What causes a dead cell in car batteries? We reveal what is happening under your hood.
The battery in your car is one of the most important parts of your vehicle. It powers your engine and provides the energy needed to start your car.
Unfortunately, a dead cell can cause a lot of problems. A dead cell means that the battery has lost its capacity to store energy and will no longer start your car. You might be able to start the motor with a jump – but it’s time to replace your battery.
In this post, we’ll take a look at what causes a dead cell in car batteries. We’ll also look at how to identify dead cells, and how to fix a dead cell in your car battery.
Why are Battery Cells important?
An auto battery consists of six cells arranged in a row. Each cell contains the chemical energy that powers an auto’s electric starter motor. The lead dioxide plate and the lead plate in each cell generate 2.1V.
All six together produce the 12.6 Volts that start the engine. When a cell is dead the battery will only produce around 10.5 Volts. This is not enough amperage to turn over the engine.
What Causes a Battery Cell to Die?
There are a few possible causes of an expired battery cell.
Sulfation is the number one cause of battery failure.  It occurs when Lead Sulfate crystals accumulate on the lead plates within the cell. This reduces the surface area available.
As the Lead Sulfate consumes the plates, the capacity of the battery reduces. Usually, this is temporary and the crystals dissipate during recharge. But if the battery is in a state of low charge for an extended period, the damage can be permanent. 
Luckily, sometimes you can resolve the problem using a battery desulfating device.
Other ways to reduce sulfation involve
- keeping the battery cool
- keeping the battery as fully charged as possible.
2. Battery stratification
If you drive your car short distances while using your heating and wipers, your battery is at risk of stratification.
The electrolyte in the battery cell is an even mix of Sulfuric Acid and water. As the battery discharges, this mixture separates into layers. The denser Sulfuric acid settles to the bottom part, whilst the top part is now mostly water. This is then reversed as the battery recharges.
If the battery is not recharged and the layers remain separated performance dips. A faulty alternator or inadequate charger can also cause this.
Solve the problem using a multi-stage battery charger with an equalizer setting. The battery recharges at a high current for a short time to break up the layers (aka a topping charge). The charger will do this after a certain preset number of charge cycles. The battery is then charged again at a lower current for longer to ensure the battery is fully charged.
If you use this method, it is important to note that during the process the inside of the battery will heat up. Hydrogen and Oxygen will release from the battery, known as ‘gassing’. This occurs during the electrolysis of the water in the cell. Hydrogen is combustible. Ensure that the process is undertaken in a well-ventilated area without ignition sources.
It also means that the cells will need topping up with water after the process. Please note that this should ONLY be undertaken on refillable batteries. Do NOT attempt it on sealed AGM or gel batteries. 
Other methods to solve the problem include allowing the battery to rest a while, or shaking or tipping it. This will remix the sulfuric acid and water.
3. Internal Cell Short Circuit
During its lifetime, a battery cell produces Lead Sulphate (PbSO4) – brown crystalline debris known as “Mud” or “Sludge”. It gathers on and then sheds from the plates in the cell. This is normal and occurs as the battery expands and contracts during the recharge cycle. It is a byproduct of the electrolysis process. The sludge can accumulate at the bottom of a cell and cause a short circuit. This results in the dead cell.
Batteries are often used in tough environments such as mining, farming, or construction. The vibrations and general beating that batteries take can exacerbate the problem. More debris is shaken off onto the floor of the battery. 
A short will also occur if the separator between the anode and cathode is weak or compromised. Anything causing a connection between the positive and negative plates will cause a short. If the battery is discharged for long enough, the Lead Sulphate crystals form dendrites which eventually grow between the anode and cathode.
4. Poor Maintenance
Sealed AGM or Lithium batteries need no maintenance. But this is not the case for wet lead acid batteries.
During electrolysis, water inside the cell converts to hydrogen and oxygen. Some of these gases escape, which in turn lowers the water level in the cell.
During your battery maintenance routine, you should check the water level. If a cell becomes dry, or the water level is too low, the cell will ‘die’. Avoid this by replenishing the water in the cell.
When you top up the water, make sure that you use distilled water. This water has had all minerals and impurities removed. Using tap water will harm your battery.
How Can I Tell Which Cell is Dead?
1. The first method is to examine the cells. After you remove the lids you should be able to see into the cells. You will see obvious signs. If you have a cell clogged with lead sulfate, it will be plain to see. This is because of the presence of the brown sludge I mentioned earlier.
2. If you charge the battery the electrolysis process will cause the cells to bubble. This is normal and is hydrogen gas release – also known as “gassing”. If a particular cell is dormant and NOT producing bubbles, that cell will be your culprit.
3. Use a multimeter to test the voltage of each cell. Connect the positive wire of the multimeter to the positive terminal of the battery. Lower the negative probe into the closest cell. Dip it into the electrolyte solution so the end touches the lead plate head within.
Your multimeter will then give you a reading for each cell. The cell closest to the positive should read 2-Volts. The next will read 4-Volts, and so on up to 12V. If one of the cells does not add 2-Volts to the total, that is the dead cell.
How to fix a dead cell
You will need:
- Protective Eye Goggles
- Neoprene Rubber Gloves
- A flat head screwdriver
- Toothbrush or wire wool
- Epsom Salt (1 pound)
- Battery Charger
- Distilled Water (1 gallon)
- A Plastic Funnel
- Baking Soda (1 pound)
1. PPE up. It’s not worth getting injured for the sake of a battery.
2. Mix up one part baking soda and 2 parts water. This is for safety as well as cleaning. If there is a spillage this solution will neutralize the acid. It will also be your cleaning solution.
3. Use the baking soda solution to clean corrosion and debris off the battery. Do this with the casing and the battery terminals.
4. Using the flathead screwdriver remove the battery cover. Then prize off the maintenance caps.
5. Tip the battery and empty the cells with great care into the bucket.
6. Add baking soda to the acid in the bucket to neutralize it. Do the same to any excess spilled acid.
7. Using your funnel, pour the baking soda and water mix into the empty battery cells. Then, replace the cell caps and give the battery a jolly good shake for at least a minute.
8. Remove the caps again and decant the contents into the bucket.
9. Mix Epsom salt into the boiled distilled water until it dissolves clear. Using the funnel, top up the cells with the solution.
10. Seal up and shake again. Then, pop the caps off again to release any pressure. then reseal.
11. Charge the battery and connect it up.
Please note that this method of battery revival can be a little inconsistent and can take time to work. The process may need to be repeated several times. Also, before you proceed, remember to test the voltage. If it is below 10-Volts the battery is no good and should be junked.
Above all think safety first, gear up with PPE, and conduct the operation in a well-ventilated area.
Symptoms of battery failure.
There are a few little clues that your battery might be dead
If there is a dead cell in the battery, it won’t supply the starter motor with enough cold cranking amps (CCA) to turn over.
While the vehicle is running, the alternator restores the battery charge. If the battery has a bad cell it will restart soon after shut down. But it will not start after leaving it overnight.
This means that you may hear the dreaded clicking sound when you turn the key.
As we said, the alternator supplies a charge to the battery. But, it first needs to flow through the battery itself. The battery regulates the amperage heading to the electronics.
If your cigar lighter, wipers, radio and electric windows are misbehaving, it may be a sign of a dead battery cell. 
The voltage is below 12.6 V
Get a voltmeter and add a regular voltage check into your maintenance schedule. Along with the tire pressure, radiator and wiper fluid, also check the battery voltage.
If the voltage is less than 12.6 it is a sign of a dead cell. The battery will not supply enough amperage to start the vehicle.
Conclusion – What causes a dead cell in car batteries?
So that is everything you need to know about what causes a dead cell in car batteries. And if getting down and dirty under the hood is not your cup of tea, remember that you can take your vehicle to your local auto parts center. They will test your battery free of charge and advise you on the next appropriate steps.