery reaches a state of full charge, the electromotive force becomes strong enough to oppose the current flow from the alternator, the amperage output from the alternator will drop to close to zero, while the voltage will remain at 13.5 to 14.5.
- http://www.carparts.com/classroom/charging.htm
Basically, the voltage regulator "sees" that the fully charged battery is capable of keeping the voltage at a higher level and reduces the amount of current generated by the alternator.

The two are very different things for very different roles.

ZV
 

Ferzerp

Diamond Member
Oct 12, 1999
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The alternator is effectively a "dumb" device. As it functions in an automotive application it simply tries to maintain a set voltage level.
This is an oversimplification of a modern vehicle's charging system. At this point, they are far more sophisticated than you have described here, in the name of fuel efficiency.
 

Zenmervolt

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Oct 22, 2000
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This is an oversimplification of a modern vehicle's charging system. At this point, they are far more sophisticated than you have described here, in the name of fuel efficiency.
As far as it applies to the battery, it's essentially correct at a layman's level.

Even on the most advanced charging systems that can cut themselves off to save fuel, you're still dealing with an alternator, a rectifier, and a voltage regulator, and it's still nowhere near as optimized for charging the battery as a bench charger operating off of the mains.

ZV
 

ummduh

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Aug 12, 2008
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Not to mention is using most alternators to charge a dead battery will kill the alternator. They're not built to charge, they're built and meant to maintain.
 

Zenmervolt

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Oct 22, 2000
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Not to mention is using most alternators to charge a dead battery will kill the alternator. They're not built to charge, they're built and meant to maintain.
Only if the battery is very heavily discharged.

While it's theoretically possible to burn out the rectifier diodes by running the alternator at full output above the rated duty cycle, it's not very likely under the conditions commonly experienced. The average case of needing a jump start is not going to be enough to burn out most modern alternators from the added load of the low battery.

These days the biggest reason to caution against using a car to recharge a battery is simply that it takes so bloody long. You'd need to drive for several hours on the freeway to bring a battery back up to full so you're much better off using a bench charger.

ZV
 

LTC8K6

Lifer
Mar 10, 2004
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Many vehicles have AGM batteries these days. I think they need a little different charging regime.
 

Zenmervolt

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Oct 22, 2000
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Many vehicles have AGM batteries these days. I think they need a little different charging regime.
AGM is not the same as a gel cell. You're thinking of gel batteries, which require a different charge profile. AGM batteries will charge just fine with the same charging profile as traditional wet cell batteries provided you keep the charging current at or under 10 amps (which is, frankly, a good idea even for traditional wet cell batteries).

Lots and lots and lots of people use the terms AGM and "Gel Cell" as though they were synonymous. They aren't. Even Optima batteries, which people often call "gel", are AGM.

While gel cell batteries are relatively common for deep cycle batteries (as they tolerate heavy discharge better than AGM batteries), you will almost never find a gel cell battery used as an automotive starter battery since AGM offers higher peak amp delivery and lasts just fine as long as it's never discharged below about 60%. Both gel and AGM batteries are leak-proof and maintenance free though, which is part of why there's confusion.

ZV
 
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tortillasoup

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Jan 12, 2011
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Only if the battery is very heavily discharged.

While it's theoretically possible to burn out the rectifier diodes by running the alternator at full output above the rated duty cycle, it's not very likely under the conditions commonly experienced. The average case of needing a jump start is not going to be enough to burn out most modern alternators from the added load of the low battery.

These days the biggest reason to caution against using a car to recharge a battery is simply that it takes so bloody long. You'd need to drive for several hours on the freeway to bring a battery back up to full so you're much better off using a bench charger.

ZV
The diodes are only one part of the issue, you also wear out the brushes with a high current load like you'd have with a heavily discharged battery.
 

LTC8K6

Lifer
Mar 10, 2004
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Chrysler issued a tsb saying not to use a regular battery charger with the AGM batteries.

http://www.wk2jeeps.com/tsb/tsb_wk2_0801510.pdf

Owner's manual also says this, which is probably to protect the vehicle charging system/electronics:

If a “fast charger” is used while the battery is in
the vehicle, disconnect both vehicle battery cables
before connecting the charger to the battery. Do
not use a “fast charger” to provide starting voltage.
 

Zenmervolt

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Oct 22, 2000
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Chrysler issued a tsb saying not to use a regular battery charger with the AGM batteries.

http://www.wk2jeeps.com/tsb/tsb_wk2_0801510.pdf
The people who actually make AGM batteries repeatedly note that there is no need for any special charging profile and that they can be charged with bench chargers on standard wet cell charge profiles.

Chrysler's tip applies to large, fast-charge stations like these. The issue is that these systems (in manual mode) can set a target voltage in excess of 15 volts for standard flooded batteries as well as charge rates of up to 60 amps (yes, just for charging). These are charge rates that would fry even traditional flooded cell batteries. The difference is that you can simply add more electrolyte to a flooded battery and mask the fact that you've shortened its lifespan significantly. However, since AGM batteries are sealed, any electrolyte that boils off in this sort of abusive fast charge is lost for good. The recommended charger has an AGM mode that stops morons from entering a target voltage above 14.6, which is as high as you should ever go even for a traditional flooded cell.

Unless you're the sort of person who spends $2,000 on your battery charger, you really don't need to worry about that sort of idiot-proofing on the manual mode because the basic $100 Sears bench charger won't ever set a target bulk charge voltage that high.

Owner's manual also says this, which is probably to protect the vehicle charging system/electronics:
A "fast charger" is going to be charging at well above the 10-amp maximum I mentioned before. The "rapid charge" mode on most chargers I've seen is 20-30 amps and the manuals usually will state that using the rapid charge mode can decrease the lifespan of the battery.

When you're just using the standard < 10 amp overnight charge mode there's no need to treat AGM batteries as special. They'll take the exact same profile as a traditional flooded battery.

The problem comes only when you get into charge rates that are abusive. Flooded cells will tolerate abusive charging better than AGM, but you're just not going to get into that abusive range with a bench charger on the standard < 10 amp setting.

ZV
 

LTC8K6

Lifer
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Well, Chrysler doesn't actually say not to use a "fast charger".

They say that when you do, you need to isolate the battery from the vehicle.

Do we also have to keep our alternator from charging at more than 10 amps?

The alternator in the GC seems to be using two different charging voltages. ~14.5 and ~13.7V.

I haven't pinned it down exactly, but it seems to be at the higher voltage when the vehicle has been sitting and first started, and at the lower voltage when it's been running for a while.
 
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steppinthrax

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Jul 17, 2006
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I guess what I was really asking (and maybe Zenner answered) is if a battery charger is superior to charging a battery v.s. the car alternator. I guess if you bought a brand new battery and that battery were in your car and the only electricity that went into the battery (to charge it) came from a battery charger (with desuflicator). v.s. how the alternator charges the battery. Would the battery that was charged by the battery charger last longer?
 

LTC8K6

Lifer
Mar 10, 2004
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I guess what I was really asking (and maybe Zenner answered) is if a battery charger is superior to charging a battery v.s. the car alternator. I guess if you bought a brand new battery and that battery were in your car and the only electricity that went into the battery (to charge it) came from a battery charger (with desuflicator). v.s. how the alternator charges the battery. Would the battery that was charged by the battery charger last longer?
I think it is likely technically true that using a battery charger would make the battery last longer, but in reality, it is not a great deal of difference.
 

tortillasoup

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Jan 12, 2011
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I guess what I was really asking (and maybe Zenner answered) is if a battery charger is superior to charging a battery v.s. the car alternator. I guess if you bought a brand new battery and that battery were in your car and the only electricity that went into the battery (to charge it) came from a battery charger (with desuflicator). v.s. how the alternator charges the battery. Would the battery that was charged by the battery charger last longer?
Yes in a lot of cases will last significantly longer. People who do a lot of highway driving with not many stops won't benefit much but those who do lots of city driving with engine on-off and do short trips, or don't drive the car everyday but do have a lot of electronics will probably see their batteries lasting not much more than 2-3 years.
 

Zenmervolt

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Oct 22, 2000
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Would the battery that was charged by the battery charger last longer?
Unless you're in the habit of leaving the car sit for very long periods (months) at a time, the difference will not be significant for the vast, vast, vast majority of people.

Using a charger would be slightly better, but not by anywhere even close to enough to justify the hassle.

ZV
 

tortillasoup

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Jan 12, 2011
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Unless you're in the habit of leaving the car sit for very long periods (months) at a time, the difference will not be significant for the vast, vast, vast majority of people.

Using a charger would be slightly better, but not by anywhere even close to enough to justify the hassle.

ZV
Depends on the car... Cars with Smart-Key equivalent system that lexus/toyota vehicles have would greatly benefit from regular charging if they're driven less than 10K miles per year (spread throughout the year). Meanwhile a Honda Civic with no alarm or keyless entry probably wouldn't benefit as much.

Alarm + Keyless entry + "Smart Key" = dead battery in a LOT of vehicles. I have no idea how bad the parasitic draw is in a modern BMW if you exclude the Alarm + "Push button start" equipment.
 
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Zenmervolt

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Oct 22, 2000
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Depends on the car... Cars with Smart-Key equivalent system that lexus/toyota vehicles have would greatly benefit from regular charging if they're driven less than 10K miles per year (spread throughout the year). Meanwhile a Honda Civic with no alarm or keyless entry probably wouldn't benefit as much.

Alarm + Keyless entry + "Smart Key" = dead battery in a LOT of vehicles. I have no idea how bad the parasitic draw is in a modern BMW if you exclude the Alarm + "Push button start" equipment.
Seriously, you are WAY too effing concerned about getting every last second out of your battery.

If the car's being driven every few days, it's going to be fine. You might prefer wringing every last bit of possible life out of your battery by obsessively monitoring it and calculating the precise best way to charge it. That's fine. The rest of us have friends and families/significant others to help us spend our free time.

ZV
 

tortillasoup

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Jan 12, 2011
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Seriously, you are WAY too effing concerned about getting every last second out of your battery.

If the car's being driven every few days, it's going to be fine. You might prefer wringing every last bit of possible life out of your battery by obsessively monitoring it and calculating the precise best way to charge it. That's fine. The rest of us have friends and families/significant others to help us spend our free time.

ZV
Some people don't like getting stranded. Cars that don't get too many miles and have high parasitic draw get the charger. $100+ for a battery is no small expense except for someone who regularly pays $100 for an oil change perhaps.
 

PhoKingGuy

Diamond Member
Nov 15, 2007
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Some people don't like getting stranded. Cars that don't get too many miles and have high parasitic draw get the charger. $100+ for a battery is no small expense except for someone who regularly pays $100 for an oil change perhaps.
Ive left my USB battery plugged into my BMWs usb port since I got it and it charges 24/7 whenever its not being used by me. Ive left it in there for 2 months without driving it.

Starts on the first button press every single time.

Seriously, I think even my POS cobalt when I couldnt turn the radio off wouldnt kill the battery even when I left it on all day.
 

tortillasoup

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Jan 12, 2011
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Ive left my USB battery plugged into my BMWs usb port since I got it and it charges 24/7 whenever its not being used by me. Ive left it in there for 2 months without driving it.

Starts on the first button press every single time.
If your criteria is whether the car cranks over and turns on, then you don't know squat! An engine can crank over with as little as 11V!
Seriously, I think even my POS cobalt when I couldnt turn the radio off wouldnt kill the battery even when I left it on all day.
and that's how I know you didn't scientifically or even just carefully notice power consumption of the devices in the car. A 50AH car battery has at MOST 600WH (watt hour) of power in it. Some european cars come with group H6 or H8 which are batteries with a sizable reserve capacity of around 150 minutes. Reserve capacity = How many minutes battery can discharge 25amps until it reaches 10.5V.

i.e. Unless you do a lot of driving in your car every day and your radio doesn't use much power, I would be very surprised your radio in your car wouldn't have killed your battery if you in fact weren't able to turn it off upon removal of the key from the ignition.
 
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LTC8K6

Lifer
Mar 10, 2004
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If your criteria is whether the car cranks over and turns on, then you don't know squat! An engine can crank over with as little as 11V!

and that's how I know you didn't scientifically or even just carefully notice power consumption of the devices in the car. A 50AH car battery has at MOST 600WH (watt hour) of power in it. Some european cars come with group H6 or H8 which are batteries with a sizable reserve capacity of around 150 minutes. Reserve capacity = How many minutes battery can discharge 25amps until it reaches 10.5V.

i.e. Unless you do a lot of driving in your car every day and your radio doesn't use much power, I would be very surprised your radio in your car wouldn't have killed your battery if you in fact weren't able to turn it off upon removal of the key from the ignition.
Many vehicles have a Battery Saver system, which does not allow the battery to be drained by accessories or lights. It turns them off after 30 or 45 minutes, or when the battery voltage reaches a certain level. Many people aren't even aware that the system saved them, because the stuff turns back on as soon as the vehicle wakes up. They think that whatever they left on, has been on for three days, and they are amazed when the car starts. What a good battery!
 

steppinthrax

Diamond Member
Jul 17, 2006
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I guess to take this thread to a different direct.

What is the general reason why batteries go bad. I generally see two types of situations.

1. Battery starts to get weaker (i.e. drains fast). If you leave the blower motor on for a few minutes after off it won't start again. Cold weather exaggerates this condition.

2. Battery goes completely bad and will not hold a charge. You jump the vehicle and drive it for a hour or so and it won't start if you cut it off.

My suspicion in situation 2 there is catastrophic damage in the battery. Plates have completely turned to sediment and/or grid is bad etc...

My suspicion in situation 1 is the acid has lost is molarity and/or the plates have desulfated and/or one plate/grid etc is damaged causing reduced capacity.

Then I see people (and I have done this myself) put in liquid concentrated epsom salt (boil distilled water and add epsom salt to a super concentrated solution) in a battery. Then put it on a charger for a few hours. The battery will "come back to life"? I'm not too sure how this works??
 

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