Am I understanding this about charging car batteries correctly?

[imzjustplayin]

I remember hearing or reading somewhere that alternators are about 50% efficient, that for 100 watts of crankshaft power, it will only make 50 watts of electricity. Also, according to wikipedia, the charge efficiency of lead acids is only 50% while the discharge efficiency is 92%. http://en.wikipedia.org/wiki/Lead_Acid

If all of this is true, this means that to get 10 watt/hrs of electricity from a car battery (light bulb), 43.478 watt/hrs of crankshaft power is needed..

Holy crap!

[Jay2TheRescue]

Gotta love the laws of Thermodynamics.

[add|ct]

Not to sound like a complete maroon(), but would driving the vehicle at a higher RPM for a sustained period of time 'help' with the effectiveness/efficiency of an alternator in any application? I remember scantly about discussions of the Civic VX having a bare bones alternator that doesn't even 'appear' to be capable of completely/truly recharging a battery to max on it's own. Sorta a rough statement but generally got me thinking towards that end with regards to the VX app having such an interesting alternator.

[theholycow]

Alternators not being able to fully charge the battery is a concept that's been discussed on many automotive forums lately, at least the ones I read. It is described as universal, not specific to a make or model. I don't know if it's true and I haven't made the effort to confirm or deny it (probably a matter of going for a decent drive, hooking up a voltmeter, then a charger, then the voltmeter again).

I never got the impression that it was a matter of efficiency, just that the charging system is not designed to fully top up the battery. I can imagine a few reasons but I'm not sure.

None of the discussions I've seen has proposed that higher RPM would help. Even if it would I can't imagine how it would be worth any potential extra fuel usage.

[imzjustplayin]

Well I do think he is onto something, that alternators typically don't make their peak amperage until 2500rpm or something of the sort.. As for why alternators don't fully charge batteries, only thing I can think of is that in order to save costs by not having sophisticated charging circuitry and not risk overcharging the battery, they keep the batteries charged at a relatively low level. Also apparently cars do not charge car batteries when idling which is typically why a person who does a lot of city driving will find themselves replacing their batteries quite frequently.

[theclencher]

NO, alternators MAINTAIN batteries fully charged BUT THEY WILL NOT charge up a battery from DEAD. It's too much current for them to do that and they may burn out trying. AND alternators work far better than old school GENerators do at low rpms. Some of these comments lead me to believe you guys think alternators don't charge the batteries fully? Nonsense.

[imzjustplayin]

Originally Posted by theclencher

NO, alternators MAINTAIN batteries fully charged BUT THEY WILL NOT charge up a battery from DEAD. It's too much current for them to do that and they may burn out trying. AND alternators work far better than old school GENerators do at low rpms. Some of these comments lead me to believe you guys think alternators don't charge the batteries fully? Nonsense.

Alright but the thing is, within the same post, you seem to contradict yourself. We hopefully all know that you shouldn't try charging dead batteries because you can burn out the alternator, however I think that's an independent thought from the fact that alternators don't fully charge batteries that are partially discharged. I mean if you don't drive a car for a week, it's already got a partially discharged but not fully discharged battery, yet we feel that overall, the car won't restore the battery back to 100% charge because of the way the alternator charges and the voltage it charges at.

[theclencher]

I think that's an independent thought from the fact that alternators don't fully charge batteries that are partially discharged. I mean if you don't drive a car for a week, it's already got a partially discharged but not fully discharged battery, yet we feel that overall, the car won't restore the battery back to 100% charge because of the way the alternator charges and the voltage it charges at.

WTH? Why wouldn't the alt fully charge the batt?

[imzjustplayin]

Originally Posted by theclencher

WTH? Why wouldn't the alt fully charge the batt?

For a couple of reasons.. 1. Batteries get hot in the engine bay... 2. The charging system in cars do not have temperature compensation... 3. Overcharging batteries can lead to excessive hydrogen production which is extremely flammable, not to mention it'll boil out all of the electrolyte in the battery. 4. Because of all of the above, car's typically keep their charge voltage under 14v, yet to fully charge a lead acid battery, you typically need to charge at 14.7v with 13.5v being only good for float charging a fully charged battery. 13.5v in cold weather will be ****ty and on the brink of not charging the battery but in hot weather, it may be on the brink of boiling the battery. In my experience, a lot of cars have their alternator voltage somewhere at or below 14v which is a safe voltage but not too low to discharge the battery. Show me a car that has temperature compensating charge mechanism for their batteries and I'll take back what I've said.

[theclencher]

And to think my stuff has been starting with weak batteries all these years

No, scratch that. And to think millions of engines have been starting in all temps all these years with weak batteries

Including the original factory battery in my '94 Ford that's still going strong today! Too bad Ford doesn't know how to make a proper voltage regulator huh.