Tuesday, February 5, 2013

Your alternator doesn't charge your battery the way you think it does.

It's a common misconception among drivers that if their battery is flat (by which I mean dead), a good long drive will charge it up. After all the alternator generates power, right? Well yes, and no. Have you ever seen or driven a car where, as you sped up, the lights got brighter? That's because the battery was dead and all the electrical power was coming from the alternator with no battery to act as a load-balancer.
Bear in mind here the distinction between a low charge, and a flat, dead battery. Yes - alternators can revive batteries that have drained somewhat, but they can't revive a dead one (or at least it's not recommended to try). When people say 'flat battery' they normally mean 'low charge'. A truly flat battery is one that is nearly dead.
In a properly functioning car, the battery is there to start the car and be an electrical load-balancer. After the initial current draw of the starter motor, the alternator takes over and runs all the loads (lights, wipers, radios and other electrical gear) whilst providing a topping-off charge to the battery. Run that battery down far enough (under about 10v) and either the alternator won't recharge it, or it will damage the cells and give them a surface charge that will draw off so fast that it makes the charge useless. The alternator can't really be relied on to recharge a fully dead battery. So much so that most alternators have a warning label like this on the box when you buy them:
The alternator in most cars is typically sized to match the demands of the running car. In other words, when the engine is on and the car is running, the alternator provides the required electrical load for all the equipment and typically doesn't offer much more than about a 5%-10% overhead for the topping-off charge. To fully charge a nearly dead battery could take a very long drive with everything electrical turned off and as stated above, could result in a near-useless surface charge (and/or a dead alternator). A quick 10 minutes down the motorway isn't doing anything for you.
That's why you should invest in a battery charger if you ever find yourself with a very low battery. Use the grid's power supply to charge your car's battery instead of trying to overload your alternator. Chargers come in many sizes from 0.75 to 1.5 amps for slow 'trickle' charging, all the way up to 200 amp quick chargers (although you'll need a dedicated electrical supply for something that beefy). Optimate, Battery Tender and Black & Decker all do reasonable chargers in the $20 to $60 range, available from most local car parts stores or online mega vendors, and using them is pretty easy.
They'll typically come with a variety of connectors - clamps, ring connectors and accessory plugs. The accessory plug might seem like the ideal choice but many modern cars don't have live 12v accessory sockets when the ignition is off, plus you're trying to charge a hefty car battery through tiny cables so I just wouldn't recommend that. Ring connectors are good if you need to charge the battery time and time again because you can wire one end of the charging cable permanently to the battery.
The most useful connectors are going to be the crocodile clips. Simple to use - connect the red one to the positive battery terminal and the black on to the somewhere on the chassis or engine, plug the charger in and leave it alone. A decent modern charger will be able to charge in three or four different modes - bulk charge, top-off and maintenance are the most common three. It should choose which mode to use based on the condition of the battery, and there'll be a light or indicator on the charger to tell you what's going on.
Two final points : some cars, notably high end Audis and VWs, have their 12v battery in the trunk or under the driver's seat. Typically they'll have charging posts under the hood somewhere for easy access - Audi put theirs under a plastic flap at the back of the engine bay, in the middle. Flip it open and there's a "+" post in there. For the negative terminal, just clamp the clip to any part of the chassis or engine where there's bare metal.
And secondly : if you have to keep charging the battery, it means either your alternator is about to give up, or more likely, the battery itself is about to give up. Expect 3 to 4 year's life out of a normal 12v car battery, irrespective of what the manufacturer or guarantee tells you. 5 years is out of the ordinary. 6 years is extreme. If you live anywhere with wild temperature swings, expect 2 to 3 years before you need a replacement.

31 comments:

Paul said...

"It's a common misconception amongst drivers that if their battery is flat, a good long drive will charge it up."

Have to disagree with this statement Chris (doesn't happen often!). Case in point, I left the lights on overnight in my rental the other day, a VW Tiguan. I couldn't open the doors remotely and the electronic dash stayed black. A very kind person had some jumper cables and we jump started the car after which I traveled to a city 2 hours away. Never had another problem, the car started flawlessly after that. I do have to add that I was up in Wisconsin and the morning temp's were around -16F so not great for batteries.

I do agree with your later statement that a quick 10 minute drive may not do the job but you specifically said "a good long drive" which I would take to mean an hour or more.

Paul

David Mackintosh said...

I keep reading this statement that batteries are only good for 2 or 3 years and honestly I don't believe it.

I owned a '96 Subaru Legacy in Ottawa Canada (annual swings -25C through +35C, plus humidity) for twelve years, and it left with the battery it shipped with. We owned a '98 Mazda 626 that we got in 2001 for eight years, and it left with the battery that it came to us with. My wife's Yaris is over 5 years old, no battery issues. And my Mazda 3 is only three years old, but I don't even think about the battery. Including my first beaters, I've owned six cars, and I think total I've purchased one battery -- that one of my beaters needed because it had been a driveway bunny for two years before I drove it.

Now maybe I'm doing something that means my batteries get extraordinary long life? -- but frankly I don't believe it.

Chris said...

David
I think it depends a lot on where you live but I'm surprised you're getting that sort of life in Ottawa. I owned a Golf, a Jetta, an Audi Coupe and an Audi Quattro when I lived in England - a temperate climate - and the batteries in those cars lasted 3 to 4 years before being totally spent. That was with a wide variety of driving including a 120 mile round trip commute every day. Since moving to the US I've had a couple of Subarus, two Hondas and two VWs. The current VW is still on it's first battery but at 2.5 years it's getting to the point where it'll barely hold a charge now. I had to put it on a charger this winter (which spurred this blog entry). The Subarus and Hondas and other VW went for about 3 and a half years each. My wife's Yaris ate it's first battery at exactly 1 day over three years. Her previous car - another Yaris was still on it's first battery at 2.5 years when she had a terrible accident. Previous to that her Subaru lunched it's first battery at 3 years and 3 months. I guess my experience with batteries is just shorter lifespan ...

Birger Delrue said...

I just bought my first battery ever. For my 2005 Suzuki Bandit. I have never had to replace a battery in any of my cars. (I've owned each for around 3 years). I live in belgium , that's about the same climate as in England.

Don Carr said...

You actually have it a little backwards, the vehicle's electrical system is actually powered directly by the alternator. The battery is just there for starting, storing extra power produced by the alternator, and delivering extra current if the alternator isn't producing enough.

There are also two related misconceptions that stem from this. The first one is that the vehicle 'ground' comes from the battery, but it is actually produced in the alternator. The second is that your stock ground connections stay sufficient to efficiently transfer power from the alternator to the battery. The ground connection usually travels through multiple brackets, from chassis, to engine block, to alternator bracket, then the alternator case itself. Corrosion builds up at these mounting points, increasing resistance, leading to excess heat and destroyed components.

If you ever have trouble with a dying battery, or dimming lights, you should run a #4 AWG battery cable directly from the rear case of the alternator to your battery negative, this gives a much more efficient path from the generation point (alternator) to the storage and distribution point (battery and fuse box).

Source: http://alternatorparts.com/do_i_need_a_bigger_battery_wire.htm

Anonymous said...

thanks for share...

Anonymous said...

Is it posible that some of you are using your cars battery the way it was intended. While the ones with shorter life batteries are adding sound systems and extra devices like GPS or phone chargers etc. I think people who add extras on their vehicles forget that the cars are designed only for an occasional plug in device. Not to run a high power amp and speakers. The battery goes bad much quicker when you have all the extras. Just think about the fact it comes designed to handle the stereo in it. You add more amps and watts being drawn off it ruins the life of the battery.

Anonymous said...

And that is exactly why they died on you quickly. 120 mile trips = overcharging, which means the batteries gas and loose their electrolyte quickly, which will kill them. 14.4v from an alternator is a standard charging voltage, but this voltage applied for long periods on a fully charged battery is too much. The alternator only puts out 1 voltage : 14.4v it care what the state of charge is. This is why RV batteries die quickly because their converters put out too high voltage 24/7 while plugged in. They dry out.

Anonymous said...

Not exactly. Most alternators in modern cars have plenty of power for extras, just nothing crazy. A 120 amp alternator in a typical modern vehicle can safely deliver 70% of that current continuously (84 amps). So say with the fan on medium, lights, ignition, small charging current to a full battery, etc there is a draw of about 45 amps. That leaves say 39 amps (550 Watts) of extra power available. SO as long as you don't have 1000w subwoofers running at max volume all the time, you should be fine.

Anonymous said...

To Anonymous:
Your statement is absolutley incorrect!

Anonymous said...

The alternator is not a battery charger. It has other functions to do amongst others to maintain a charged battery ,the s.g levels( specific gravity ) water acid ratio can only be corrected by placing the battery on a charger at a set a set amp hour rate .If you do a longish drive the alternator can put some charge into the battery to start the vehicle, but don't be surprised when the car does not start the following day. Good tip; charge a car battery irrespective every now and again especially if its more than a year old, expecting an alternator to charge a flat battery can lead to damage in the charging system

Anonymous said...
This comment has been removed by a blog administrator.
Anonymous said...

With electrical currents, there's voltage and current. Both are independent. If an alternator pushes 14.4 volts, it depends on the current at any particular time...not just voltage. In the case of smarter chargers, they vary the current over the period of a charge. For example, the C-Tek chargers, though more expensive than simple cheap ones, vary the current over the charge to manage things such as sulphating of the battery, topping up the charge and finally maintaining the charge. It's never as simple as asking your alternator to 'charge' your battery.

Anonymous said...

I have a question or more rather a combined statement. My truck was dead when I first picked it up. the lights wouldn't even come on. We jump started it there at the dealer and I drove it home approx. 80 miles. It's now 10 months later and I have yet to jump start it again and I have also ran the reserve down while washing and detailing said truck. It has yet to die again. So question is, how would you explain that? I'm not disagreeing with your article, I'm just curious.

Anonymous said...

This is ridiculous. Even a short drive can partially recharge a dead battery. Growing up poor we constantly had cars in poor repair. If a battery was too low to start the car we would get it jumped then usually just the drive across town back home was enough that it would start on it's own thereafter. Less than an hour long drive will nearly fully charge a dead battery. This article is totally wrong. The whole "batteries only last 3 or 4 years" thing is a myth. They "might" only last that long but often last much longer. Same as needing to change your oil every 3k miles, you can if you want to but you hardly need to.

Anonymous said...

This article and some of the commenters are full of BS. Notice how the alternator is part of the "charging system" - I wonder why they call it that...hmm....

Chris said...

You'll notice I'm talking mostly about dead and nearly-dead batteries in the original article. If alternators are good for charging dead batteries, someone should probably tell the people who make alternators to stop putting this warning on their boxes and in their instructions....
http://tinyurl.com/lykese8

Anonymous said...

"In a properly functioning car, the 12V electrical system is mostly reliant on the battery itself. The alternator provides a 'topping off' charge to help maintain the battery, but can't really be relied on to charge it."

This is absurd. The alternator provides you with all the current you need under normal conditions. (This is why your car while continue to run even if you had to jump start a dead battery.) Whoever wrote this is an idiot with no mechanical understanding of an automobile.

Anonymous said...

I think checking and cleaning the battery terminals periodically is a good idea too. Your alternator can;t charge your battery if the connections are corroded. I usually squirt a little WD-40 on each terminal after cleaning it. It seems to resist the battery corrosion much better. I use a cool program Automotive Wolf car care software to keep track of all my car maintenance tasks including checking the battery connectors and belts. It also keeps a great vehicle maintenance log so I know when I last replaced my battery which can be useful if you need to replace it and its still under warranty.
Thanks for the great car tips! I love your site.

Anonymous said...

Complete and utter nonsense, a flat battery condition, caused by lights left on etc, is completely cured by a 10 minute drive after a push to get it going, via the alternator, or is it some magical over the air energy transmission to the battery? Are you selling battery chargers perhaps?

Anonymous said...

There is no doubt the alternator WILL charge the battery. No doubt whatsoever.

mary said...

Yesterday I drove my car for ten minutes then for whatever reason shut if off and started it up again. It started but sounded odd and the radio stopped working. It wasn't running right I felt and the abs and brake light went on. I stopped at a gas station because I knew I was low although the gas gauge ceased to work. After I filled it up the car wouldn't start. I called a mechanic who tested the alternator and said a two foot spark shot out of it. His testing showed it was working barely enough but not great. Or maybe he said variable. The battery was pretty new.

My question is can I trust the car to take me a couple miles here and there today and get it fixed Monday or should I take it in today. And should I let the car idle or specifically not let it idle before I try to head out.

Unknown said...

This article is completly wrong and false. Whoever wrote this clearly has no clue what they are talking about and should probably do the world a favor and get a lobotomy so they are unable to spread this stupidity. This article needs to be taken down immediately.

Duck life said...

A stock car with no power draining accessories that get driven all year round will have a battery last 5 years plus. Easily. Don't let your battery drain, which kills normal car batteries. That's why boats and RV uses deep cycle batteries. Don't treat your regular battery like a deep cycle one.

Shane Kirby said...

I live in michigan and have a 89 gmc sierra thats had the same battery in ot since 1997 still starts every day so 3 year life on batteries bull sh#!

TA28Dutch said...

I only recently came across this site and it looks like there's quite a pragmatic approach to car engineering in the various 'bibles'.
Some of the comments on the blogs are quite different though. Amazing to see that people can ignore basic engineering facts and believe in ghost stories and fairy tales.
The comments on car batteries are a prime example.
I'm a mechanical engineer and have driven cars for about forty years now. I had batteries die on me within a year and some lasted more than eight. The common denominator seems to be though the more (heat) stress you put a battery through, the shorter the life span. Not rocket science, if you look at the inner workings of a battery.
I've lived in the ME with average summer temperatures exceeding 50 degr. C, and in Kazachstan, with -40 degr. C in winter and +40 in summer. Those are battery killers. Western Europe is more forgiving to batteries, as the climate is moderate.
Also a well maintained engine starts quickly and puts less stress on a battery. It all adds up.
Oh, and of course, sometimes you can just be lucky or unlucky and get more or less expected life out of one.
Keep up the good work, Chris!

Anonymous said...

This article is absolute nonsense. A car battery should last 6 or 7 years even with heavy charge/discharge cycles. If you are buying batteries that aren't lasting at LEAST five years, you need to get your batteries somewhere else. Also, a 75amp vehicle alternator running at an engine rpm of 2200 WILL in fact recharge a 675CCA car battery in 30 minutes at 10 degrees Celsius ambient.

MrMan said...

1. Correct, driving around for a while is a very bad way to try to recharge a completely dead battery (defined as one that you had to jump start to get your car going). The battery isn't just a bucket into which you "pour electricity". There
is a complex chemical behavior going on and a dead battery will not be properly restored by just driving around. You will get some charge on it, probably even enough to start the car later, but no matter how long you "drive charge" it, it won't have nearly the capacity and reserve that it is supposed to have until you put it on a charger.

2 Incorrect things peppered through these comments:
a) You cannot overcharge a battery jusy by taking it on long trips all the time
(On the face of it, this is ridiculous. If it were true, every long-haul
trucker would be killing batteries left and right)

b) An alternator does NOT just put out one fixed voltage. There is a voltage
regulator in it (or as a separate part), which limits the voltage, but you
will find, if you put a voltmeter on that battery right after start-up, that
the alternator is charging it at something like 14-15 volts.
After it warms up and goes into idle, it will probably drop to something like
13.

When you use a charger, be sure to disconnect the battery from the car. Otherwise, the charger could be powering "sleeping" devices in the car that are always on. Even though these are usually very small loads, they will make the charger "think" that more current is flowing to the battery than really is, (because some of it is
going to those other devices). Battery chargers limit the current while charging, so this will make your battery recharge slower, and not as completely.

Fierce Freddy said...

Well I had my AWD car sit for a almost a year. Gave it a boost and let it run for a few hours. Car wouldn't start when shut off. Brought a battery charger and charged the battery and the car starts after being charged. There's a limit to what the alternator can do with a dead battery. My car was completely dead.

Anonymous said...

I rebuild alternators for a living since 1990. I always tell customers to charge their battery before re-installing an alternator so the dead battery doesn't kill their alternator. On the other hand I built a battery charger with a 1/2 HP motor driving an old Delco alternator. I had a battery in my truck that seemed to be going bad so I charged it once a week with my smart charger for about three weeks and got another year out of it. I guess what I am trying to say is there is a little truth to all the above blogs. It doesn't hurt to charge your battery occasionally. Your alternator will charge your battery. A weak battery can possibly kill an alternator.

Robert C said...

I have owned a 2000 Suburban since 2006 and I do not remember ever replacing the alternator. About two years ago I bought a new Interstate Megatron II battery with 875 CCA and a “72-month” warranty. Lately the Suburban has been hard starting but once she’s going, all is well. Engine runs smoothly, headlights are bright etc. However about a week ago, I measured a “steady state” voltage (key OFF and all components that I had control over --- OFF) of 10.5V.

So I checked the battery terminals and found them clean except the negative terminal was loose. When I tried to tighten the post bolt, I found it was stripped. I also put my ammeter in series between the (-) terminal and the negative cable and measured a steady state drain of about 400 mA which is not surprising for this car since the electrical system was customized before I bought it (stereo/amp etc).

A few days ago I brought the battery to O’Reilly for diagnosis. They had to charge the battery enough to run the test but it checked out good. So I bought a new set of post bolts and fixed the negative terminal.

Unfortunately my old Schumacher SE-82 trickle charger (2A/6A) cannot charge this battery. There is no significant increase in SS voltage after almost 24 hours on the charger. But I have a newer (hopefully better) charger on order --- a Schumacher 5212A (50A/10A/2A). It is due to arrive any day now.

Today (March 31, 2017) I took the Suburban for a short drive at sub-freeway speeds, maybe 1500 RPM average, got back and measured SS voltage at 10.75V --- getting a little better.

Is it possible that I have a weak alternator? I imagine that the brushes are fairly worn by now. If so, I would like to try rebuilding it. Any ideas on where to find a rebuild kit? I think it is 130A.

Thanks,
Robert