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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?

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    I replaced the battery in my 1976 VW in 2010, so that's a battery life of 34 years. That's long enough for me. – Chenmunka Feb 6 '17 at 12:39
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    Car batteries (like everything else) don't last for ever, but you should never just throw out a failed battery. It contains a lot of lead, and the acid is also strong enough to cause damage to property, as well as to plants and animals. If you buy a replacement battery from anywhere, they should take the old battery to be recycled, not thrown away. – alephzero Feb 6 '17 at 13:44
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    Car batteries are already the most recycled object on the face of the earth, with something like 98% going into the recycle stream. You would have to work pretty hard to actually get one into a landfill, as curbside scavengers and everyone in waste management would be on the lookout and snatch it out of the waste stream on sight. There's enough mineral value in them to be wrorth the trouble. – Harper - Reinstate Monica Feb 6 '17 at 20:15
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    How old is this battery? Is the car in regular use? – MrWhite Feb 7 '17 at 9:04
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    @Chenmunka 23 or even 34 years not surprising? Most batteries don't last that long, either because of the cold, deepcharges, parasitic draw or whatever. – Bart Feb 7 '17 at 16:33
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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.

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    Where you say "open circuit", do you mean "short circuit"? – Gremlin Feb 6 '17 at 13:41
  • Essentially yes, something is drawing power when the vehicle is switched off describes what I mean. – Steve Matthews Feb 6 '17 at 13:55
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    Ok, because "open circuit" is a term in electrical engineering that means almost exactly the opposite - it's when there's a break (an "open") in the circuit, and no current can flow. I suggest that you change it to "closed circuit" or maybe "active circuit". – Gremlin Feb 6 '17 at 14:09
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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.

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    I don't believe the 1 kW for 10 seconds part of this answer is entirely correct. Firstly, a typical gasoline engine starts in 1 second unless it's freezing cold outside. Secondly, 1 kW is only about 80 amperes at 12 V. Does that start the engine? If the engine is very small and already at operating temperature, it might, but a large diesel in freezing conditions will not start with 1 kW. No downvote, though, because the answer is mostly correct. – juhist Feb 6 '17 at 12:48
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    Oh, and the number of 100% charge/discharge cycles for lead-acid starter batteries is very low: probably around 10, definitely under 100. If left discharged for a long period of time, even one discharge cycle can destroy the battery. – juhist Feb 6 '17 at 12:50
  • Fair enough, I saw a starter motor for a VW 2.0 TSI engine's rated at 2 kW. re: charge cycles: incomplete cycles count too. Every time the vehicle's parked and then started again is one partial discharge, use the car twice a day and you have 700 partial discharges a year. I've amended my answer. – Hobbes Feb 6 '17 at 13:23
  • @juhist While the main load on starting happens during the cranking a diesel engine is a bit special: The glowplugs usually heat the combustion chamber for some seconds after the engine starts running to smooth up the combustion process (10-20 A per plug). Also some other things put additional load on the battery/alternator: Some electric heaters come to mind – Martin Feb 6 '17 at 17:19
  • @juhist: definitely. For many cars even one complete discharge is able to essentially wreck the battery and leave it unreliable. – whatsisname Feb 6 '17 at 19:57
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Lead-acid batteries are rechargeable. Have you tried recharging it? That often will do.

Lead-acid batteries fail after 4-7 years

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.

Lead-acid batteries hate deep cycling

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 drain it dead, you'll get 5-30 cycles before battery death
  • If you drain to 50%, maybe 20-200 cycles
  • if only 25% depth-of-discharge (using 25% of capacity) many hundreds of cycles
  • 15% DOD thousands of cycles.

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.

They don't like being left discharged, either

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.

Surely there must be a better battery

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.

  • Lead Acid batteries have disadvantages, but they remain the best option for cars due to their features, which are high discharge rate on demand, low maintenance, temperature tolerance. The downside of weight is relatively outweighed by the vehicle - a LA battery of 5 kilos is about 0.2% of the car's mass. – Criggie Feb 7 '17 at 1:43
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    I wouldn't agree, but I would agree if COST is a factor. Only recently have other batteries overtaken their ability to deliver starting impulse, which has always been their trump card. None of them can do it for a 2-digit price tag, not even close. Cost is always a factor, isn't it? – Harper - Reinstate Monica Feb 7 '17 at 2:42
  • There are youtube videos with various demos of fixing a battery with alum or epsom salt. Are they legit according to any reliable sources? e.g.: youtube.com/watch?v=Supe1a3LW2U Guy claims to fix deep cycle battery that won't hold a charge using alum water but the video doesn't include clear before and after testing evidence. – Matthew Elvey Feb 7 '17 at 5:37
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    In any case, lead acid batteries are getting pretty close to obsolete for most mobile uses - Li Ion batteries are common on performance motorcycles, available for cars and close to becoming cost competitive. For marine or stationary use where weight and density doesn't matter much, they still have the edge though. – Matthew Elvey Feb 7 '17 at 5:40
  • Li Ion batteries are also making jumper cables largely obsolete - you can use one of these instead - amzn.to/2kBWUCt - a lightweight portable jumpstarter. – Matthew Elvey Feb 7 '17 at 6:19
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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.

  • It isn't "in constant use when the car is operating". After starting, the alternator recharges the battery in a short time. After that, the alternator also powers the rest of the car's electrical. Once it's recharged the battery doesn't receive appreciable current, except to make up for self-discharge losses. If this weren't true then cars with dead batteries would not be able to run after a jump-start. – Jamie Hanrahan Feb 7 '17 at 17:47
  • (contd.) Oh, in some cars the alternator may not have enough output while the car is idling, esp if eg the headlights + the HVAC blower + the really loud stereo are all going at the same time. At those times the battery makes up the difference between alternator output and the car's electrical load, and (again) is recharged after you get moving again. – Jamie Hanrahan Feb 7 '17 at 17:48
  • @JamieHanrahan Thank you for your explanation. My answer was intended to be very basic. I did not state that the battery provides all power, all the time. My answer states that it is "in constant use", to which you reinforced with "battery makes up the difference between alternator output and the car's electrical load". Would you agree the battery is a continuously connected part of the electrical system and is in various states of charge and discharge throughout it's operational life? – CharlieRB Feb 7 '17 at 19:04
  • No, I would not. Most of the time, even though an ammeter will show a small current flowing to the battery (because the alt. output is of slightly higher voltage than the battery) the battery will not be "charging"; this current will simply be turned to heat. Re what you claim is my "reinforcement", I said that that only occurs in rare circumstances (better be, or the battery would soon be drained), You quoted it without that qualification. As I actually wrote it, it does not reinforce what you wrote. I think you should rewrite your second paragraph. – Jamie Hanrahan Feb 7 '17 at 19:46
  • This must be some new meaning of the word "constant" that I wasn't previously aware of. – Jamie Hanrahan Feb 7 '17 at 23:38

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