Because of differences in plate types, manufacturing conditions, and usage methods, batteries fail for various reasons. In summary, lead-acid battery failures fall into the following six cases.

1. Imbalance

Most lead-acid batteries are not used alone but in packs of several. If one or two lagging batteries appear within a pack, the remaining good ones cannot work properly either. This is called imbalance.

2. Water loss

During charging, water is electrolyzed to produce oxygen and hydrogen, so water is lost in the form of hydrogen and oxygen; this is also called gassing. Water plays a very important role in the battery's electrochemical system. A reduction in water lowers the activity of the ions taking part in the reaction and reduces the contact area between the sulfuric acid and the plates, raising internal resistance and worsening polarization, which finally causes capacity to drop.

3. Irreversible sulfation

When a battery is over-discharged and stored in the discharged state for a long time, its negative plate forms coarse lead sulfate crystals that can hardly accept a charge; this phenomenon is called irreversible sulfation. Mild irreversible sulfation can still be restored by certain methods; in severe cases the electrodes fail and the battery cannot be charged.

4. Plate softening

The plate is a porous material whose specific surface area is far larger than the plate's own surface area. During repeated charge-discharge cycles, as different substances on the plate alternately transform, the plate porosity gradually declines. In appearance, the surface of the positive plate changes from firm at the beginning to soft and loose and finally pasty. At this point the drop in surface area causes the battery capacity to fall. High-current charging and discharging and over-discharging all accelerate plate softening.

5. Short circuit

In a circuit, if current bypasses the load and connects the two poles of the power source directly, the source is short-circuited. Because the wire resistance is very small, the current in a short-circuited circuit becomes very large. No battery or other power source can withstand such current, and the source will be damaged. What is more serious is that the excessive current raises the wire temperature and, in severe cases, may cause a fire.

6. Open circuit

An open circuit means that a break somewhere in the circuit makes the resistance so large that current cannot pass normally, so the current in the circuit becomes zero. The voltage across the break equals the source voltage, and it generally does not damage the circuit. Possible causes include a broken wire or a load (such as a burnt-out bulb filament) becoming disconnected from the circuit.