How do car batteries work?

Question

Hi all,

I always wondered how a car battery is able to charge cell phones, radio, dashboard and all other things that requiere electricity, and not need to to be recharged for a couple years. Why does it not die out sooner? Can someone please on how it’s able to achieve this curious feat?

Thanks!

In: Engineering

Answer 1

Your car battery is only used when the engine is not running, and principally it exists to deliver the amperage necessary to power the high-torque electric starter motor that cranks over the engine, which starts the self sustaining combustion cycle.

After the engine is started, the combustion cycle drives what is called an engine accessory; these are all the components driven by the belts off the front of the engine. Most of these components are pumps: the AC condenser pump, power steering pump, water pump – for the engine cooling system, sometimes a smog pump for emissions…

One such accessory is the alternator. This is a device that produces an alternating current, and it produces more than enough power to both recharge the battery, and run the entire electrical system of the car – most importantly the ignition system, but also the very energy hungry headlights. Alternators are self-regulating, in that they produce nominal power output once they spin at a minimum speed, and from there will lower or raise their output to compensate for changes in their rotational speed. This is why, when using jumper cables to start another car, it doesn’t help to rev the engine of the supplier car.

Some of this power produced by the alternator will be used to recharge the battery. The power in the battery is stored chemically, and those reactions are reversible to a point. The average life of a battery that sees regular use is around 7 years.

Cars that won’t start have an old battery that can’t deliver the amperage necessary to turn the starter motor adequately. If a running car dies due to an electrical failure, seemingly cured by a jump, the alternator itself has failed, and the vehicle is actually just running off the battery. You can get a few miles out of just a battery.