Car Batteries: More Than Just Wet Lead

Working on car electrical systems used to be easy. The battery simply provided power for the car’s starter motor when starting or to run the small number of accessories when the engine wasn’t running. The rest of the time, the alternator charged the battery and provided power for the rest of the vehicle and the ignition system. While very early cars didn’t have batteries, and some old cars had 6 V positive ground systems, most of us have lived our entire lives where car batteries come in several sizes (controlled by Battery Council International) and cars have a 12 V, negative ground system.

Times have changed. Cars don’t have distributors anymore, they have computers. They also have lots of gadgets from GPS to backup cameras and cellphone chargers. Batteries have had to get beefier and the modern trend is to also require less maintenance So, today, you’ll find that there isn’t just one kind of car battery. But how do these other batteries work and what was wrong with the good old lead acid wet cell?

For the purposes of this post, I’m not talking about electric car batteries which is a whole different topic — and most of them have a regular car battery, too.

In the Realm of the Practical

A 6-cell lead acid battery. CC BY SA 3.0 by [Genetics4good]
Back to reality, you don’t need to pay $800 for a car battery, but there are a number of choices. The traditional battery we all grew up with is a flooded-cell lead-acid battery. These batteries have a number of desirable characteristics. They stay charged for a long time. They can handle large surges like the ones that occur when you crank the starter motor. They can survive being lightly discharged and recharged many times. In fact, most of these batteries don’t like being totally discharged and recharged.

These batteries are simple enough. Each cell has a grid with a lead-based alloy for the cathodes and lead oxide anodes sitting in sulfuric acid. A typical battery has six cells to get to 12.6V, and there was a cap that allowed the user to add water — preferably, distilled water — when needed and also would vent out hydrogen gas generated during charging.

Evaporation and Shedding

The scourge of all this is evaporation of the electrolyte battery. With old batteries, you could actually spill the contents. Battery operation also removes water. Heat can evaporate liquid. Less liquid means less surface area exposed to electrolyte which reduces the battery’s capabilities. It also can lead to sulfation, where the electrodes are coated with lead sulfate which weakens the battery and requires careful recharging.

However, the primary wear on this type of battery is how part of the material sheds lead sulfate during operation, which sinks to the bottom of the battery.  Deep cycle batteries will have thicker plates and more room at the bottom for waste to accumulate. Eventually, if enough waste material accumulates at the bottom, the battery will fail.


In the day when you had to check the water in the battery like checking the air in tires, there were additives like VX-6 that you could dump into the battery cells that used cadmium sulfate to prevent sulfation. At least, that was the claim that some disputed.

But clearly, you would like to have a battery that lasts forever and doesn’t require any maintenance. You can’t have everything, of course, but you can try. New alloys reduced the amount of water decomposed, so batteries can have enough liquid in them to last over their useful service life. That’s why most batteries today are sealed and only have vents for gas to escape during charging.

Enhanced flooded batteries were another innovation and managed to largely supplant conventional batteries. These use a polymer separator that is porous to electrolyte but prevents the plates from shorting together. These batteries last much longer than conventional batteries and have far greater tolerance to deep discharge.

Glass Tacks

AGM battery opened up. CC BY SA 3.0 by [Bullenwächter]
The major battery today, however, is the AGM or absorbent glass mat battery. These are also known as valve-regulated lead-acid or VRLA batteries or sealed lead-acid batteries. Technically, VRLA or sealed lead acid can also refer to gel cells, but in automotive applications, you usually see AGM batteries due to the fast charging and long service life they provide.

The key innovation to AGM batteries is that the electrolyte isn’t a liquid. Instead, it is held in a fiberglass mat woven from very thin glass fibers. During manufacturing, the mats are soaked in acid, wrung out to remove excess liquid, and then installed in the battery where they keep sufficient acid in contact with the electrodes for the life of the battery.

AGM batteries require little maintenance and, unlike liquid batteries, can operate in any orientation. This is especially important in all-terrain vehicles and motorcycles where the battery may be tipped or flipped. They also emit less hydrogen gas which makes them safer.  They do not require any periodic maintenance, but they may or may not last longer than a properly maintained conventional battery. Note, though, that if you don’t maintain the normal battery properly, all bets are off. Then again, overcharging is harder on AGMs.

While a mat seems to imply a flat pad-like structure, AGM mats can be rolled or put in other configurations depending on the battery. For example, below you can see a comparison of a conventional battery and a spiral AGM battery. Another option for semi-solid electrolytes is the gel cell, but you don’t see these often in automotive applications because the gelled sulphuric acid doesn’t perform well at colder temperatures.

There are ways to fix some types of problems in unsealed batteries, some of which involve big voltage spikes. Thomas Edison liked using nickel-iron batteries in his electric car, and there may be a bit of a revival of this technology in some cases, too.


Featured image: “Walmart EverStart Car Batteries, Battery” by Mike Mozart

47 thoughts on “Car Batteries: More Than Just Wet Lead

    1. any scrap yard in the US will give you money for any kind of lead/acid battery commonly available (though most stores will charge you more than that if you don’t trade in the old battery when buying the new)

      1. Yes. But interestingly, most stores requiring a core return don’t care what size battery you return as the core, as long as it is a lead acid battery of some kind. Meanwile, the scrap yard pays based on weight of the battery. Thus, I always save up small batteries for the core return process and send my big heavy batteries to the scrap yard.

  1. Interestingly, in my experience auto batteries of years past had much greater lifetimes that the ones offered today. In fact, it is really tough to find an auto battery that does not have some level of pro rated warranty. I have had several AC Delco old batteries (lifetime warranty no less!) that would last 10 or so years and would not require any special treatment even if left outside in -40C temps for weeks on end… except to ensure that they were fully charged at the beginning and mounted securely (vibration can be a killer for batteries). To those that say that newer cars and their battery demands are the culprit, I say hogwash… as the same old vehicle where the old AC Delco’s were used are seeing lifetimes of maybe 3-5 years with batteries bought today.

    As for AGM batteries… so far my experience has been so-so at best. I am also finding that these batteries last maybe 3 or so years before they starting losing the ability to maintain voltage. To be fair my sample set is small… 4 different vehicles, they all use AGM and the lifetimes have all be similar despite the battery manufacturer.

    1. this is true. my dad had a 2.0L Renault 18 with a National brand battery (that’s Panasonic in most markets) and it lasted well over 10 years of daily use. My Nissan March’s lasted only 5 years (1 year into pandemic with barely any use except for weekly start and run for 10 minutes). I was told the replacement would be even worse and I’d be lucky if it lasted 3.

      1. Is this surprising? That seems far too little to maintain charge on a car battery.

        I’d imagine the battery was being run deeper into discharge than it’d like.

        Same with the engine it’ll be full of condensation and not hot enough for the water to be driven off and oil circulated properly.

        It’s also means your oil degrades due to age faster than actually gets to work.

        Overall those city miles are awful for cars.

        Is that how your dad drove too?

    2. The factory-installed battery in an ’04 Alero (therefore ACDelco, which I think is really a rebadged Johnson Controls part) went a good 5-6 years in Las Vegas before it finally conked out. Subsequent replacements (usually whatever flooded-cell battery was available) were only good for maybe 3 years each.

      For a while, maintenance-free EverStart batteries (Walmart’s house brand) were also sourced from Johnson Controls, according to the label. Placed next to an equivalent ACDelco battery, the only difference I could see was the color of the plastic used and the branding. I ran a ’77 Cutlass Supreme with one of those for I don’t remember how long…probably at least 4-5 years, if not more. Whether this is still the case today, I couldn’t say.

      OTOH, two Duralast AGM batteries (AutoZone’s house brand) only lasted a couple years each in a ’12 Rogue…this after a succession of flooded-cell batteries (including the factory original) that lasted about three years each. The second one was provided free under warranty, but when it started behaving like time was getting short, I replaced it with a Duracell AGM battery. (I don’t know who really makes them; they’re just what Sam’s Club carries nowadays.) That was just four months ago, so the jury is still out. I’ve stuck with AGMs here because the last flooded-cell battery dissolved the vehicle’s positive terminal almost completely away, and replacing the terminal (and the fuse block connected directly to it) is rather expensive.

    3. True, Ive had Diehards last up to 6-7 years in the 70s. Had a $69.00 premium Diehard in the late 70s. At 10,000 in Big Bear I was the one giving everyone a jump in the morning after spending the night in freeing temperatures! That battery lasted about 7 years.

  2. I’m adamantly trying to get Lead Acid and spinning disks out of my life, so I wait not so patiently for our Battery overlords to give us LiFePo car batteries (I already got one for the motorcycle, it’s wonderful). Though by the time they get here I’ll probably have to switched to an electric car.

      1. Not all EVs do. Because an EV doesn’t require a huge pulse of 12V energy at once for a starter, they can get away with a different battery technology and some are using lithium batteries for the 12V system. There are also lead-acid replacement lithium batteries for ICE cars. I’ve seen this as a performance option on some high end cars because it is lighter. But honestly this seems weird and I’d worry it would result in a situation where the car can’t get enough current to start.

    1. Your car will still be a rolling generator of toxic waste even without a lead battery, even with an electric car.

      You are still generating mass quantities of rubber dust from your tires and wipers, and your washer fluid is still polluting the groundwater. And then there is the issue of road salt.

      All of this is much worse than battery lead which gets recycled 100% and is not spraying from your car as you drive.

      You won’t even make a dent in your pollution profile until you deal with the big issues.

    2. Heartily agree – & aside from their electrochem. features LiFePO4 batteries are much lighter than comparable Pb systems. Such weight saving is very beneficial in portable & automotive applications!

      1. Except for the point where LiFePO4 absolutely refuses to charge below 0 C, and it takes more than half an hour to heat the battery up from dead cold before it accepts any current in the winter, so you’ll run the battery down in the winter.

        They come with built-in battery heaters, but the battery never warms up enough in average commuting, so you have to keep recharging the battery separately, which is a major inconvenience.

        1. I have to plug my car in overnight every day anyway (cheap Nissan Leaf EV), so half an hour to heat seems like no big deal. Besides, I live in hot AZ — we get maybe one or two nights of below 0C per year? I replaced my heavy lead acid 12V battery with a tiny, lightweight LiFePO4 and it’s been fantastic so far.

          But maybe you go Lithium Titanate Oxide (LTO) instead. LTO can charge well below 0C, perhaps as low as -40C. It also handles higher current than LiFePO4, so again seems more suitable for replacing lead acid for starter batteries. Unfortunately, they cost a lot.

    3. I’ve seen some models that use lithium chemistries online. They package all the charge control and lithium cells into a form-factor borrowed from the ol’ lead-acid standards.

      The devices essentially are a drop-in replacement offering a nice weight reduction – they are pricey though!

      I’ve long since decided it’s not worth it for my commuter automobile where Noise Vibration and Harshness concerns take priority, leading to many pounds of sound deadening material.

      Now if I was building a track or autocross car, I’d be seriously looking at it.

      1. I’ve been using LiFePO4 in my miata commuter for awhile now and am quite happy. More cranking amps with significant weight and space reduction (admittedly, that matters more for a miata than most cars). Battery also has charge/discharge protection with a “jump start” button to allow starting your car if the battery was inadvertently drained. As far as cost, they claim a 10y life so if I pay 2x the price of lead acid, price per year might be similar.

        1. I was looking at these recently and while they claim a 10y life, they only warranty them for the same 3-5y as any other battery. That doesn’t give me much faith in their claims. The ones I found were also more like 4-5x the cost, which just didn’t seem worth it.

          1. Why bother providing 10y if the competing product is only providing 3-5y? Why do you care about 10y warranty anyway, when in all likelihood, the company will be gone or moved on by the time this battery goes bad?

            Look at the history of Nickel Iron (NiFe) batteries. Way back in history, Thomas Edison produced rechargable NiFe batteries that literally last forever. But making a product that lasts forever means losing your shirt in business because eventually all the people who were willing to pay for your product already have your product. Lead acid batteries won in the end largely because they absolutely fail 3-5 years out from when you bought it. You return the dead lead to the company as a core, and they sell you a newly formed battery from recycled cores. This is a totally sustainable business model compared to the NiFe business.

            I see LiFePO4 as a modern day NiFe. It’s way better in that it doesn’t self discharge much at all, it has excellent coulombic efficiency, and it’s capable of higher current. Sure, it’s only rated for ~5000 charge/discharge cycles, so not forever like NiFe, but it’s still REALLY good. Even at 5000 cycles, LiFePO4 isn’t completely useless like a 3-5yr old lead acid battery — it’s only down to 80% of it’s original rated capacity, so still completely useful in a less demanding application.

            As for cost, LiFePO4’s designed to be a fully drop in replacement cost a ton, but the underlying LiFePO4 cells themselves don’t cost anywhere near that. My car expects a group 51R shape/size lead acid battery. I found a generic LiFePO4 on eBay that actually cost LESS than Walmart’s ready-to-go 51R lead acid. However, I had to make my own circuitry and custom mount to make it compatible with my car. YMMV.

          2. To K9spiud: edison didnt invent anything. He was a terrible prick who stole other peoples idead and patented them. He also electrocuted innocent animals to discredit alternating current transmission and distribution pioneered by Nikola Tesla, George Westinghouse and several engineers including Charles F. Scott and Benjamin G. Lamme.
            The nickel-iron battery was invented by Swedish cmehist Waldemar Jungner.

          3. @szakallat: “To K9spiud: edison didnt invent anything. ”

            K9spud (that’s ‘spud’) never mentioned Edison inventing anything. To quote: “Way back in history, Thomas Edison produced rechargable NiFe batteries that literally last forever. ”

            Before you go nitpicking others’ posts, re-read your own.

  3. In north America the vast majority of lead acid batteries are manufactured by a company called Clarios. (Formerly Johnson Controls) The plant is not far from my house located in Geneva Illinois USA. I applied for a job at the facility a number of years ago and found it to be a scary place. They draw blood once a year to test your blood lead levels. If your levels are too high you have to wear a respirator while in the plant. No matter if you buy a Menards brand battery or a Diehard gold they all come from the same assembly line. The only difference is the sticker that goes on the outside. You read that right. They all come off one assembly line (dependent on the group size, e.g. they run one group size until enough are warehoused and then change size) and there are carts at the end of the line with the appropriate stickers. When its time to chang brand one cart is rolled away and another cart is rolled in. Same batteries different stickers. They give you one free battery per year if you work there and calling out cause your batteries dead is not acceptable. The best advice the maintenance manager gave me when I was interviewing there was “Always buy the cheapest battery you can find and replace your battery every 3 years even if it still works.”

    1. A nephew of mine worked at one of the few competing lead-acid battery plants. Even with the yearly blood testing, he suffered neurological damage that was bad enough that he qualified for lifelong disability. It’s hard to talk to him now, he can’t keep a train of thought and often doesn’t make any sense. You couldn’t pay me enough to work in one of those plants.

        1. the thing with lead is that there is NO safe limit of exposure to lead. it will mess with your brain to the point of “schools near airfields and racetracks have lower IQ scores”, because of the lead present in fuel. lead exposure is well understood and documented.

    2. It’s not quite that bad; but close. Clarios makes different quality grades of lead-acid batteries; but most of their customers just want “cheap”. The market is also full of low quality Chinese imports. Like so many things, the bad drives out the good.

      We know how to make better lead-acid batteries; and some suppliers still do. Deep-cycle batteries from Trojan or US Battery, or AGMs from Enersys are still very good. East Penn Manufacturing (Deka and Intimidator brands) also have some above-average batteries. Industrial lead-acid batteries still last for 10-20 years or more, if cared for properly.

      With regard to short battery life in modern cars: It’s a combination of things. Downsized batteries to save cost and weight, higher under-hood temperatures, lots of always-on parasitic loads, lack of a way to add water that’s lost during normal use, etc. One trick I’ve used is to add a small PV panel to trickle charge the battery when parked. Lead-acid batteries last a lot longer when kept fully charged.

      1. One problem is never getting the battery full. Lead acid batteries are balanced by “overcharging”, or indeed trickle charging them until the cells equalize. When people drive short distances with all the parasitic loads etc. and the charger can’t get the battery back up, the cells drift apart and soon after the battery fails.

        1. @Dude I believe you are correct on maintaining a full charge and drive time has a lot to do with that. My wifees 2011 corrola had it’s original battery until 2021. Her commute was about 40 minutes each way until 2010 when it went to about 5 minutes each way. The battery failed shortly thereafter. 50ma is the normal max for parasitic draw although modern vehicles may draw more for up to 30 minutes after shut down due to systems “staying awake”. Also in many cases “sealed lead acid” batteries can have thier caps popped off to check electrolyte levels and be topped off to extend battery life.

  4. @Kirby This is your perception playing tricks on you.
    We ask MUCH more of our batteries nowadays. It is no wonder that we need to replace them more often.

    A 70’s car used it’s battery like, 30 seconds a day? If that? The ONLY thing it did was turn your starter motor.

    Today’s batteries are required to run a BUNCH of electronics all the time. A few weeks without running your car is BEGGING them to be empty when you try to use them.

    1. Yeah, doesn’t help all the IoT crap getting added has cranked the parked power consumption sky-high.
      Keyless entry is also a big sinner, that’s much worse than the remote key fob.

      1. Also forgot to mention, they’re more aggressively downscaling battery sizes to reduce weight and make room for other things.
        So it’s really a cocktail of things.
        Even early 90’s econobox had a larger battery from the factory than a 00’s one that had essentially same engine.

      2. Ah ! The aftermarket Keyfree Boyo product is a pure crap : just got one to install on mycar and this student piss poor job is just a microchip, sinking a 20mA@12v non stop!
        I contacted them without any response…
        Just avoid this product even if it look great and easy to install behin windshield.

  5. Somewhere out there on the web is a site with all the text and images from an old book (1920’s or 1930’s) that details how to take apart, rebuild, and reassemble lead acid car batteries. The batteries covered have tarred wooden cases. The cells are inside rubber “jars”. The cells are connected with exposed lead bars and everything is held into the wood case and sealed over the top with tar. Steam was used to soften the tar to be able to take the battery apart.

    But the especially interesting part of those old batteries was they used thin wood veneer between the lead plates. The manual details assembling a stack of interleaved positive and negative lead plates with veneer sheets between them. The stacks were squeezed in a special press then put into the “jars”.

    AGM isn’t really a new technology. Whomever came up with it dipped way back into the history of the lead acid battery and replaced wood with glass fiber. The main difference is there’s enough glass fiber in an AGM battery to hold all the electrolyte while the wood veneer was mainly there so the plates wouldn’t short.

    With the development of injection molded cases, all thought of physical separators between the plates went away for decades. The cases had slots molded in to hold the plates apart.

    As for an alternative to AGM, NEXcell is currently having a bunch of their customers test prototypes of their lithium iron phosphate replacement for the AGM 12V battery used in Toyota hybrids that have a NiMH main battery, or have had that replaced with NEXcell’s LiFePo4 module kit.

    For a 2004-2009 Prius, losing 40 pounds of traction battery weight and gaining more usable battery capacity helps the MPG. NEXcell offers kits for all the Toyota hybrids that use the same NiMH modules. Shedding a few more pounds from the right rear corner would help even more.

  6. “The major battery today, however, is the AGM or absorbent glass mat battery.”

    ?? Define “major”

    I think (feel free to correct me if wrong) that flooded batteries are vastly more common in the US and AGMs are a premium upgrade.

    1. Not in the US so cannot fully comment but here in the UK, AGM is essentially mandatory on vehicles with stop start. And most every new vehicle has stop start. This is also a big contributor to battery failure these days as stop start is naturally very hard on the battery, not just in cranking but running all ancillarys when the engine is off.

  7. Very recently discovered my 2013 Volvo V40 has a 2nd smaller battery just for the stop/start function. My stop start has been pretty much dead for the last 2 years, reporting ‘battery charging’, so assumed the main battery is wearing down.

    I then got a suprise to learn about this 2nd battery. Even more surprising is that to access it you need to remove the front bumper?! A quick calculation in my head gave savings of about €50 (at todays [inflated Swedish] fuel prices over the last 10 years (really the stop start was working for about 8 years) from the stop start. I don’t travel in traffic much and plan ahead (not racing to a red light) so it’s used maybe once or twice a day.

    Somehow even if I fit the battery myself, it’s going to be loss making to even buy a replacement battery. There’s progress!

  8. Apparently BMW has a totally different technology? I’m only passingly aware of it, but something to do with it never really being charged up that much. Or maybe it just has a DRM or something. Either way, I was told that you cannot replace an OEM one with a generic. Anyone know about this? thanks.

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