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Back when I was a kid, car batteries used to be huge heavy lumps of plastic filled with lead and acid. They used to weigh almost as much as a mobile phone (slight exaggeration there, sorry).

45 years later, car batteries still look the same and weigh the same.

So, in this modern age and emphasis on fuel economy, why do batteries still weigh 40 lb? Why have advances in technology not been able to make them lighter and more efficient?

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    \$\begingroup\$ Well, all the technology from 45 years ago isn't obsolete as of now. \$\endgroup\$ – dim Nov 18 '16 at 10:00
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    \$\begingroup\$ @dim? 45? More like 120 years... but yeah. We still build bridges out of steel, our concrete has gotten better, but still is essentially concrete, we use Asphalt for roads, copper is still our favourite conductor, the most commonly found amplifier technology in everything that isn't basically low-frequency is a bipolar-transistor based class A/B amplifier, we still burn oil to keep our homes warm, and our refrigerators still aren't based on Peltier elements, but on compressing more or less dangerous fluids. \$\endgroup\$ – Marcus Müller Nov 18 '16 at 10:12
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    \$\begingroup\$ Imagine someone drives their car into yours and fractures your lead-acid battery. What do you get? You get a bit of acid leaking out onto the floor. Now imagine that with a Li-Ion. What do you get? You get a huge fireball that engulfs you and your family. Which would you choose? Ok, maybe that's an exaggeration, but you get the idea ;) \$\endgroup\$ – Majenko Nov 18 '16 at 11:28
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    \$\begingroup\$ Because the rest of the car is still 2000lbs of iron. If we were making 200lb cars of graphite/kevlar/epoxy and titanium, then 40lbs of battery would become more significant. \$\endgroup\$ – Brian Drummond Nov 18 '16 at 11:30
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    \$\begingroup\$ Well.... the price, oh, the price, and... the price \$\endgroup\$ – J... Nov 18 '16 at 12:48
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So, now after the answer to your literal questionto your real question, that you sadly didn't ask

Battery technology has moved so far in the last 100 years. The lead-acid starter battery became common in cars in 1920, lead is essentially poison, and sulphuric/lead acid isn't any less dangerous. They tend to fail in cold temperatures, especially if not regularly maintained, and even though they're obviously cheap as hell to produce, the whole handling of them, including legal requirements to take back old batteries, must be a nightmare.

Why hasn't the industry just drawn a line and switched to things like LiIon or good ol' NiCd or NiMH batteries, now that electric cars have shown you can reliably drive years based on those?

The NiCd batteries are simply worse in every aspect but energy density than lead acid. NiMH is better, but much more expensive, and still has a higher rate of discharge, typically (unless you make them even more expensive). And still pretty hard to properly dispose of.

Lithium batteries aren't that easy handle. You need to protect them against all sorts of failures, and some of them are pretty fatal: don't overheat your lithium Battery. It will explode. And heat is a serious problem inside a motor compartment (in fairness, a battery doesn't have to be in there, but it's pretty handy).

The main reason really is cost. The battery in my last car, a 1999 Fiat Punto, supplied max 100 A (when I tried to estimate the actual short circuit current, around 43 A, but still a lot. Let's say P=U·I=12V·40A=480W) current, and had a nominal capacity of around 30 Ah (that's an energy of 12V·30Ah = 360Wh). It cost me 25€. So, rough guess, it's cheaper than 10€ to produce.

So, let's take a lithium battery type that is mass-produced and hence cheap. The commonly found round cells that make up many laptop battery packs are around 3€ each (let's say 1€ in production) for around 3Ah (11.1Wh), supplying up to 5A (tops, don't do that for long) at some 3.7 V. That says a single cell of these can supply 18.5W. So to reach the estimated 480W of my cheapo car battery, you'd need 26 of them. They'd cost 26€ in production, not counting the Euros you spend on control, charge and protection circuitry, on encasing them in something rigid and safe, and the fact that the minerals needed to produce some of the rare-metal components in Lithium batteries aren't currently getting cheaper, and equipping cars all over the world with those will definitely speed up that market mechanism.

Let's assume cost scales with capacity. My 26-cell lithium battery has 26·11.1Wh=288.6Wh energy. So we need to scale that by 1.25 to achieve the same 360Wh as the lead-acid battery.

Such a cell weighs around 90g. So the weight of the cells is 26·90 g = 2.34 kg. Ok, I don't have the exact weight of my cheap car battery in my head, but let's say it was 15 kg. So we saved weight by a factor of about 6.3, if our casing, and electronics are lightweight (they're not – as far as I can tell, you'll need a hefty switch mode power supply to be able to efficiently charge these using your car's generator, and those mainly consist of a pretty bulky coil of copper, and maybe some ferrite core that isn't exactly lightweight, either).

That leads to a cost factor of about 3.5 between component A and component alternative B, with handling disadvantages, lesser reliability and supply chain changes. No wonder the car industry isn't pushing in that direction. (And, by the way, they have excellent lobbying.)

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    \$\begingroup\$ I think 100A is uncommonly low for a car battery. Even 200A is small for a subcompact small car. Compacts and medium cars will easily be outfitted with batteries that can push 400-500A and higher, especially in cold weather climates where they need to still produce sufficient cranking amps at -30C or -40C. The high-current burst application is also very hard on lithium cells (unlike lead, which is extremely tough under this type of load). The list goes on... \$\endgroup\$ – J... Nov 18 '16 at 12:35
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    \$\begingroup\$ I think the WH of a starter battery isn't all that important.. nor the continuous discharge rate - rather the burst or 10s discharge (just enough to start your motor) would rather be the best measurement - I also agree with J in that 100A seems low, especially to start off with - still having100A after ~10 years is a lot more reasonable \$\endgroup\$ – user2813274 Nov 18 '16 at 12:56
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    \$\begingroup\$ I wot that a LiFePO4 battery will do what is required at acceptable whole of life cost BUT at higher initial capital cost - which makes it unattractive to car manufacturers. Cycle life is >> that of LA. Spike through the heart will not cause the issues a LiIon has. Max acceptable charge rates are higher, temperature range better, recharge efficiency better. \$\endgroup\$ – Russell McMahon Nov 18 '16 at 12:57
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    \$\begingroup\$ @MarcusMüller Surely, but likely your battery was old and rather heavily sulfated - certainly near EOL with performance like that. The point was that this is getting close to an absolute minimum of performance required in a replacement battery. The real demands of the application, with tolerance, aging, temperature, and safety factors, are undoubtably much higher. This is only to say that your estimate should be considered to fall on the cheapest and easiest side of the true requirement. \$\endgroup\$ – J... Nov 18 '16 at 15:31
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    \$\begingroup\$ @J... no questinos about that! bought it for 25€ four years before that. It barely worked, but it worked (lest things got too cold or I left the car standing for too long). So, I really picked the "worst lead acid battery that money could've bought four years before comparison". \$\endgroup\$ – Marcus Müller Nov 18 '16 at 15:34
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So, obvious answer first:

why do batteries still weigh 20kg?

Because they're still the same lead-acid batteries. Simple as that. No other technology came near the low cost per Ampere (and ampere-hour) of those, near the reliability and near the ease of handling. 20kg isn't that heavy, if you consider that "fuel economy" still means your average new car carries around dozens of kilogram of "comfort" functionality, and weighs around 1 Mg for the metal parts alone.

45 years later, car batteries still look the same and weigh the same.

45? More like 120 years... but yeah. We still build bridges out of steel, our concrete has gotten better, but still is essentially concrete, we use Asphalt for roads, copper is still our favourite conductor, the most commonly found amplifier technology in everything that isn't basically low-frequency is a bipolar-transistor based class A/B amplifier, and our refrigerators still aren't based on more efficient means of heat transport, but on compressing more or less dangerous fluids.

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    \$\begingroup\$ You're right, but the snark is a little unnecessary. \$\endgroup\$ – pjc50 Nov 18 '16 at 9:55
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    \$\begingroup\$ You should think a little more about the meaning of "why". It is unavoidably subjective, depending on the asker's starting viewpoint/premises. Why is the book in this position? Because John placed it there. Because it hasn't been put back on the shelf. Because I'm lazy. Because the table is 1.2m high, else it would have been at a different height (and therefore position). So many different "correct" answers. To truly answer a why question, you first have to know why* it was asked, and for that you have to somehow detect and understand the asker's perspective. (* yes yes..) \$\endgroup\$ – Museful Nov 18 '16 at 14:00
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    \$\begingroup\$ This answer gets a +1 from me due to Mg reference. \$\endgroup\$ – AndrejaKo Nov 18 '16 at 14:01
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    \$\begingroup\$ @Johnny I assure you, I've never replaced a car battery after 1-2 years, and I've never spent $150 on one, and I don't think that's the production cost of a battery :) \$\endgroup\$ – Marcus Müller Nov 18 '16 at 19:40
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    \$\begingroup\$ @LightnessRacesinOrbit well, not everyone burns oil, indeed. But it's still a very convenient, save, reliable and cheap way of heating, and thus, very common. (maybe not where you are from – outer space – (or where your SO avatar is from – Krypton)) \$\endgroup\$ – Marcus Müller Nov 19 '16 at 18:58
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The latest batteries are much lighter and cost less over a vehicle lifetime than ones of yore. But they do not use LA (lead acid) chemistry.

A LiFePO4 (Lithium Ferro Phosphate) battery will do what is required at acceptable whole of life cost BUT at higher initial capital cost - which makes it unattractive to car manufacturers.

Low initial capital cost seems to be the main reason to prefer lead-acid to LiFeO4 and it's not obvious that there are any other really good reasons.

Cycle life is very much greater than that of Lead Acid, which allows whole of life cost to be lower than lead acid.

Unlike LiIon (Lithium Ion) a "spike through the heart" will not cause the issues a LiIon has.

Charging control is "easy enough".

Compared to lead-acid:

Allowed depth of discharge, & max acceptable charge rates are higher,

Temperature range is better

Recharge efficiency is better.

Self discharge performance is better.

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Lithium Ion / LiIon:

It's worth commenting on LiIon batteries as they often get "bad press" with respect to safety.

Compared to lead-acid, LiIon chemistry offer substantially better mass and energy densities (lighter & smaller), somewhat longer cycle life, higher capital cost and probably somewhat superior whole of life cost. Properly managed, charging control is easier. Temperature ranges are better, charge/discharge efficiency is somewhat superior. Disdavantages relating to safety are largely not an issue - see below.

In many applications LiIon batteries are the battery of choice - from Dreamliners to Samsung phones to "Hoverboards", Mars Rovers to laptops and smartphones to MP3 players and more. The first three applications above were selected for their known spectacular failures. But anything used in a Mars Rover is chosen for its suitability in a long life, hostile environment, must not fail task. And there are hundreds of millions of LiIon batteries in everyday use in people's pockets and homes and cars and more.

Given the ways in which LiIon batteries CAN fail, the numbers that DO fail in a spectacular manner are very rare. Failures that are widely reported are quite often due to some systemic failure that affects a batch or model of battery that has been produced and distributed in vast quantities OR lower volume bu high profile applications. In such cases a design or manufacturing fault or shortcoming causes or allows failures whose consequences are exacerbated by the LiIon chemistry's unforgiving behaviors.

Examples are well publicised "vent with flame" events in some past Apple laptops, Samsung phones, self-balancing "hoverboards" and similar. In the 1st two examples usually competent manufacturers allowed a design fault to exist uncorrected and/or unnoticed or cut corners in manufacturing to the extent that safety margins caught up with them. In the case of the "hoverboards" the cause is unknown to me but is as liable to be low quality low cost manufacture and poor charge control as anything else. In consumer equipment LiIon battery failures often result from a short circuit occurring in a cell due to inadequate clearances and either consequent impact sensitivity or hitting the far end of statistical manufacturing tolerance variations. These are design and manufacturing errors that can be avoided at the cost of extra $ - something high volume manufacturers would love to avoid.

In the case of the Boeing Dreamliner battery failures I've not seen a final root-cause report BUT while a number of well publicised failures occurred (and maybe a few unpublicised ones) in a very small product volume, the consequences were astoundingly well contained.

A detailed examination of LiIon failures and modes and consequences shows that they are almost invariably nowhere near as violent as popular 'myth' suggests and that while the energy release is substantial, containment is relatively easy in engineering terms. Containment adds weight and volume and cost and is unlikley to be found in laptops or pocketable / portable devices. It IS found in Dreamliners and could easily be used in automotive single battery (ie non-EV) applications while keeping weight and volume still well below lead-acid levels and at modest extra cost. In electric vehicle applications the problems seem to have been solved or accommodated "well enough". I have ni expertise in vehiclar safety regulatory areas, but am confident that the regulations that bring us spectacular crash-dummy footage and allow the catting of high volatility petroleum fuels in passenger vehicles also address the safety issues around LiIon power sources. I have not heard of a 'Tesla' car being immolated through battery failure - although it may have happened - and I imagine that Musk and co believe they have this risk area "adequately in hand".

I have never, somewhat to my disappointment, seen a LiIon vent-with-flame event and do not personally know anyone who has. Occurrences are common enough to occasionally make the NZ news (NZ population is under 5 million).


LiIon versus LiFePO4:

Compared to LiFePO4, LiIon chemistry offers somewhat better mass and energy densities (somewhat lighter & smaller), substantially LOWER cycle life, slightly lower capital cost (per energy capacity), and substantially inferior whole of life cost. Charging control is about the same but LiFePO4 are significantly harder to damage in marginal cases. Temperature ranges are not as good, charge/discharge efficiency is about the same. LiFePO4 are far less subject to safety issues.

In areas where smallest size and weight and lowest capital cost matter (with electric vehicle use being a good example) LiIon are superior to LiFePO4.

In almost all other areas and applications, LiFePO4 are better or much better than LiIon and I'd consider them the current battery technology of choice for high energy long lifetime, high cycle count energy storage.

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    \$\begingroup\$ Basically, all of the above is true, but simply doesn't offer enough of a compelling reason to spend literally billions of dollars replacing the entire battery supply chain for the auto industry. LA batteries have survived because they do the job well, and all the infrastructure to use them is in place. Even in motorbikes, where weight is far more a consideration than cars (~0.2 vs ~2 tons gross vehicle weight) LA is still king. \$\endgroup\$ – Leliel Nov 21 '16 at 1:24
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    \$\begingroup\$ @Leliel A common enough means of introduction of new technologies is for early innovators to bypass the "supply chain" of the old tech and compete head on. Sometimes the attempt fails but often enough not. eg The 40V+ automotive supply proposals of a while back seem to have vanished without trace. Side-valve technology may be still found on Harleys (is it?) and lawn mowers and such but is otherwise long gone. | As above "I wot that a LiFePO4 battery will do what is required at acceptable whole of life cost BUT at higher initial capital cost - which makes it unattractive to car manufacturers." \$\endgroup\$ – Russell McMahon Nov 21 '16 at 8:16
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    \$\begingroup\$ It's not just the capital cost that's putting the auto manufacturers off. It's that the capital cost is huge, and benefits slim to none. LA is simply good enough, and already in place. Li doesn't offer enough benefit to justify the costs. \$\endgroup\$ – Leliel Nov 21 '16 at 18:19
  • \$\begingroup\$ @RussellMcMahon The 48V technology is implemented in the Audi SQ7. The SQ7 has a electrical compressor in addition to a turbocharger and needs so much energy that they needed more than 12V. But that is the only car I knew with 48V. \$\endgroup\$ – Sunzi Nov 21 '16 at 19:42
  • \$\begingroup\$ @Leliel, it appears you are saying the exact same thing as Russel. If all auto manufacturers switched to LiFePO4, it wouldn't be much more expensive LA. Like you (and Russel) said, the huge startup costs make it unfeasible. That said, if a great Tesla-like company came up with drop-in replacements that would literally last 3-5 times as long as LA and could afford to market it and sell at a slight loss, they could make it. \$\endgroup\$ – Joshua Nurczyk Nov 22 '16 at 11:23
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Lithium starter batteries exist, primarily for racing or other performance or luxury applications where the weight savings or bragging rights are worth the cost.

As others have noted, however, the demands of the application are rather extreme and lithium technology needs a lot of special development and care to be able to reliably and safely fulfil the role of a starter/accessory battery in a motor vehicle. The prices are extremely high - easily ten to twenty times the cost of a normal lead battery. Most people don't want to pay $1000 for their car battery, so they don't.

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  • \$\begingroup\$ If the battery lasts 10x the time (so, 20-30 years), I would be interested. Though really only if I was buying a new car. \$\endgroup\$ – Wayne Werner Nov 21 '16 at 14:40
  • \$\begingroup\$ Taking in to account the number destroyed in accidents, the proportion of cars lasting 20~30 year is tiny. And a decent LA battery will last a lot longer that the 2~3 years your comment implies (battery in my car is the original one, and is over 9 years old) \$\endgroup\$ – Kickstart Nov 21 '16 at 17:17
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    \$\begingroup\$ @Kickstart That depends heavily on the economy of the country, but yeah, 30 years is still pushing it. And importantly, LA battery maintenance is quite trivial - when the communists ruled here, you didn't just throw out the battery - you wouldn't get replacement parts! Instead, you replaced the acid, cleaned the battery and it was as good as new. Try doing that with a lithium battery (disclaimer: don't - made of explodium). \$\endgroup\$ – Luaan Nov 22 '16 at 9:35
  • \$\begingroup\$ If you live in a really cold or really hot environment (neither of which are good for LA, and both of which I've lived in) only top of the line batteries last more than 3-5 years, and even then I've never had one last more than 6. \$\endgroup\$ – Joshua Nurczyk Nov 22 '16 at 11:31
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The answer is very simple: Because we haven't found anything better.

A car battery needs to hold its charge over a long period of time, be able to deliver a huge current and fit into a small space. And it would help if it's not too expensive.

Lead acid is still the best solution for those requirements.

You could use a Lithium based chemistry, they can hold the charge and deliver large currents. They are also far more expensive, temperature sensitive, require more care electrically, and are more spectacular if mishandled electrically or mechanically.
The extra costs and complexity are simply not worth the benefits of a < 1% reduction in the cars final mass.

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    \$\begingroup\$ As you say, 1% mass reduction in car, not worth it. 1% mass reduction in airplane, worth it, if you can stop it catching fire as a consequence! \$\endgroup\$ – Neil_UK Nov 18 '16 at 9:50
  • \$\begingroup\$ As a clarification: Li-based batteries are less cold-sensitive than Lead-acid, but more heat sensitive. \$\endgroup\$ – Joshua Nurczyk Nov 22 '16 at 11:25
  • \$\begingroup\$ Li based batteries don't work at -40°, or even -30°. Automotive batteries are required to work at these temperatures - admittedly not many places need that. \$\endgroup\$ – Kevin White Feb 6 '17 at 23:00
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I saw that you added a new question to the end of your post:

Why have advances in technology not been able to make them lighter and more efficient?

Because that's not how chemistry works.

Capacity in a single type of battery is pretty much defined by the amount of ions that you have - and that is, in the case of lead-acid batteries, pretty much the mass of lead you need, plus some to keep the structure intact.

Now, other battery types suffer from a lack of surface or a limited ion mobility that limit those battery's ability to source a high current, but there's not much you can do to increase that for the lead acid battery – water is an excellent carrier for the chemicals involved, and the current sourcing ability of a lead acid battery is pretty much at its maximum.

Hence, it's simply a mature technology. Just like we haven't made cheap construction steel much better in the last 80 years, there's not much that can be done about lead-acid batteries to make them better without abandoning the lead-acid principle, with all the problems my second answer explains.

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Using supercapacitor as starter battery is fully feasible and was tried by enthusiasts in practice, see example. Aside from higher price, some examples of practical difficulties are reported:

  • Supercap alone, while starting the car more easily than lead battery, will discharge in about half a hour of listening to radio if not continuously charged.
  • Directly connected lithium battery+supercap combo does not suffer from above but ended up damaged when he used it to jumpstart a lawnmower - the Li battery would need extra electronics to prevent this.
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Mainly one reason: price. There are technologically better alternatives, like lithium-ion batteries used in electric cars, but they are also much more expensive. These batteries are absolutely needed in electric cars where you need a huge capacity without increasing a lot the weight of the vehicle (lead batteries would be too heavy if they had to replace the fuel tank as the only energy supply for the car), but in fuel powered cars the weight of a single classical lead battery that is used just for starting the motor, compared to the car weight is not significant, while the price/capacity ratio is dramatically lower. It's a cost/efficiency issue: they're cheaper, they provide enough energy for the car needs, and its weight is not relevant. Weight and size only become relevant compared to price when you increase some thousands times the electric capacity needs.

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  • \$\begingroup\$ Electric cars traction batteries are not directly comparable to starter batteries. \$\endgroup\$ – sharptooth Nov 22 '16 at 15:07

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