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Today I had a bit of car trouble where the culprit turned out to be the battery. I had driven my car just 5 minutes earlier without issue, but upon attempting to crank the vehicle it made a final effort at turning over and abruptly stopped, as if the battery leads were suddenly removed and all power was cut-off. After a bit of troubleshooting I solved the problem, but several questions came to mind.

I measured the open-circuit voltage to be 12V, but when the battery was wired-in (vehicle remained off with key removed) the same test measured only 8-9V. I am not sure exactly what kind of load remains on a car battery when the vehicle is off, but there certainly was a significant voltage difference. So

  • What causes a battery cell to die?
  • Are there symptoms of a dying battery or do they often quit as this one did (immediately, like a switch was flipped)?
  • The behavior described above leads me to believe the battery was working fine right up until something caused it to abruptly stop working. What "something" might this be?
  • If a cell was indeed "dead" as I was told, why did I measure these voltages? I would expect lower voltages if an entire cell was not working properly.
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In my boat, I have had a couple of lead-acid batteries fail with a shorted cell. In one case, the shorting may have been caused by the impact of a minor collision, but the batteries were 6 years old, so reaching end-of-life anyway. The shorted cell reduced the battery voltage by 2 volts, and apparently had enough internal resistance to prevent the battery from providing enough power to start the engine.

In both cases, the boat had two batteries in parallel, so the battery with the shorted cell prevented the good battery from being fully charged. These were all "flooded" batteries, and after trying to recharge the batteries for some time, the good cells in the bad battery all needed water as they were being over-charged, while the shorted cell did not need any water, nor did the good battery.

A battery can develope a shorted cell because the cell plates shed debris over time, and this debris falls into cavities at the bottom of the cell. When the cavity is full, the debris will contact the bottom edge of the cell plates, shorting the cell.

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Open circuit voltage and voltage while connected to the car should not be very different. There are generally 6 cells each with a nominal voltage of 2 volts a piece in SLA 12v, AGM, and other lead driven batteries. If a single cell is dead or internally shorting, expect to see leakage, because like the other answer, cells will be overcharging by a lot to make up that 2v.

My first thought is the regulator/bridge rectifier/diode bridge on your alternator is dying/dead, putting a load of current through the alternator windings. If it's a more modern car, some of these electronics are integrated with the ECU, and there may be an issue there as well.

Easy way to tell: See if it happens with the new battery, then buy a new battery and a new alternator.

Cheap way to tell: Buy a current clamp probe, hook it around your positive battery lead.

Cheaper way to tell: Take it to a local auto parts store that will test your alternator WITH THE BATTERY OFF. Most will test alternator with battery present, and that won't isolate the issue as well as they think it does.

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  • \$\begingroup\$ I guess I didn't answer your 4 questions: 1: Generally in lead acid, sulfur deposits that plate onto the cells causes issues leading to their demise. 2: It really depends. Lead acid has everything from long, slow deaths to catastrophic failures. 3: I think the diodes on your alternator are super dead. But if that's not it, probably sulfation. 4: Even a critically damaged lead cell can put out the correct voltage. But putting any load on it could cause such a drop \$\endgroup\$ – MildotPhil Mar 12 '15 at 4:57
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good answers. another reason may be that if battery is not properly fitted, it cause vibration which may break internal cell lead parts which is a cause of dead cell. In hot seasons, hot sulfuric acid may effect plates and make them dead

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  • \$\begingroup\$ Unless an answer is commentary on another post, a thank you answer, a question or comment DO NOT flag it as low-quality. Downvote it. \$\endgroup\$ – Voltage Spike Jan 25 '17 at 16:59

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