After Jump starting a car; why do you need to drive it around?

Factual Questions
1

Quick question - I just bought an old toyota for a beater car for the winter. It’s an 87’ Camry. It runs great, no noises no nothing, but I made the mistake of leavings the lights on for 24 hours… and I had to jump it the other day… now if I leave it for more than 24 hours it needs a jump. Whats up with that? Did I not drive it around enough after jumping it initially? What are the mechanics behind this? The battery is brand new but I have no idea about the alternator?

Come on Mechanics what gives? Any Ideas?

2

Lead-acid batteries are not designed to be deeply discharged; it may be that by running yours flat, you have compromised its capacity to hold a proper charge.

3

When you’re running the car, the engine is powering the alternator, which recharges the battery. If for some reason the alternator isn’t alternating, then your battery will die often. A loose alternator belt is a very common culprit and easy to fix. A bad alternator is another possibility, which will set you back a few bills.

4

Typically after a jump you want to let it sit running idle for 5-10 minutes. Theybattery may have just enough charge to keep it running, for a little while, but not near enough to start it. But if you do have a bad alternator, it will die again shortly after the jump. And it’s better to have it still at home then on the road if that happens.

You can tell pretty easily when the battery is up to charge by reading the voltage across the terminals with a meter. When it is charging the voltage is higher than normal, but drops to normal when it is good again. The problem though is that you have to take a baseline for each car/battery to determine what normal is before you have the problem. Even though a battery is supposed to be 12 volt, it’s pretty common for normal to be anywhere between 10.5-13.

5

You’ve probably damaged the battery. I’d look into a new one sooner than later. Like Mangetout said, car batts aren’t desigend to be deeply cycles. This means if you more or less fully discharge them, they never really can be restored.

6

Well the alternator belt would be no problem to fix. Is there any way to “test” an alternator? Now putting another alternator in would be a different thing all together… I’m quasi mechanical…and I’m glad this car is from the 80’s…because if it was from today I’d be scratching my head…

7

IIRC an 87 Camry might have a voltage regulator than can be replaced seperately from the alternator(Today most are intergrated and considered the same peice), Which might be the problem as well.

8

That’s not entirely correct. It’s true that automotive lead-acid batteries don’t take well to being deeply discharged, but that’s due to the design of the lead plates. Automotive batteries have thin lead plates, because by using thin plates, you can pack more of them in the same space, thereby gaining a greater current capacity. The tradeoff is that the thinner plates will be completely sulfated–converted to lead sulphate–by the time the battery is fully discharged. Essentially, with little or no lead left, the process cannot be reversed, at least not easily (or completely). Deep-cycle batteries, such as used for marine applications are also lead-acid batteries, but these have much thicker lead plates. They don’t need to (and cannot) deliver the high current that automotive batteries can, but because of the thicker plates, the sulfation doesn’t completely consume all the lead, so that a much deeper and lengthier state of discharge can be tolerated. For recreational marine use, this is more important than high current, since the batteries will often sit for most of the year before being used infrequently.

9

I’m aware of the existence of deep-cycle lead/acid batteries, but thanks for the clarification.

10

An alternator is there to maintain the battery charge level and build mild discharges back up. When the battery has been drained to zip, an external slow charge is the preferable way of restoring voltage.

What is the age of the battery? If it’s around 5 years, plan on replacing it. Are the cells filled? Dry batteries don’t hold charges. Alternators can be tested-most auto parts stores will perform an output test and waveform analysis free of charge, as they hope to sell you a replacement if one or more diodes or the voltage regulator is toast.

If you can borrow a trickle charger, put it on the battery overnight. Provided you can get the battery to hold a charge, the auto parts store can also perform a load test on it, but if a cell(s) is/are shorted or sulfated, it’s time for a new battery.

11

My mechanic once told me that when a completly discharged battery is subject to freezing the acid (I think) can freeze and cause the lead plates to warp and short to each other thus ruining the battery. He pointed out the bulging sides on our battery in question.

If this is infact true, check your battery, are the sides bulging out.

12

Well, it wasn’t so much for your benefit anyway, y’know. :slight_smile:

13

If you have a voltmeter, measure the voltage across the battery with the engine running - ideally, at slightly above idle revs. If the alternator is working, the voltage should be about 15-16V. If it’s down around 12-13V, then the alternator isn’t charging the battery.

14

I understand, but I still appreciate the correction of my unqualified statement.

15

An alternator is not designed to charge a flat battery. See question 15.2 in jnglmassiv’s link

This is a true statement.

This is false. As a battery is recharged the voltage with the charger (or alternator) will rise as the battery is charged, not fall. The amperage output of the alternator will taper off as the battery charges, not the voltage.

This is incorrect. Any 12V automotive system will give results that are, as they say, good enough for government work.

Boy it had better never read 10.5 this voltage is nowhere near normal*. 10.5 volts is almost to the point where the electronic systems will start shutting down for lack of voltage.
With a fully charged charged battery, normal voltage is about 13.92 volts with the engine running. Some alternator systems run a little higher than this, some a little less. As a quick and dirty test, 13.5-14.5V with the engine running is considered normal.**
Battery voltage varies with state of charge. Each cell in an automotive battery produces 2.12 volts when fully charged. X 6 cells gives 12.72 volts fully charged. Any voltage
> 12.6 is considered 100% charged.
12.45 V is 75% charged
12.20V is 50% charged
12.00V is 25% charged
These numbers are measured with the engine off, and the surface charge removed from the battery.
The internal resistance of the battery requires a slightly higher voltage to recharge a battery. About .2V per cell or 1.2V for the battery. So if you look at the voltage with a charger is running and subtract 1.2V from that reading, you will have a good guess as to the state of charge of the battery. Looking at alternator output will be less accurate, but it will still give you an idea of the state of charge.

Getting back to the OP. My gut is telling me that your alternator is doing exactly it is supposed to do, but the battery needs to be recharged. I would have the battery recharged (slowly please) and tested. Only if the battery tests good, would I start looking at the alternator and voltage regulator.

*Funny you should mention 10.5 V. Today I had to buy a battery for an old car with a dead battery. The local chain auto parts store sold me one for $50. When I got it in, the dash lights barely glowed. Car would not crank at all. I dug out my voltmeter and read 10.7V across the terminals. When I pulled the battery out, the date code made it about 1 year old. It obviously had never been recharged while sitting on the shelf at the parts house.

** test conditions, engine running, about 2,000RPM, all electrical consumers turned off, A/C off normal is considered 13.5-14.5V

16

15 volts would be pushing the upper end of the system. 16V would indicate an overcharge condition on every car I have ever worked on.
As I said before 13.5-14.5 is considered the normal range for a unloaded alternator at about 2,000 RPM.

17

(On preview, I see Rick has given an excellent account, but I’ll chip in my two bits with a slightly more detailed description of a quick and dirty alternator test.)

There is a simple way to get a good idea if your alternator is charging properly. It won’t tell you everything, but if the results are good the alternator is probably ok, and if it fails you know the alternator (or regulator) is definitely bad.

You just need a simple volt meter. Set it to read voltage for DC voltage, in the lowest range that will read at least 18 volts.

With the car not running, put the positive lead from the voltmeter on the positive terminal of the battery, and negative lead on the negative terminal. You should read about 13 volts or a little under.

Start the engine, and check the voltage again. If the alternator and charging circuit are working correctly, you should see the voltage move up to somewhere in the range of about 14 to 15 volts. (I think the official range for most cars is 13.7 to 15 volts). For example, my dash gauge sits just a little under 14.5 volts pretty much all time.

If your alternator reads in this range, it should charge the battery without any problem, keeping in mind it may take a few minutes.

If the voltage is below that range, the battery won’t charge correctly. If it’s higher, the battery can be overcharged and damaged permanently, and even explode!

In either case, the alternator, regulator, or something else in the charging circuit needs to be looked at. It could be something simple like a loose belt, or many other things.

Good luck!

18

I thought the test was to measure how many cranking amps it could deliver under load, and maintain voltage, not just the voltage.

19

There is such a test, called a load test, but that’s not what Rick was referring to. He was talking about rest voltage. Proper rest voltage is an indication that a battery is fully charged. A load test, which is only valid on a fully charged battery, is used to see if the battery can perform its work, as you described.

Back to the OP:

While it’s true that the deep discharge your battery experienced will shorten its life, it did not necessarily make it unusable. The fact that it’s a new battery gives reasonable hope that it will work for several years. The way to be sure (as sure as possible, anyway) is to have the battery properly tested, which requires special equipment.

Alternators are designed to top up a battery’s charge after starting, and to maintain it while operating the vehicle. They are not designed to charge dead batteries, and often a battery will never be fully charged by the alternator – though the alternator’s life may be shortened by its trying to do so. Do yourself a favor, and measure the battery rest voltage after it has sat for at least several hours (overnight is ideal). If it’s less than 12.6, charge the battery with a battery charger, not the alternator.

The basic alternator testing method advocated by trade journals is at idle under load. The load should be blower on high (but A/C compressor off, which may mean don’t select defroster) and headlights on. At idle speed, expect to see 14.0-14.5. Some cars do all right with slightly lower voltage, but if the voltage is below 13.5 and dropping – well, that’s what it will be doing at night in the rain.

20

If you are trying to test for state of charge, testing for voltage is fine. If you are trying to test the battery’s condition, then a load test is called for.*
In this particular case we are dealing with a fairly new battery, and I am not too concerned about its condition. I am very concerned about its state of charge. That is why I suggested using a voltmeter. That and the fact that very few consumers own a $2000 variable load carbon pile tester means that I am going with a voltmeter. :smiley: If your car came to a shop where I was working I would do both tests. Since it is possible to have a battery that indicates fully charged, but won’t start the car.
*For your general fund of information, not that it means much here, but batteries are rated in cold cranking amps. That is the number of amps that can be pulled out of the battery at zero degrees F for 30 seconds and still have 7.2V. Needless to say this would not be an easy way to test a battery in the field. When a technician load tests a battery he loads the battery to 1/2 of the CCA rating for 15 seconds and looks at the voltage at that time. At 70F minimum spec is 9.6V, a new battery should hold 10.0-10.2V