Why are batteries so heavy

Why are car batteries still so heavy?

When I was a kid, car batteries were huge, heavy lumps of plastic filled with lead and acid. They used to weigh almost as much as a cell phone (a bit over the top, sorry).

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

Why do batteries still weigh 40 pounds in this day and age where fuel economy is paramount? Why couldn't advances in technology make them easier and more efficient?


So now after answering your question about yours real question which you unfortunately did not ask

Battery technology has advanced so far over the past 100 years. The lead-acid starter battery became common in automobiles in 1920, lead is essentially poisonous, and sulfur / lead-acid is no less dangerous. They tend to fail in cold temperatures, especially if they are not regularly maintained, and while they are obviously cheap to produce, all of them handling, including legal requirements to take back old batteries, must be a nightmare.

Why didn't the industry just draw a line and switch to LiIon or good old NiCd or NiMH batteries, now that electric cars have shown that you can drive reliably with these batteries for years?

The NiCd batteries are worse than the lead-acid energy density in every respect. NiMH is better, but much more expensive, and usually still has a higher rate of discharge (unless you make it even more expensive). And still pretty difficult to dispose of.

Lithium batteries are not that easy to use. You need to protect them from all types of failures, and some of them are pretty deadly: don't overheat your lithium battery. It will explode. And heat is a serious problem in an engine bay (in fairness, a battery doesn't Has to be there, but it's pretty convenient).

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

So let's take a type of lithium battery that is mass-produced and therefore cheap. The common round cells that make up many laptop batteries cost around € 3 (let's say € 1 in production) for around 3Ah (11.1Wh) each, and some deliver up to 5A (peaks, don't do that long) 3.7 V. That is, a single cell of these can deliver 18.5 W. So to get the estimated 480W of my cheap car battery, you'd need 26 of them. They cost $ 26 to produce, not including the dollars you spend on controls, charging, and protection circuitry to wrap them in something rigid and secure, and the fact that the minerals needed to make some of the rare metal components in lithium become batteries not currently cheaper, and equipping cars around the world with them will definitely accelerate this market mechanism.

Let's say the cost depends on the capacity. My 26-cell lithium battery has 2611.1Wh = 288.6Wh of energy. So we have to scale that to 1.25 to get the same 360 ​​Wh as the lead-acid battery.

Such a cell weighs about 90 g. The weight of the cells is therefore 26 x 90 g = 2.34 kg. Ok, I don't have the exact weight of my cheap car battery in mind, but let's say it was 15 kg. If our case and electronics are lightweight, we've saved 6.3 times the weight (that's not the case - as far as I can tell, you need a strong switching power supply to efficiently charge these devices and your car's generator is made up of it mainly from a rather bulky copper coil and a ferrite core, which is also not exactly light.

This leads to a cost factor of around 3.5 between component A and component alternative B with disadvantages in handling, lower reliability and changes in the supply chain. No wonder the auto industry is not pushing in that direction. (By the way, they have an excellent lobby.)

So, obvious answer first:

Why do batteries still weigh 20 kg?

Because they're still the same lead-acid batteries. It's that simple. No other technology has come close to the low cost per ampere (and ampere hour), reliability and ease of use. 20kg isn't that heavy when you consider that "fuel economy" still means that the average new car contains dozens of kilograms of "comfort" functionality and weighs around 1mg for the metal parts alone.

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

45? More like 120 years ... but yes. We're still building bridges out of steel, our concrete has gotten better, but it's still concrete. We use asphalt for roads. Copper is still our favorite conductor Our refrigerators are still not based on more efficient means of heat transfer, but on the compression of more or less dangerous liquids.

The latest batteries are much lighter and cost less than they used to be over the life of the vehicle. However, they do not use LA chemistry (lead acid chemistry).

A LiFePO4 battery (lithium ferro-phosphate battery) does what is required at an acceptable total cost, but at a higher acquisition cost - which makes it unattractive for automakers.

Low cost seems to be the main reason why lead is preferred over LiFeO4, and it is not obvious that there are other really good reasons as well.

The lifespan is much longer than that of lead acid, making the total cost of living less than that of lead acid.

In contrast to LiIon (lithium-ion), a "spike through the heart" does not cause the problems that a LiIon does.

The charge control is "simple enough".

Compared to lead:

Permissible depth of discharge and maximum permissible charge rates are higher.

Temperature range is better

The charging efficiency is better.

Self discharge performance is better.


Lithium-Ion / LiIon:

It is worth commenting on LiIon batteries as they are often "poorly pushing" when it comes to safety.

Compared to lead-acid, the LiIon chemistry offers much better mass and energy densities (lighter and smaller), a slightly longer lifespan, higher capital costs, and probably slightly better overall costs. Properly managed, charge control is easier. Temperature ranges are better, the charge / discharge efficiency is a little better. Disadvantages in terms of security are largely not an issue - see below.

LiIon batteries are used in many applications the Battery of choice - from Dreamliner to Samsung cell phones to "hoverboards", Mars Rovers to laptops and smartphones to MP3 players and more. The first three applications mentioned above were chosen because of their known spectacular flaws. But everything used in a Mars rover is chosen for its suitability for a long-lived, hostile environment that no task should fail. And there are hundreds of millions of LiIon batteries in daily use in people's pockets, in homes, in cars, and more.

Given the way LiIon batteries CAN fail, the numbers that fail in spectacular ways are very rare. Frequently reported failures are often the result of systemic failures affecting a batch or battery model that was manufactured and sold in large quantities OR lower volume in high profile applications. In such cases, a design or manufacturing error or defect causes or enables errors, the consequences of which are exacerbated by the relentless behavior of LiIon chemistry.

Examples of this are well-known "vent with flame" events in some previous Apple laptops, Samsung phones, self-balancing "hoverboards", and the like. In the first two examples, the manufacturers responsible generally allowed a design error to occur uncorrected and / or unnoticed, or cut the corners in production to such an extent that the safety margins caught up with them. With the "hoverboards" I don't know the cause, but there is a risk that they lead to low manufacturing costs and poor cargo control. In consumer devices, LiIon battery failure is often due to a short circuit in a cell due to insufficient spacing and the resulting impact sensitivity, or to deviations in statistical manufacturing tolerance at the extreme end.

In the case of Boeing Dreamliner battery failures, I haven't seen a definitive baseline report BUT while a number of known failures (and possibly some unpublished ones) occurred in a very small volume of product, the consequences were surprisingly well contained.

A detailed examination of LiIon faults and modes and consequences shows that they are almost always nowhere near as violent as the popular "myth" suggests, and that while the energy release is significant, containment is technically relatively straightforward. Containment increases weight, volume and cost and is rarely found in laptops or portable handheld devices. It can be found in Dreamliners and can easily be used in self-contained (i.e. non-EV) automotive applications, where weight and volume are still well below the lead-acid content and have little additional cost. In electric vehicle applications, the problems appear to have been solved or solved "well enough". I have no expertise in vehicle safety, but I am confident that the regulations that give us spectacular crash dummy material and the catting of highly volatile petroleum fuels in passenger cars will also address the safety issues with LiIon power sources. I've never heard of a Tesla car that was set on fire by a battery failure - although this may have happened - and I imagine Musk and Co. believe they have this area of ​​risk "in sufficient hands" .

To my disappointment, I have never seen a LiIon event with an open discharge and do not know anyone personally who has done this. Occurrences are common enough to get the New Zealand news out the occasional (New Zealand population is under 5 million).

LiIon versus LiFePO4:

Compared to LiFePO4, the LiIon- Chemistry slightly better mass and energy densities (slightly lighter and smaller), one much lesser Lifetime, slightly lower capital costs (per energy capacity) and significantly lower total costs. The charge control is about the same, but LiFePO4 is much more difficult to damage in borderline cases. The temperature ranges are not that good, the charge / discharge efficiency is about the same. LiFePO4 is far less relevant to safety.

In areas where the smallest size and lowest weight, as well as the lowest cost of capital play a role (for example when using electric vehicles), LiIon is superior to LiFePO4.

In almost every other area and application, LiFePO4 is better or much better than LiIon, and I would consider it the current battery technology of choice for long life, high cycle count energy storage.

Lithium starter batteries are primarily used for racing or other high performance or luxury applications where the weight savings or the bragging rights are worth the cost.

However, as others have noted, the application requirements are rather extreme and lithium technology requires a lot of special development and care in order to reliably and safely fulfill the role of a starter / auxiliary battery in a motor vehicle. The prices are extremely high - easily ten to twenty times the cost of a normal lead-acid battery. Most people don't want to pay $ 1000 for their car battery, so they don't.

The answer is very simple: because we haven't found anything better.

A car battery has to stay charged over a long period of time, deliver a large amount of current and fit into a small space. And it would help if it isn't too expensive.

Lead acid is still the best solution for these needs.

You could use some lithium based chemistry that can hold the charge and deliver large currents. They are also far more expensive, more temperature sensitive, require greater electrical care, and are more spectacular if improperly treated electrically or mechanically.
The added cost and complexity just isn't worth the benefits of reducing the final mass of the car by <1%.

I saw that you added a new question at the bottom of your post:

Why couldn't advances in technology make them easier and more efficient?

Because that's not how chemistry works.

The capacity of any particular type of battery is essentially defined by the amount of ions you have - and in the case of lead-acid batteries, that is essentially the amount of lead you need plus some that keep the structure intact.

Now other types of batteries suffer from a lack of surface area or limited ion mobility, which limits the battery's ability to generate high currents. There is not much you can do to increase the capacity of the lead-acid battery, however - water is an excellent carrier for the chemicals involved, and the current sourcing ability of a lead-acid battery is as good as at its maximum.

So it's just a mature technology. Just as we haven't made cheap structural steel much better over the past 80 years, there isn't much that can be done to improve lead-acid batteries without abandoning the lead-acid principle, with all the problems that my second answer explained .

The use of a super capacitor as a starter battery is quite feasible and has been tested in practice by enthusiasts, see example. Aside from higher prices, some examples of practical difficulties are given:

  • Supercap alone, while the car is easier to start than the lead battery, will discharge in about half an hour after listening to the radio if it is not continuously charged.
  • The directly connected lithium battery + supercap combination does not suffer as a result, but was damaged when he started a lawnmower with it - the Li battery would require additional electronics to prevent this.

Mainly one reason: price. There are technologically better alternatives like lithium-ion batteries in electric cars, but they are also much more expensive. These batteries are absolutely essential in electric cars, which require enormous capacity without adding significant weight to the vehicle (lead-acid batteries would be too heavy if they had to replace the fuel tank as the only power source for the car) fuel-powered cars The weight of a single classic Lead-acid battery, which is only used to start the engine, is insignificant compared to the vehicle weight, while the price / performance ratio is dramatically lower. It's a cost / efficiency problem: they're cheaper, they provide enough energy for the vehicle's needs, and their weight doesn't matter.

We use cookies and other tracking technologies to improve your browsing experience on our website, to show you personalized content and targeted ads, to analyze our website traffic, and to understand where our visitors are coming from.

By continuing, you consent to our use of cookies and other tracking technologies and affirm you're at least 16 years old or have consent from a parent or guardian.

You can read details in our Cookie policy and Privacy policy.