Glenlivet wrote:
Seeing as a common vehicle operating voltage is 13.8 V., are you saying that a production car's battery, as designed, never achieves a full state of charge?
Yes, that's what I'm saying.
(Though my old '76 Ford campervan bus voltage averages closer to 14v.)
Voltage regulated alternators (~constant voltage charging systems) aren't really designed to charge batteries at all. They are designed to supply and maintain a more or less steady bus voltage, while supplying whatever loads happen to be drawing down the bus.
The typical cranking battery won't be discharged much by starting the vehicle. Say 250a starter draw for 3 seconds. Ignoring Peukert for the moment:
250 amps x 1 hour would be 250 amps per hour or 250ah
250ah / 60 minutes would be 4.1 amp*hours per minute
4.1ah / 60 seconds would be .07ah per second
.07ah * 3 seconds would be .21ah
So the cranking battery, on average, could be expected to be drawn down by one fifth of an amp*hour each time it was used.
The voltage regulated alternator system is designed to hold the bus voltage, and by doing that, can be expected to replenish the teeny-tiny amount of amperage drawn from the cranking battery.
But, that battery will be overcharged if held at too high a voltage for too long, so it's better to hold the bus voltage down to a level which will not cause damage to the battery over a long exposure time.
That would seem awfully inefficient and would represent awfully negligent design from decades of automotive engineers.
:h
I disagree. It would be only slightly inefficient, and would represent a design philosophy of "best compromise" by the engineers.
The design engineers do have better solutions - such as multi-stage voltage regulators often used in marine applications. You don't often find those used in automotive applications however.
There are also alternator/voltage regulator systems for automotive applications which utilize PWM technology - though even they are still simply single-stage constant voltage systems designed to maintain bus voltage.
The PWM allows for a much tighter, more well-regulated bus voltage - but really does nothing to improve battery charging, which is proper, since the voltage regulated alternator is not designed to be a battery charger in the first place.
The internet is chock with advice and information about battery charging, with most lore being similar but variations existing. The most telling phrase I encountered is: 'You talk to different engineers, even at the same company, you get different answers.' :B
Not surprising, of course, because there is a reason why it happens:
If you study the recommended charging specs for a large assortment of lead-acid batteries from a large assortment of manufacturers, you will find mostly similar trends, but each with their own specific minor variations.
This is largely due to the fact that a lead-acid battery is a big sloppy chemical device and not a precise electronic component.
Basically, it's the St. Bernard of the electronics world.
For the most part, "close enough is good enough" when dealing with such a sloppy and imprecise apparatus.
I'd have to think that the finished and stabilized voltage of an out of circuit wet battery will dictate what the level of charge it containes, and when the full charge voltage is 12.7 and the battery, removed by several hours from a charger, reads 12.7 volts then it is 100% charged, whether it got that way from being on the vehicles' own charging stystem, from being 12 hours on a 'maintainer' that generates no more than 13.8 volts, or whether it got that way from being on a three stage charger for a shorter period of time.
It would seem so.
However that doesn't exactly agree with the manufactures recommended charging routines, which generally (and pretty much always in the case of deep cycle), specify an extended time at a higher voltage (absorb stage) to achieve full charge.
Personally, I agree with you: Hold a battery at a low float voltage for long enough (in some cases, a few days) and it will eventually reach a full charge.
(An analogy would be simmering a sauce at a very low heat as opposed to simmering at medium heat.)
But then...by not pumping it up to a full charge quickly, sulfation certainly will result, and what started as a 100ah battery might only be a 99.75ah battery at that point.
But yes, it will be filled...to the limit of its new lower capacity.
