Car battery charging Q: how long?

Factual Questions
1

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.

2

How do you know that it didn’t just die for good? How old is it?

3

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.

4

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.

5

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.

6

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.

7

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 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 = I12.6*T/2

Solving for T:

T = 2E/(I12.6) = 22000000/(1.212.6)

T = 264550 seconds ≈ 3 days

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

8

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?

9

[nitpick] 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)[/nitpick]

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 :slight_smile: )
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. :slight_smile:

10

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.

11

Crafter Man :smiley:

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

12

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.

13

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.

14

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.

15

Perhaps it is just pining for the Fords?

16

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.

17

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.

18

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.

19

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.

20

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. :smiley: