The battery plates can fracture and fall off their mounts, creating a short in their cell, which takes out some or usually all of its capacity. Every plate makes 0.3 volts or something, so when you lose one (actually 3) then your cell is down to 1.2 volts instead of 2.1... and the battery now has only 11.7 volts.
I dunno if battery plates have become thinner over the years (weight and cost) or it they're plated or made with some kind of lead substitute (calcium or whatever) that makes the plates more brittle and liable to fracture... or whether car electrical systems just need to power a telly, a fridge-freezer, sat-nav, Blue and Me and whatever, so the batteries just have more work to do.
In the old days all you had to worry about was sulphation. And Lucas points ignition.
Sulphation is when the lead dissolves in the acid, which it does if the battery isn't charged up enough. Little pieces of lead sulphate fall off the plates and accumulate at the bottom of the cell. When the lead suplhate accumulation reaches the bottom of the cells, it shorts them out.
You can "fix" the problem with some tablets of <something> that dissolves the lead sulphate.. and turns it into something soluble and/or not conductive.. but there is no way to return the lead to the plates, so a sulphated battery generally has thinner (dissolved away) plates that are also more brittle and likely to fracture. So keep your battery charged up, even if you don't use it.
Lucas points ignition could be cured by a lot of fiddling with the distributor cap and/or swearing a lot. A strobe light was also needed.. but I have no idea what for..
Ralf S.
Interestingly, I was in Halfords on the weekend to buy a new battery charger, since the old one had suffered from being melted inadvertently.. (don't ask).
The new charger looks like a regular battery charger but it says it's not suitable for Stop and Start batteries, which is a bit of a chore since the Younger Mrs S' Rasputin has the S&S.
It looks like the charger is just not "smart" so it just charges at a default 4 amps/hour regardless of the battery, whereas the "suitable for S&S" charger was £30 smarter and seemed to offer an automatically variable charging rate.
Obviously, I will use the regular charger on the Rasputin battery if it comes to it.. but it could be the S&S styley batteries (which seem to be just bigger than non-S&S, rather than containing some special alchemy) don't like to be over-charged, too quickly.
That suggests they have the cell plates very close together (makes sense...equals more amps) and these can overheat, warp, touch and short out if you overdo the juice.
So.. it seems we have to charge our batteries in an ice-bath to keep them cool... and at as low a rate as possible...
but it maybe points to the new-styley batteries are essentially being worked quite hard these days, compared to the old times, when Joe Lucas aka The Prince of Darkness - was resposible for the electric components.
Ralf S.
Sorry Ralf but these two posts present a woeful understanding of batteries.
Firstly, in this case i'd assume we are talking about a stop start battery thats failed known as AGM (absorbent glass mat) In AGM batteries the plates are packed tightly together and separated by a fiberglass material. If they did fracture they could not come into contact with one another as they are insulated. you would get a drop in current in a cell with a fractured plate (potentially) but not a drop in voltage.
because AGM batteries are so tightly packed they are more powerful (they are also more expensive) and they less susceptible to damage from vibration or being knocked or dropped. They are also sealed so can be mounted in any orientation without leaking.
Battery containing calcium.
If you where to make a battery plate out of calcium, then surround it with sulphuric acid, the plate would dissolve quickly converting to calcium sulphate and leaving you with no reaction, as calcium sulphate won't react with sulphuric acid. No reaction = no battery.
This leads me on to your comments about battery Sulfation. Sulfation is when the battery reaches a very low state of charge or is never charged properly over a longer duration, the sulfur drops out and breaks up in the sulphuric acid and forms free crystals, which build up on the negative battery plate. over time if this build up of crystals continues, the plate will become smothered and as the sulphate
does not conduct electricity
the plate will not work and the battery will eventually fail but over time its power (current) will get worse and worse.
In old lead acid batteries the sulfates could build up to such an extent the battery would bulge or split, it could also reshape or push the plates around causing short circuits by pushing them together. it however does not in any way shape or form work in the way you described.
You could in some cases with sulfation break it down with a very powerful and long charge which would break down the crystals and the charged acid would reabsorb the sulfate. However this depends on the duration that the battery has been left at a low charge.
I assume this is possible in an AGM battery but the effects would be very different as there is not much acid in an AGM battery.
Finally because AGM batteries are sealed, they have to be charged with lower currents for longer periods. an old battery would be able to take a big charge. The battery releases hydrogen when charging and in an old battery this gets vented.
In an AGM battery the hydrogen is not vented, it is completely sealed and the idea is that the hydrogen gets absorbed back into the electrolyte if given time, however if you produce too much too quick (by trying to charge it with a high current) then the pressure in the battery will build up until the battery blows its pressure release valve at which point it becomes scrap.
finally you can put epsom salt into a battery to dissolve the sulfation, however this often doesn't work and while it may improve battery voltage it won't help a very old battery and it won't help much in improving the cranking amps, as you dissolve the sulfates but then the acidity is much reduced.
Battery Technology is in essence all a chemical reaction and so understanding the chemistry is important, but also knowing the differences in the types of battery would help, rather than speculating about how something does or doesn't work.
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In the instance of this thread its possible that an AGM battery has vented and lost its pressure because of an internal failure due to age or over charging and its simply died.
