You may already know that a failed car battery means that the engine won't crank or turn; however, when the battery begins to get run down, this can also cause the car to stall while on the road. This is because many parts under the hood run on electrical power, so that when the battery dies, certain pumps and motors stop functioning, and the car stalls. Typically there signs you can note that your car's battery may need replacing, so you can get that done before it actually dies completely.
Check engine light
The dashboard's check engine light may come on for a variety of reasons, including a low battery; if this light alerts in your car, have the battery checked. This is especially important if you note any of the other signs of a failing battery mentioned below.
Inspect your battery every month or so and note if it seems bloated or swollen. This happens when fluids inside the battery release vapours that don't escape, usually due to exposure to extreme heat. Over the years, these vapours trapped inside can cause the battery case to actually bloat and swell. This swelling is a sign that the battery is old and may be weakened, and needs replacing.
When inspecting your battery, look for corrosion along the connectors. This happens when vapours or battery fluid leak and cause damage to those bolts. You can disconnect the battery and clean it and note if the connectors are still solid; if they show any signs of damage due to that corrosion, the battery should be replaced. Too much corrosion can interfere with the battery delivering power to the electrical systems of the car and they may then fail.
If you notice a rotten egg smell in the car, this might be from the air conditioner holding mould and mildew. However, if you notice this smell from around the battery, this often indicates a leak; the corrosion mentioned above can sometimes cause a rotten egg smell around the battery case. If you notice this smell, have the battery tested for power and checked for leaking.
Age and wear
Batteries only have a particular lifespan, even if you don't often drive your car; check the manufacturer's recommendation as to when your car's battery needs replacing. A battery may also wear down sooner than you expect if exposed to high heat and extreme cold, or if you have added electrical demands, such as a plug-in phone charger or GPS system.