My car battery is completely dead and I was wondering if they are meant to be thrown out or re-chargeable.
The reason why I ask is because I keep reading that a car battery should last a few years, but if car batteries are re-chargeable, shouldn't they last a lot longer than a few years? I figure a good working alternator operating in parallel, would not require the battery to be recharged often, so wouldn't the expected life of a car battery be much higher?
Car batteries are not only rechargeable but are constantly being recharged by the alternator. Simply starting the car takes a lot of power reserves from the battery, then electrical systems such as the headlights, screen heaters, ignition and injection systems all draw on the battery. When the engine is running, the alternator is near constantly providing a charge to counter the power drain on the battery.
If a battery has gone completely dead, this could be because there has been an electrical fault in the vehicle leading to parasitic battery drain whilst the engine is not running. It could be that the alternator or control pack on the alternator has failed so the car hasn't been recharging the battery. It could be that the battery simply needs topped up, they typically have removable plugs in the top of the case although some batteries are sealed for life or it could be that a fault has occurred within the battery, for example one of the lead plates has warped or fractured.
If the battery can be charged from a charger and retain power, check the operation of your alternator and check that there aren't any circuits within the car draining the battery. A car in use without the alternator will cause the battery to become flat within a few hours of operation. This will happen quicker if systems such as lights, blowers and wipers are in use.
As Steve has explained, the battery is rechargable and is constantly charged and discharged when it's in use.
This process of charging and discharging is not perfect. Over time, the components in the battery that hold the charge (the plates and the electrolyte) degrade, reducing the battery's capacity (amount of charge it can hold) and its maximum current. After ~6 years, capacity has degraded so much that the battery can't supply enough power to start the car any more. Then it's time to replace the battery.
Starting the car is the critical point: the battery has to supply several hundred A for a few seconds (diesels and large petrol engines require more power, new/well-maintained engines require less time to start). This is pretty close to the maximum current a good battery can supply. No other operation in a car requires this much current, so the cold start is the first operation to be compromised when battery capacity decreases.
Other batteries have this problem too: all rechargeables slowly lose capacity. Batteries have a lifetime specified in charge/discharge cycles. This number is often around 300-1000 cycles. Cell phone batteries last about 3 years because of this.
For lead/acid batteries this is a bit more complicated because discharging them too far quickly damages them, while lots of 'shallow' discharges are less of a problem. Still, every time the battery is used, it degrades a bit.
Lead-acid batteries are rechargeable. Have you tried recharging it? That often will do.
The simplest reason is there's acid in them. But some other battery types don't do any better, look at any cell phone battery.
If you draw out most of the power of a battery, and then recharge it, that is called a "deep cycle". This is different from starting a car normally, where you take a small amount of energy "off the top" for cranking, and then quickly charge it back up once the alternator is running.
A unique curse of the lead-acid chemistry is that deep-cycles damage the battery. Other battery types don't have that problem. Deep-cycle-rated lead-acids, like Optima yellowtops, do better -- but they still will be destroyed by deep cycling, just will get more cycles before they do (on the upper range of the numbers I'm about to give).
If you design with batteries, e.g. off-grid solar systems, nobody will tell you this, it's just something you're expected to know... SMH...
How does that happen in a car? When you forget and leave lights on, or have a wiring fault which drains the battery, and find the car dead and jumpstart it or put it on a battery charger to get it going again... and that becomes a habit.
Another thing that kills car batteries is leaving them in a state of discharge for awhile. Suppose the battery gets drained and you just store it drained instead of recharging it. Or if the car is parked for months and has an electrical drain (as some modern cars do). Lead-acid batteries must be stored at 100% charge.
Another killer is winter cold plus discharge. Since the chemical reaction is lead vs acid, at 100% charge, the acid is very strong. At full discharge, the acid is very weak. Strong acid freezes well below minus 40 degrees. Weak acid is mostly water and is much more vulnerable to freezing at common winter temperatures. When it freezes, it cracks the battery's case, and it's done.
Once upon a time, there were a variety of batteries on the market, and all of them are fine with deep discharge.
The famous "Edison Cell" was made for electric cars, is nickel-iron, and is virtually immune to abuse, having none of the above problems, and last 40+ years. It's not good at huge amounts of surge current, though, that's the one thing lead-acids are better at.
It evolved into the nickel-cadmium, either in a traditional wet-cell or a sealed AA or D-cell. They solved the surge current problem, in fact they can start jet engines - that's how jetliner APUs start. (and in the linked video, that's a really old battery.) These batteries last 20 years too. Of course they're more expensive.
Now, lithium batteries are literally exploding onto the battery marketplace. Every cell phone, most laptops, and the Boeing Dreamliner use them, and they could make a viable car battery. However nothing indicates they'll last any longer than lead-acids.
Automotive batteries only have a given service life and are meant to be recycled at the end of that life. When a battery reaches the end of its service life, you need to replace it. In most countries, when you purchase your new battery, you will need to turn in the old one as core.
The battery is in constant use when the car is operating, with the alternator keeping it charged. This is why they only last so long, typically 3 to 5 years. Even batteries which are not in use lose their charge over time.
