When a modern car battery gets old, why does it die all at once? There’s no period of obvious low cranking power anymore, it just quits. Is this due to changes in the batteries themselves? Or does the car starting system refuse to engage below a certain voltage/amperage? (Maybe to protect itself?) Why don’t car manufacturers include a warning light to let the driver know the battery is getting low?
Tire pressure monitors. How do they work? I’ve just gotten a new car with this feature, and I can’t see anything different about the valve stems (I would assume any pressure sensor would be there). They look normal, at least from a casual glance. Is there something in the tire, or stem that signals the car? Or does the transmission somehow recognize a continuous difference in rotation speed (right .vs. left)? If it’s in the tire(s), does the system just look for a low pressure, or does it react to a significant difference between pressures? I realize some of this may be brand-specific, but any info is welcome.
Re: batteries, they can fail gradually. It’s just that modern cars start so quickly and reliably that you don’t notice as easily that the battery is weak.
Though, when they do fail suddenly, it’s usually due to one of the cells having a break in the circuit, or the plates that’ll contacting each other. The two top killers of lead acid batteries are
vibration/mechanical shock (sudden failure)
repeated deep discharging building up a layer of lead sulfate on the plates (gradual).
I know that (at least some) tire pressure monitors work by measuring the rotation of each wheel, as you guess. Any difference between rotation speed is taken as an indication that one of the tires is flat. Rotation sensors are simple, nearly foolproof, and cheap. They’re also needed for anti-lock brakes, traction control systems, and the like, so most modern cars already have the necessary electronics. Tire pressure monitoring is just another few lines of code and an additional indicator light.
I suppose you could engineer some sort tire pressure monitor that actually measures air pressure in each tire, but off the top of my head that seems much more complicated. You’d need a pressure gauge that handles the rotation and general beating that a tire receives, and then somehow transmit the information from the sensor to the car’s computer. That’d require some sort of power source, like a battery that needs regular changing, or a dynamo which adds more cost and complexity.
Your battery isn’t really “dead”, it’s output voltage has just fallen below a minimum voltage that the auto manufactuer designed into the system electrical circuits to keep it from damaging components with a low voltage.
I beleive those tire pressure sensors communicate by radio frequency with the cars computer control and they anit cheap. It cost me $125 to replace one because I put a chrome cap on it to replace the ugly plastic one it came with; turns out the cap caused the valve stem to corride the electronics (due to dissimilar metals) and it wasn’t covered under the warrenty because there was a disclaimer hidden away in the owners manual telling you not to do that.
It doesn’t always. Like many electrical parts, it can fail suddenly or fail gradually. Keep in mind that the handful of dead batteries that you have personally observed is but the tiniest fraction of all the car batteries out there.
It’s not the kind of thing that lends itself to a simple warning light. While some cars have voltmeters that can indicate a battery being low on charge, that’s more commonly due to an alternator problem or excessive drain on the battery (e.g. from a light left on) than to a faulty battery. To get useful knowledge about the condition of the battery itself requires a test sequence that takes time and/or uses specialized equipment and is not feasible for an onboard monitor.
Yes. Google on tire pressure monitor sensor under “Images” to see what they look like.