A car battery goes bad primarily due to sulfation, a process where lead sulfate crystals build up on the plates and prevent the battery from holding a charge. Other major causes include excessive vibration, deep discharges, extreme temperatures, and simply reaching the end of its natural lifespan, which is typically 3-5 years. Regular maintenance, like keeping the terminals clean and ensuring the charging system works correctly, is the best defense against premature failure.
The most common killer of batteries is sulfation. This occurs when a battery is left in a partially or fully discharged state. The lead sulfate, which is a normal byproduct of discharging, hardens into large, stable crystals that cannot be converted back into active material during charging. This permanently reduces the battery's capacity.
Extreme heat is another major factor. High temperatures accelerate the chemical reactions inside the battery, leading to faster corrosion of the internal plates and evaporation of the electrolyte fluid. This is why batteries in hotter climates often have shorter lifespans than those in colder regions. Conversely, extreme cold doesn't directly kill a battery but exposes a weak one. Cold weather thickens the engine oil, making the engine harder to crank, which demands more power from a battery whose capacity is already reduced by the low temperature.
Physical damage and vibration can also cause internal short circuits or break the connections between the plates. A loose battery hold-down clamp can lead to excessive vibration that damages the internal components over time. Finally, parasitic drain from a vehicle's electrical system—like a trunk light that doesn't turn off—can slowly drain a battery if the car is left unused, leading to a deep discharge and subsequent sulfation.
The following table outlines common failure causes and their typical impact on battery lifespan:
| Failure Cause | Description | Impact on Lifespan |
|---|
| Sulfation | Hardened sulfate crystals on plates reduce capacity. | Can reduce life by 50% or more. |
| Heat Damage | Accelerated internal corrosion and fluid loss. | Shortens life significantly; high heat is a primary factor. |
| Vibration | Internal components break from physical shock. | Can cause immediate or gradual failure. |
| Deep Cycling | Fully discharging a standard starting battery. | Damages plates; 4-6 deep cycles can ruin it. |
| Overcharging | Excessive voltage from a faulty alternator/charger. | Causes overheating and rapid fluid loss. |
| Low Electrolyte | Fluid level drops below the top of the plates. | Exposed plates sulfate and are damaged. |
| Parasitic Drain | Constant small draw when vehicle is off. | Leads to chronic discharge and sulfation. |
| Normal Aging | Gradual degradation of active materials. | Expected lifespan of 3-5 years. |
