# Car Battery Went Bad After 20 Months **URL:** \ **Category:** In My Humble Opinion\ **Created:** [June 11, 2015, 3:03pm UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256 "2015-06-11T15:03:19Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [June 13, 2015, 12:01am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/21 "2015-06-13T00:01:34Z")
> [@bump](#): > > I did; my point was that if your voltage was somewhere between 11-13, it may be an issue of how charged the battery is, but that if it’s way off from 11, there’s something seriously wrong with the battery. Not only did you not read your own cite you didn’t read or comprehend my post either. Read the following carefully: **If your battery is anywhere near 11 volts, as in 12.0 or below you have a battery problem. Period.**. Now it might be you left the lights on and it just needs charging, or maybe there is a draw draining the battery, or perhaps the alternator is bad, or there is one of a half a dozen or so other issue causing a battery problem. Or it could be just a bad battery. But the bottom line is there is a battery problem. The cause is up for discussion the fact there is one isn’t. > [@](#): > > While he has the voltmeter/multimeter out, he may as well check the charging voltage - while idling, put the positive probe on the positive terminal, and the negative on ground (NOT the negative terminal) It should read 14 volts. Why pray tell would you not use the battery negative terminal when checking the charging voltage? You do realize that the battery negative is the one ground forged by the elves to rule all the other other grounds. If you look closely inside the negative cable terminal there is an inscription. Here it is: One Ground to rule them all, One ground to find them; One ground to bring them all And on a British car there is an extra line: and in the darkness bind them. ---
**Author:** ![Mr.Me](https://avatars.discourse-cdn.com/v4/letter/m/d2c977/32.png) [@Mr.Me](https://boards.straightdope.com/u/Mr.Me)\ **Post date:** [June 13, 2015, 2:20am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/22 "2015-06-13T02:20:31Z")
> [@Rick](#): > > :snip: > If you look closely inside the negative cable terminal there is an inscription. Here it is: > One Ground to rule them all, > One ground to find them; > One ground to bring them all > > And on a British car there is an extra line: > and in the darkness bind them. :golf claps: Bravo![RIGHT]I am so stealing this[/RIGHT] ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [June 13, 2015, 2:36am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/23 "2015-06-13T02:36:16Z")
> [@Rick](#): > > Not only did you not read your own cite you didn’t read or comprehend my post either. Read the following carefully: > **If your battery is anywhere near 11 volts, as in 12.0 or below you have a battery problem. Period.**. Now it might be you left the lights on and it just needs charging, or maybe there is a draw draining the battery, or perhaps the alternator is bad, or there is one of a half a dozen or so other issue causing a battery problem. Or it could be just a bad battery. But the bottom line is there is a battery problem. The cause is up for discussion the fact there is one isn’t. A dead battery means… your battery is dead, not that there’s necessarily something wrong with it. Voltage under 12 means that it’s dead. Voltage significantly lower than that does mean something’s wrong with it- car batteries are made of cells connected in series, and if you have one go bad, your voltage will drop by 2-3 volts, even though the charging system may be working fine. So a totally full battery with a dead cell would read something like 10-11 volts- at any rate, lower than the “dead” battery reading of under 12.0. THAT is the point I’m making. And no, the negative terminal is NOT the ultimate ground- if you hook your voltmeter up across the two terminals, you aren’t necessarily getting the real charging voltage, because you’re measuring your voltage across the battery, not between the positive terminal (coming from the alternator) and actual ground. I don’t know if it makes a practical difference, but there is definitely a difference between ground and the negative terminal. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [June 13, 2015, 3:18am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/24 "2015-06-13T03:18:50Z")
Unfucking believable. ---
**Author:** ![Cara\_mel](https://avatars.discourse-cdn.com/v4/letter/c/22d042/32.png) [@Cara\_mel](https://boards.straightdope.com/u/Cara_mel)\ **Post date:** [June 13, 2015, 3:51am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/25 "2015-06-13T03:51:38Z")
> [@bump](#): > > A dead battery means… your battery is dead, not that there’s necessarily something wrong with it. Voltage under 12 means that it’s dead. Voltage significantly lower than that does mean something’s wrong with it- car batteries are made of cells connected in series, and if you have one go bad, your voltage will drop by 2-3 volts, even though the charging system may be working fine. So a totally full battery with a dead cell would read something like 10-11 volts- at any rate, lower than the “dead” battery reading of under 12.0. > > THAT is the point I’m making. > > And no, the negative terminal is NOT the ultimate ground- if you hook your voltmeter up across the two terminals, you aren’t necessarily getting the real charging voltage, because you’re measuring your voltage across the battery, not between the positive terminal (coming from the alternator) and actual ground. I don’t know if it makes a practical difference, but there is definitely a difference between ground and the negative terminal. I was a little confused about this ground thing so I showed it to my boyfriend who works on cars. When he got done laughing about the inscription in the cable terminal, he said why of course the battery negative post is the ultimate ground in the car, without that there is no ground. He said ask this guy if he disconnects the battery negative lead if the car will start? Why not? The answer he said is of course it won’t, you don’t have a ground. Anybody that works on cars knows that. ---
**Author:** ![Jeff\_Lichtman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jeff_lichtman/32/1531_2.png) [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)\ **Post date:** [June 13, 2015, 5:11am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/26 "2015-06-13T05:11:11Z")
> [@bump](#): > > I don’t know if it makes a practical difference, but there is definitely a difference between ground and the negative terminal. The ground cable has effectively zero resistance, so where do you think this difference comes from? ---
**Author:** ![edwardcoast](https://avatars.discourse-cdn.com/v4/letter/e/cab0a1/32.png) [@edwardcoast](https://boards.straightdope.com/u/edwardcoast)\ **Post date:** [June 13, 2015, 5:30am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/27 "2015-06-13T05:30:50Z")
> [@ethelbert](#): > > I have a 2014 Mazda 6 GT. On Monday I went to start the car and I got nothing. The battery was completely drained. I jumped the car and drove it on the highway for about an hour. The same thing happened the next day. When I say I got nothing, by the way, I mean that even the little red/green light on the start button is out. I jumped it again and took it to the dealer. They told me the battery was bad and installed a new one (I had to pay 50% of the cost of the battery). > > Now I understand that modern batteries do not react well to being completely drained, and I suspect that is what happened, although I have never seen a battery go south as quickly as this. > > I am more concerned about why the battery drained. I am pretty careful about leaving the ignition in an “off” state when I leave the car (I pay attention when the car beeps at me as I am getting out and I always ascertain why). I can’t see how it is possible to leave the headlights on (and I hadn’t been driving after dark the day before anyway). My suspicion is that there is a short in the electrical system that is draining the battery. Anybody have any thoughts on this? I have gone through batteries and couldn’t figure out what was going on, and neither did the repair shop. Then one day, in the middle of the night I notice my headlights come on. I didn’t leave them on. So I go out there and find out I can’t turn them off no matter what I do. After a bunch of trouble-shooting with this intermittent problem, it turns out the fuse box under the hood needed to be replaced. A relay (I think) or something like that was the problem, and it would randomly put the headlights on, drain the battery and then I needed a jump in the morning. After that, it would sometimes go for months without there being a problem. In summary: Have the dealership run diagnostics on the electrical system. Otherwise, in all the cars I have had, I had to remember to replace the battery every 5 years. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 13, 2015, 11:31am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/28 "2015-06-13T11:31:10Z")
> [@Jeff\_Lichtman](#): > > The ground cable has effectively zero resistance, so where do you think this difference comes from? There is enough of a difference for some car ammeters to get a differential voltage. It’s small but it’s there. The battery negative terminal defines and is the basis for the ground in a car. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 13, 2015, 11:38am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/29 "2015-06-13T11:38:29Z")
> [@Rick](#): > > I did read the OP. > Many modern alternator systems will NOT charge a dead battery. The are designed to keep a fully or nearly fully charged battery fully charged. There is a difference. … I know you have been saying this for quite some time, but your sentences do not follow, nor do I feel your logic does either. What is correct is: > [@](#): > > The are designed to keep a fully or nearly fully charged battery fully charged. The ‘opposite’ is also true: The are not designed to fully recharge a deeply discharged battery However your statement: > [@](#): > > Many modern alternator systems will NOT charge a dead battery. I do believe is false, yes they are not **designed** to do this, but given enough time they \*\*will \*\*do this. This may take 12 hours or more of continuous driving, far more then the average Joe will be doing, and just driving a 1/2 a hour at a time would do more harm as you are very slowly recharging the battery which will leave it in various states of discharge for weeks or even months which is damaging, but technically if making a cross country trip most if not all properly functioning alternators will fully charge a discharged battery. Practically that does not happen often. ---
**Author:** ![Jeff\_Lichtman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jeff_lichtman/32/1531_2.png) [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)\ **Post date:** [June 13, 2015, 3:57pm UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/30 "2015-06-13T15:57:13Z")
> [@kanicbird](#): > > There is enough of a difference for some car ammeters to get a differential voltage. It’s small but it’s there. > > The battery negative terminal defines and is the basis for the ground in a car. Ammeters measure current, not voltage. The current at the two ends of the ground cable must be exactly the same. Not even a tiny bit different. There’s no other place for the current to go. Kirchoff’s laws, and all that. Battery cables are designed to carry hundreds of amps. The resistance must be low enough that they don’t overheat when the starter is running. You could probably measure a voltage drop in the ground cable when the starter is actually cranking. In the conditions we’re talking about, with the car and all the lights shut off, the voltage drop in the ground cable wouldn’t be measurable. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [June 14, 2015, 12:29am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/31 "2015-06-14T00:29:35Z")
> [@Jeff\_Lichtman](#): > > The ground cable has effectively zero resistance, so where do you think this difference comes from? You are making a huge and often incorrect assumption here. [Do you think this cable end has zero resistance?](http://themustangsource.com/forums/attachments/f804/101242d1329018691-2012-battery-issue-wp_000196.jpg) [Look at the cable closely](http://s3.amazonaws.com/f01.justanswer.com/JACUSTOMER3u5cwqah/2011-12-08_181408_terminal_connection_to_cable.jpg). See how it is swollen near the cable end? That is corrosion inside the cable. [Here compare a new cable to a corroded one.](http://twrite.org/shogunnew/files/chargingcable/c2-corroded%20cable2.jpg) > [@kanicbird](#): > > > [@Rick](#): > > > > I did read the OP. > > Many modern alternator systems will NOT charge a dead battery. The are designed to keep a fully or nearly fully charged battery fully charged. There is a difference. … > > I know you have been saying this for quite some time, but your sentences do not follow, nor do I feel your logic does either. What is correct is: > > > [@Rick](#): > > > > The are designed to keep a fully or nearly fully charged battery fully charged. > > The ‘opposite’ is also true: > The are not designed to fully recharge a deeply discharged battery > > However your statement: > > > [@Rick](#): > > > > Many modern alternator systems will NOT charge a dead battery. > > I do believe is false, yes they are not **designed** to do this, but given enough time they \*\*will \*\*do this. This may take 12 hours or more of continuous driving, far more then the average Joe will be doing, and just driving a 1/2 a hour at a time would do more harm as you are very slowly recharging the battery which will leave it in various states of discharge for weeks or even months which is damaging, but technically if making a cross country trip most if not all properly functioning alternators will fully charge a discharged battery. Practically that does not happen often. Actually I totally agree with you. Modern alternators don’t charge a fully discharged battery, but when I have said that in past right here on this board, I have had responses that are along the lines of “I jumped my battery and drove 2 blocks to the qwicky mart and it was fully charged.” that I have modified my comments to prevent those responses. True story. This was 1991, car sales were slooooow. Guy buys a brand new Volvo in Santa Monica. Battery is dead in the new car because it has been sitting on the lot for who knows how long. They jump it and tell him it will charge up on the drive home. He proceeds to drive to his home in Apple Valley. This is a bit over 100 miles and took him about 3 hours. He gets home, goes to show off his new car and it won’t start. At all. Barely a click. He calls roadside and they tow him to me, I’m close it is only 90 miles or so to my shop. I put a battery in the car and test the charging system. All is OK, so I release the car. Now here is the kicker. I decided to see if I could resurrect this battery. Believe me I tested it and it was a dead battery. It was pineing for the fjords. It was an ex-battery. So I put it on a very slow charge (less than 1 amp) for about 3 or 4 days. After that it tested good, so I put it in my personal car where it lasted another 5 years. > [@Jeff\_Lichtman](#): > > Ammeters measure current, not voltage. The current at the two ends of the ground cable must be exactly the same. Not even a tiny bit different. There’s no other place for the current to go. Kirchoff’s laws, and all that. > > Battery cables are designed to carry hundreds of amps. The resistance must be low enough that they don’t overheat when the starter is running. You could probably measure a voltage drop in the ground cable when the starter is actually cranking. In the conditions we’re talking about, with the car and all the lights shut off, the voltage drop in the ground cable wouldn’t be measurable. Umm try Ohm’s law. If the resistance increases due to a bad cable (totally NOT uncommon) amperage and voltage will be effected. Ohms law is not only a good idea, it is the law, and not subject to repeal anytime soon. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [June 14, 2015, 12:37am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/32 "2015-06-14T00:37:56Z")
> [@Mr.Me](#): > > :golf claps: > > Bravo![RIGHT]I am so stealing this[/RIGHT] I just looked inside the cable on my new car, and here is what it said This ground and no other Was made by the elves Who’d pawn their own mother to grab it themselves Ruler of sensor, injector and pump This is a sleeper that packs quite a bump if broken or busted your car won’t start If this happens, Call roadside service. (yeah, not as good as the first one, but 5 points to the first person who gets the reference.) ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [June 14, 2015, 12:46am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/33 "2015-06-14T00:46:14Z")
> [@bump](#): > > And no, the negative terminal is NOT the ultimate ground- if you hook your voltmeter up across the two terminals, you aren’t necessarily getting the real charging voltage, because you’re measuring your voltage across the battery, not between the positive terminal (coming from the alternator) and actual ground. I don’t know if it makes a practical difference, but there is definitely a difference between ground and the negative terminal. As **Cara mel** ’s boyfriend pointed out without a ground connection at the battery, you do not have a complete circuit. So saying the saying the chassis is the ultimate ground is just silly. Good connections to the chassis, + no connection to the battery = no complete circuit. Resistance in the cable between the chassis and the battery can result in low charge or no charge to the battery. I think where your problem is you are trying to equate the alternator amperage output (which you won’t measure with a voltmeter) with the system voltage. If you wanted to know the amperage out put of the alternator you would measure it at the heavy wire at the back of the alternator, but if you are talking about voltage, the battery negative is the ultimate ground. Forged by the elves and all that. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 14, 2015, 12:56am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/34 "2015-06-14T00:56:07Z")
> [@Jeff\_Lichtman](#): > > Ammeters measure current, not voltage… Automotive ammeters actually measure (very small differences in) voltage not current. They are not true ammeters but assume a amperage from the voltage difference. ---
**Author:** ![excavating\_for\_a\_mind](https://avatars.discourse-cdn.com/v4/letter/e/e8c25b/32.png) [@excavating\_for\_a\_mind](https://boards.straightdope.com/u/excavating_for_a_mind)\ **Post date:** [June 14, 2015, 2:07am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/35 "2015-06-14T02:07:41Z")
> [@kanicbird](#): > > Automotive ammeters actually measure (very small differences in) voltage not current. They are not true ammeters but assume a amperage from the voltage difference. Actually, ALL AMMETERS measure voltage, not current. They measure a voltage drop across a (usually high resistance) shunt; the value of the resistance and the value of the voltage determining the amount of current. At least that was what I was taught in Electrical Engineering. ---
**Author:** ![Kenm](https://avatars.discourse-cdn.com/v4/letter/k/bc79bd/32.png) [@Kenm](https://boards.straightdope.com/u/Kenm)\ **Post date:** [June 14, 2015, 2:33am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/36 "2015-06-14T02:33:29Z")
Borrow a two-year-old kid from a neighbour, toss the kid into the trunk, close the lid and ask if the light went out. Do the same with the engine compartment, if the kid can squeeze between the air filter and the block, presuming in each case there’s a light. (But if the rear-seat backs go down, checking the trunk light that way might prevent a SWAT team from showing up.) ---
**Author:** ![Jeff\_Lichtman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jeff_lichtman/32/1531_2.png) [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)\ **Post date:** [June 14, 2015, 3:41am UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/37 "2015-06-14T03:41:48Z")
Wikipedia describes [several types of ammeters](https://en.wikipedia.org/wiki/Ammeter). One type measures voltage across a shunt resistor. Most of the other types measure the electromagnetic field caused by current flow. One type measures the heat expansion of a calibrated wire. Viewed as a black box, most ammeters are pretty much the same. They are low-resistance devices that you connect in series with a current source, and they have some mechanism for displaying the measured current. The only type I know of that doesn’t work this way is [the inductive type that mechanics often use](http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482372000P?sid=IDx01192011x000001&kpid=03482372000&pla=&kispla=03482372000P&mktRedirect=y), which measures the change in magnetic field around a wire without being hooked into the circuit at all. The type of ammeter that uses a shunt resistor measures a voltage difference inside the ammeter. It doesn’t measure voltage anywhere else. It certainly doesn’t measure the voltage drop in the ground wire. All ammeters measure current, including those that measure the voltage across an internal shunt resistor. An ammeter doesn’t measure voltage external to the ammeter. As for the location of true ground: the arguments about this seem to me to be pedantic and pointless. If there’s nothing wrong with the ground cable and its connections, the negative battery terminal and the other end of the ground cable will be at the same potential - that is, they will be electrically identical. If there’s a resistance in the ground cable or its connections, you’ve got a problem - just like you’d have a problem with an unwanted resistance anywhere else in the circuit. Ground is actually a potential rather than a place. In a car, the entire body and everything connected to it is at ground (or should be). If there is a problem with the ground cable that interferes with the charging of the battery, you’re better off measuring the voltage across the battery terminals than measuring the voltage between the battery and the other end of the ground cable. If you do the former, you’ll see whether the alternator voltage is reaching the battery. If you do the latter, you won’t. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 14, 2015, 12:04pm UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/38 "2015-06-14T12:04:47Z")
> [@Jeff\_Lichtman](#): > > … > > The type of ammeter that uses a shunt resistor measures a voltage difference inside the ammeter. It doesn’t measure voltage anywhere else. It certainly doesn’t measure the voltage drop in the ground wire. … While this is true that voltage difference has to come from somewhere, and that is the resistance in cabling system on the positive side between basically the alternator and battery. Practically is not placed right at those 2 points, but one closer to the alternator and one closer to the battery. There has to be a voltage drop over the cable for the ammeter to get a voltage difference across the shunt resistor. ---
**Author:** ![Jeff\_Lichtman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jeff_lichtman/32/1531_2.png) [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)\ **Post date:** [June 14, 2015, 3:22pm UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/39 "2015-06-14T15:22:56Z")
> [@kanicbird](#): > > There has to be a voltage drop over the cable for the ammeter to get a voltage difference across the shunt resistor. There’s a voltage difference across the shunt resistor because there’s current flowing through it. That’s what a resistor is. There’s no such thing as a resistor that depends on a voltage drop somewhere else in a circuit to create its own voltage drop. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 14, 2015, 10:34pm UTC](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256/40 "2015-06-14T22:34:28Z")
> [@Jeff\_Lichtman](#): > > There’s a voltage difference across the shunt resistor because there’s current flowing through it. That’s what a resistor is. There’s no such thing as a resistor that depends on a voltage drop somewhere else in a circuit to create its own voltage drop. It’s ‘there is a current flowing through the resistor because there is a voltage difference’, not the other way around. The current through the resistor is dependent on the voltage difference. That voltage difference comes from 2 parts of the positive leads. The way you are stating it makes no sense as they would be no current at all flowing if there was no voltage difference. And you state nothing to drive the current through the resistor, which needs to be a voltage difference. Remember a ammeter is really a volt meter measuring the difference between 2 potentials. [Previous page](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256.md?page=1) [Next page](https://boards.straightdope.com/t/car-battery-went-bad-after-20-months/722256.md?page=3)