PDA

View Full Version : Car battery charging Q: how long?


commasense
09-07-2005, 08:08 PM
Well, the battery in my car died. I think I know what caused it and have removed (what I hope is) the cause of the drain.

But I had no luck jump starting it from either of my other cars. So I brought the dead battery into the house, and have put it on the old charger I bought 25 years ago.

My question is, how long should it take?

Now, it's a trickle charger that won't blow up the battery by overcharging it. I just want to have a good idea of when it's done. Here are the details:

* Die Hard Gold battery, rated at 640 cold cranking amps. (AFAICT it is stone cold dead.)
* Charger: 12 volt, 1.2 amp output.

What do you think? (Showing your work will enhance your credibility.)

Thanks.

Shagnasty
09-07-2005, 08:13 PM
How do you know that it didn't just die for good? How old is it?

Q.E.D.
09-07-2005, 08:23 PM
If your battery is stone cold dead as in zero volts across the terminals, it is most likely gone permanently. A complete discharge is almost always death to a starter lead-acid battery, especially if it's been used a lot. Deep cycle or marine batteries are a bit more tolerant of complete discharge, but these are rarely used in automotive applications.

commasense
09-07-2005, 09:06 PM
The battery is only a couple of months old. The same thing happened once before, so I replaced the 2.5-year-old OEM battery with this new one. The first time, I thought I had left the dome light on for a couple of weeks. Now I realize--hope--that it may be a GPS puck I had put in just before the first dead battery. In both cases, the car sat unused for at least a week or two before turning up dead.

Q.E.D., I didn't think to check the voltage before I started charging (d'oh!), but I just checked it after exactly two hours of charging, and it's showing 12.1 volts now. So maybe it wasn't completely dead.

Assuming it's not, what about my original question?

Of course, it's under warranty, so I should be able to get a replacement if worse come to worst. Unless there's some fine print that lets Sears weasel out of it.

Crafter_Man
09-07-2005, 09:49 PM
If it's truly a trickle charger, you may need to keep it on there for two or three days. Then check the voltage. It should be 12.6 V.

But Q.E.D. is right... completely discharging a standard lead-acid battery is very bad on it. Based on my experience, you can (usually) get away with completely discharging a lead-acid battery twice. After the third time, the battery is toast.

snailboy
09-07-2005, 10:32 PM
I somehow managed to completely discharge mine two nights ago. (I think I didn't close the door good and the dome light stayed on.) I even jumped the battery terminals with a wire and there was no spark at all. I put the charger on it yesterday on 2 amps and today, it cranked as well as it ever has. This battery is about 5 years old by the way. So it's apparently possible to charge a battery just after it died at least once, although I don't doubt it's bad for it. The longer you leave it dead, the worse it is too, because the sulfation on the terminals tends to harden making the process irreversible.

Crafter_Man
09-07-2005, 10:50 PM
I did some math on it. I'm not sure if it's correct. Perhaps a Doper math whiz could double check it for me.

We first need to figure out how much energy a car battery can store. If the battery is 640 CCA, then it must be capable of sourcing 640 amps at 7.2 V for 30 seconds. This is a power of 4608 W, and the energy produced over these 30 seconds is 138240 J. But the battery doesn't go dead after 30 seconds, so the total energy in the battery must be higher. I found out the "reserve energy" is a better number to use. According to this (http://muller.lbl.gov/teaching/Physics10/chapters/19-ElectricityMagnetism.html) page, a typical car battery has around 2,000,000 J when you calculate the energy based on reserve energy. So let's use that figure.

Now let's assume your car battery is completely dead. This means it contains no energy, and the voltage is 0 V. How long will it take a charger to pump 2,000,000 J into the battery using a current of 1.2 A?

Let's assume that, while we're charging the battery at 1.2 A, the battery voltage linearly increases over time. Let's also assume the battery is fully charged (and thus the energy in the battery is 2,000,000 J) when the voltage reaches 12.6 V. The time it takes to fully charge the battery is T seconds. We need to solve for T.

We know that

P = I*V

and thus

p(t) = I*v(t) where I = 1.2 amps.

When t = 0, v(t) = 0 V. And when t = T, v(t) = 12.6 V. Assuming a linear increase in voltage over time,

v(t) = (12.6/T)*t, where t is in seconds.

Therefore

p(t) = I*(12.6/T)*t

Energy is the integral of power:

E = 2000000 = ∫ p(t) dt, where the limits of the integral are t = 0 to t = T.

E = ∫ I*(12.6/T)*t dt

E = I*(12.6/T) ∫ t dt

E = I*(12.6/T)*((T^2)/2 - (0^2)/2)

E = I*(12.6/T)*(T^2)/2 = I*12.6*T/2

Solving for T:

T = 2*E/(I*12.6) = 2*2000000/(1.2*12.6)

T = 264550 seconds ≈ 3 days

This assumes, of course, the battery will take a charge.

commasense
09-08-2005, 12:08 AM
Wow, Crafter_Man, that's way more math than I expected would be needed. Thanks for the effort.

It also explains why my attempt to recharge the original battery may have failed. I only charged it for six or eight hours. Even if I hadn't killed it completely, it probably would have taken many more hours to revive.

This one has had five hours so far. I'll check it at twelve hours, and see how it looks.

Is there any way of telling how it's doing before putting it back in the car, other than checking the voltage?

Rick
09-08-2005, 12:38 AM
I did some math on it. I'm not sure if it's correct. Perhaps a Doper math whiz could double check it for me.

We first need to figure out how much energy a car battery can store. If the battery is 640 CCA, then it must be capable of sourcing 640 amps at 7.2 V for 30 seconds. snip...
Actually CCA is what a battery at -0 degrees F can deliver for 30 seconds, and still have 7.2V remaining.
So your numbers are all off by a bit. A warmer battery will deliver more amps (up to a point)

My suggestion is this. The open circuit voltage of a battery will give you a pretty good idea of it state of charge. The internal resistance of the battery is .2 volts per cell. On a 12V battery the charger will run at 1.2V over the open circuit voltage of the battery. So with the charger running, measure the voltage across the terminals with a volt meter. Subtract 1.2V and compare to these numbers (Don't think that you can shut off the charger, and save yourself the math, the surface charge will screw with your readings! Measure the voltage with the charger running, and do the math. It really isn't that hard :) )
12.6V and above 100% (fully charged is 12.72, but anything above 12.6 is considered fully charged)
12.45V 75% charged
12.25V 50% charged
12.00V 25% charged
Example: Voltage measured with charger running 13.6V. 13.6-1.2=12.4 about 75% charged
I would not replace the battery back into the car until its charge level is 75% +. Alternators are not large battery chargers, they just trickle charge a battery. If you put the battery back in too soon, it may never get fully charged via the alternator, and will fail early.
When you do get the battery up to 75%+ report back here so we can check Crafter Man's math. :)

Crafter_Man
09-08-2005, 06:34 AM
Actually CCA is what a battery at -0 degrees F can deliver for 30 seconds, and still have 7.2V remaining. So your numbers are all off by a bit. A warmer battery will deliver more amps (up to a point)Yea, I knew someone would catch that! But at you can see from my post, I didn't even use the CCA info at all. I was trying to figure out how many Joules of energy a typical car battery could store, and I learned it couldn't be calculated from CCA info. I do not even know how accurate the 2,000,000 J number is.

Rick
09-08-2005, 08:12 AM
Crafter Man :D

As far as the Joule number goes, beats me. That is way more math than I would care to invest a battery recharge.

commasense
09-08-2005, 09:26 AM
Thanks, Rick, for the further suggestions. It's been charging for about 14 hours now. I'd follow your suggestion, but unfortunately, I accidentally left my VOM on overnight, and killed its batteries. (!) I have an older meter, but it's reading below 12 volts (with the charger disconnected) and I don't trust it. As I said, last night the digital VOM was reading 12.1--12.2.

So I'll keep the charger on and get some new batteries for the meter and check back in. I may also buy a newer and more powerful charger while I'm at it.

Thanks again, guys.

CookingWithGas
09-08-2005, 09:44 AM
I have a charger with an ammeter built in and the directions say to charge until the current is half the rated current for the charger (something in the single digits range, can't remember). If you have a bare-bones trickle charger you probably don't have the ammeter, and I would agree with others that you probably want to go a couple of days.

A trickle charger is intended to keep a battery topped off during idle periods rather than charging it up from dead, so that's why it's more than a couple of hours.

CookingWithGas
09-08-2005, 09:50 AM
I have a charger with an ammeter built in and the directions say to charge until the current is half the rated current for the charger (something in the single digits range, can't remember).

A trickle charger is intended to keep a battery topped off during idle periods rather than charging it up from dead, so that's why it's more than a couple of hours.

Ike Witt
09-08-2005, 12:44 PM
So maybe it wasn't completely dead.
Perhaps it is just pining for the Fords?

commasense
09-08-2005, 09:46 PM
After charging overnight (14 hours) it was still at only 12.2 volts. So I went out and got a 2/6/10 amp automatic charger, and put the battery on it for about five hours, and now the charger shows it fully charged.

I'll put it back in the car tomorrow and with any luck everything's okay. I'll know for sure by the end of the month, because I'll be out of town for a couple of weeks, and if I haven't correctly IDed the drain, the battery will be dead again when I get home.

The bad news is that because I seriously underestimated the charging time for my old charger, I probably could have saved the original battery I traded in when I got the new one. Oh, well. Live and learn.

The good news is that I now have a cool new charger (that with any luck I'll never need again).

Thanks for all the help guys.

Rick
09-09-2005, 09:55 PM
About your new charger, follow this rule of thumb, if you discharged the battery fast, you can charge it fast, if you discharged it slow, charge it slow.
If you left eh lights on, go ahead and use the 10A setting. If the battery sat in the car for 3 months and it went dead from a parasitic draw, use the 2A setting.
Trust me on this you battery will thank you for it.

commasense
09-09-2005, 10:11 PM
Thanks for the advice. I took the middle course. Since my old charger was 1 amp, and according to Crafter_Man's excellent math it might have taken three days at that rate, I opted not to use the 2 amp setting on the new charger. However, I did resist the temptation to use the 10 amp setting, and used 6 amps.

It took about four or five hours to charge fully @ 6A, on top of the 16 or 17 hours it had gotten on the old charger. And when I put the battery in the car this morning, it worked fine. In about three weeks it'll get a real test, after I get back from being out of town for two weeks. We'll see then how it has held up.

Thanks again.

Q.E.D.
09-09-2005, 10:33 PM
...if you discharged the battery fast, you can charge it fast, if you discharged it slow, charge it slow.
I do not believe this to be true. From what I understand of the chemistry of lead-acid batteries, there is no mechanism to create a difference in the battery characteristics based upon discharge rate. As I understand it, the ideal charge rate for lead acid batteries is normally given to be C/10 or 1/10th the nominal ampere-hour (Ah) capacity of the battery. For example, a battery with a capacity of 40 Ah should ideally be charged with a current of 4 A. I've heard the claim you make before, but I've never seen an explanation that I could accept. If you have such an explanation (the more detailed, the better), I'd appreciate it. My own research has turned up nothing pertinent.

Rick
09-10-2005, 01:23 AM
QED It has to do with the sulphation on the plates of the battery. As the battery discharges the eletroyte's specific gavity gets less and less as the SO4 get deposited onto the plates creating PbSO4. If the battery stays discharged long enough, or is discharged slowly, the the PbSO4 hardens and it is very difficult to reverse via a battery charge. If too high of a current were applied, the battery will get hot, and the localized heat could cause a warping of plates, and possible destruction of the battery.
Look up a 3 minute charge test. This test tests the ability of a battery to accept a charge, you start with a battery that is less than 25% charged. Apply a charge approaching but not over 40A, after 3 minutes, measure the voltage at the terminals with the charger running. If under 15.5V reduce to a standard charge rate and continue, if over 15.5V charge at a very low rate (2-3A) for upto 48hrs to reverse sulphation.
Why the reduction of charge rate if the voltage goes over 15.5? Because the battery is not accepting full charging current, the voltage in the charger will spike and the temp in the battery will also rise, sometimes dangerously (for the life of the battery anyway) also the battery will gas excessively. Both of these are not good things. However if you charge at a low rate, the chemical reaction may just reverse without the boiling of electrolyte, and warping of the plates. So causes some batteries to pass a 3 minute charge test and others to fail? It has to do with how old / hard the PbSO4 is on the plates. If it is soft and spongy, the battery can take a high charging current. if it has become hard and brittle, only a low current will do. The 3 minute test gives you a way to find out the condition of the battery, and how high a rate of charge it will take.

The above was explained to me by an field engineer from a battery company, when I was on a task force working to lower battery warranty claims in our company. My group changed some procedures, issued a service manual, issued a special low output battery charger for sulphated batteries, issued a new battery testing procedure (and tester), and a few paperwork changes. Net after all of this was a savings of over $4 million dollars per year in warranty expense.

It's late, I'm tired and writing this after a couple of glasses of good Scotch. So if it makes even less sense than my usual posts, I will be happy to come back tomorrow and try again when I am rested / sober. :D

Q.E.D.
09-10-2005, 09:51 AM
QED If the battery stays discharged long enough, or is discharged slowly, the the PbSO4 hardens and it is very difficult to reverse via a battery charge.
All-in-all, a very nice post, Rick. However, the part I bolded above is the bit I have trouble with. I fail to see how a slow discharge is significantly different than a battery with at least a significant fraction of a full charge sitting on a shelf, self discharging. Although the self-dishcarge rate is very low (on the order of a few milliamps, IIRC), even a much larger (but still "slow") discharge, say a few hundred mA won't cause much in the way of measurable heating or anything else that I can think of that would cause the sulphation to harden. Batteries can sit, charged, on the shelf for a few years, and AFAIK, the sulphation doesn't begin to harden until the level of discharge becomes severe. Can you explain the mechanism by which a slow discharge causes the hardening? I've always taken the sulphation hardening to be caused by the amount of sulphation more than the time the sulphation is allowed to sit. Otherwise, new batteries which sit on the shelf for several months before being purchased would be almost worthless.

Gary T
09-10-2005, 03:25 PM
Otherwise, new batteries which sit on the shelf for several months before being purchased would be almost worthless.
My experience is that battery manufacturers rotate their vendors stock so that no batteries are on the shelf for more than three months. The old (longer than 3 months) batteries are reclaimed or refurbished at the plant. Where are you finding batteries that sit for years and are still offered for sale?

Thudlow Boink
09-10-2005, 07:16 PM
Can anyone explain why the jump-starting attempts wouldn't have worked, if the battery was still rechargeable?

commasense
09-10-2005, 08:25 PM
Yeah, now that you mention it, what about that?

It happened the first time too, which is one reason I bought a new battery. I figured I had killed the old one.

Rick
09-10-2005, 08:54 PM
All-in-all, a very nice post, Rick. However, the part I bolded above is the bit I have trouble with. I fail to see how a slow discharge is significantly different than a battery with at least a significant fraction of a full charge sitting on a shelf, self discharging. Although the self-dishcarge rate is very low (on the order of a few milliamps, IIRC), even a much larger (but still "slow") discharge, say a few hundred mA won't cause much in the way of measurable heating or anything else that I can think of that would cause the sulphation to harden. Batteries can sit, charged, on the shelf for a few years, and AFAIK, the sulphation doesn't begin to harden until the level of discharge becomes severe. Can you explain the mechanism by which a slow discharge causes the hardening? I've always taken the sulphation hardening to be caused by the amount of sulphation more than the time the sulphation is allowed to sit. Otherwise, new batteries which sit on the shelf for several months before being purchased would be almost worthless.
Q.E.D. thanks for the compliment.
Now as far as the thick /thin vs hard soft discussion goes, at first I thought you might have a point with the thickness of the PbSO4, but its thickness would be the same regardless of if the headlights were left on all night or the car was parked with a 25mA draw for 3-4 months. I can tell you from experience that if you left the headlights on all night, the battery (all other factors being equal) will soak up the chargre like a sponge. The car that got its dead battery from parasetic draw will only take a very low current over a very long time. To try and fast charge it will cause the voltage to spike, heat and gas to be generated. So I can't go with the thick/thin theory. In the industry the term sulphation is used to describe a battery that is discharged and either will not take a charge at all, or only take a very low charge current. That battery charger I mentioned in my last post is a 5A charger. [mini rant] Most commerical battery chargers are sold for the wrong reasons, and with the wrong abilities. When you go to a tool truck, or a store and they have a new charger, if you ask about it the response is always the same "This bad boy puts out 600A!" This is the booster setting and in no way represents the best way, or for that matter any way to charge a battery. Trying to charge any automotive battery @ 600A would destroy it. [/mr]
I can't speak for other people that sell batteries, but my company requires a test and re-charge on any battery that sits on the shelf for more than 6 months. Places that sell lots and lots of batteries like Sears get weekly battery deliveries and carry a 1 month supply at the most. For small repair shops*, and odd size batteries that sell very slowly, the batteries are shipped dry, and eletrolyte is added when the battery is sold.
*Interstate batteries (and other compies like this) will put batteries in a shop on consignment basis, and rotate the stock every few months.
I will ask a few of the really smart guys around the company and see if I can get you a better answer.

Can anyone explain why the jump-starting attempts wouldn't have worked, if the battery was still rechargeable?
Because alternators are not battery chargers. Even though the alternator in your car might be rated at 150A or more (the alternator in my car is rated at 180A, and will put out almost 200A :D ) the control circuit is designed to supply the car's electrical demand, and act as a trickle charger for the battery itself. if you read up to Crafter Man's post it can take days to trickle charge a battery. You probably would not / could not drive long enough to charge a dead battery using the alternator. Alternators and the voltage regulators are designed to keep a fully charged battery fully charged, and bring up to a full charge an almost fully charged battery.
True story, customer buys a new car in Santa Monica. When picking up the car, it won't start (sat on the lot too long and had a dead battery) Salesman jump starts car and says you live a long ways off, the alternator will charge the battery. Customer drives off to Apple Valley (Way the hell out in BFE, over a 100 mile drive from dealer) gets there, goes to take wife for a drive in his new car. Guess what? it won't start, battery is dead. Not 100% dead, but no where close to starting the car. He calls for a tow, and they bring the car to me. I replace the battery and all is fine and good.
Bottom line, alternators do a different job than a battery charger.

commasense
09-10-2005, 09:18 PM
Rick, I think you've misunderstood my use of "jump start." I didn't mean rolling it and popping the clutch. (I'd call that bump starting, not jump starting.) I meant using jumper cables from another car.

Both times this happened, I tried jumping from another car, whose engine was running, and the car with the dead battery still wouldn't start. In the most recent case, I tried to jump from two different cars, just in case. It would crank once, and then stop, lights out.

I don't get why, when I've got a good battery (in car B) in parallel with the bad one (in car A), the good battery shouldn't start car A, regardless of the condition of the bad battery. Presumably, if I had removed the bad battery from the circuit, car B's battery would have started car A. But the dead battery somehow prevents this.

Crafter_Man
09-10-2005, 10:06 PM
Even though the alternator in your car might be rated at 150A or more, the control circuit is designed to supply the car's electrical demand, and act as a trickle charger for the battery itself.I wouldn't call it a trickle charger. If the battery is low, and there aren't many accessories running, the alternator is capable of pumping quite a few electrons into the battery over a short period of time. The amount of current going into the battery will depend on the battery's Voc, the maximum current output of the alternator, and other loads connected to the alternator.

http://www.uuhome.de/william.darden/carfaq5.htm
The vehicle's electrical load is normally satisfied first by the charging system and then any remaining power is used to recharge the battery. For example, if the total electrical load is 14 amps and the charging system is producing 35 amps at 2500 RPM, then up to 11 amps will be available for recharging the battery, which will take approximately six minutes. If the charging system is operating at say a maximum capacity of 90 amps at 5000 RPM, then the battery usually will be recharged within two minutes. Now let us assume that the engine is idling and the charging system is only capable of producing 10 amps. Four amps from the car battery are required to make up the difference to satisfy the 14 amp electrical load and the battery is being discharged further. This is why making short trips, driving in stop-and-go traffic, or during bad weather when there is a heavier electrical load, the starting battery may never get recharged and may even become "completely" discharged.

Q.E.D.
09-10-2005, 10:12 PM
I wouldn't call it a trickle charger.
Neither would I. How you got ME quoted for something Rick posted is beyond me. Unless you did it manually and had a brain fart. ;)

Rick
09-10-2005, 11:58 PM
Neither would I. How you got ME quoted for something Rick posted is beyond me. Unless you did it manually and had a brain fart. ;)
Crafter man from your link
9.2.13. Alternators are not designed to recharge dead (or flat) batteries and the stator can be burned or diodes go bad if used in that manner...
A vehicle charging system is made up of three components, an alternator (or DC generator), voltage regulator and a battery. Usually when a vehicle is jump started, it is NOT driven long enough to fully recharge the battery. The length of time to fully recharge the battery depends on the amount of discharge, the amount of surplus current that is diverted to the battery, how long the engine is run, engine speed, and ambient temperature. An alternator is sized by the vehicle manufacturer to carry the maximum accessory load and to maintain a battery and NOT to recharge a dead battery. For example, if 300 amps were consumed for two seconds to start a car from a fully charged battery, it will take an 80 amp charging system approximately 18 seconds to replace the power used. If 25 amps are available to recharge the battery, it will take 60 seconds and 40 minutes at one amp. With a dead 120 minute RC battery, it would take approximately 45 minutes at 80 amps, 2.4 hours at 25 amps, or 60 hours at one amp to obtain a 90% State-of-Charge (SoC). More information can be found in Dan Landiss' Car Batteries Are Not 12 Volts on http://www.landiss.com/battery.htm for more information about vehicle charging systems.
I stand by my comment that an alternator is not designed to charge a dead battery, but rather to keep a fully charged battery fully charged.
Oh by the way on that 180A alternator on my car? It produces 50A at idle when requested by the voltage regulator. So at some point you gotta ask, how does Rick know this? Simple when I am teaching starting and charging systems we test he output under various conditions. We attach a amp meter and a carbon pile load to the car start and let idle. At idle we increase the load until the voltage drops to 12.7 (max A output) and read the result. about 50A at idle. I would never expect to see 50 if I just hooked up an amp meter, regardless of the condition of the battery. Why? The control circuit in the voltage regulator. A lot more sophisticated then the days of yore. Hell some of our cars now have computer control of alternator output voltage. So what a system can do, and what it does do may be two different things.

commasense Yes I did misunderstand you, sorry about that. Jumper cables come in many flavors from complete shit for less than $10. to very good for over $50. If your battery had almost enough power to start your car (car cranked slowly, but it did crank) a cheapo set might and I repeat might do the trick. If your battery is left the headlights on all night, stone dead even the really good sets of jumper cables might have to stay attached for a couple of minutes to pump enough amps over into the dead battery to have a chance.
To give you an idea of price varrations I paid about $50 bucks 20 years ago for my set of jumper cables.

Q.E.D.Neiner, neiner, neiner. My post got mistaken for one by Q.E.D. Woot! :D

Crafter_Man
09-11-2005, 09:08 AM
Neither would I. How you got ME quoted for something Rick posted is beyond me. Unless you did it manually and had a brain fart. ;)Yipes, how did that happen? :smack:

Rick:

I was simply responding to the statement, "the alternator acts as a trickle charger for the battery itself." I guess when I see the term "trickle charger" I think of something that produces a fairly low current (around 1 A or less). Perhaps this is incorrect.

We're in agreement that an automobile's charging system is not designed to fully recharge a dead battery is a short period of time.

Rick
09-11-2005, 10:43 AM
Crafter Man I don't think we are as far apart on this as it may seem. My use of trickle charger is maybe a bit over the top, but when dealing with students that are sometimes aah shall we say thick as a brick, someitmes you tend to resort to hyperbole.
Let me try and clear this up, just a bit. When dealing with a fully charged or almost fully charged battery, the alternator will put out a ton of juice to recharge the battery in short order. However when dealing with a flat battery, the alternator will only allow a few amps at a time into the battery hence my trickle charger comment.