My car alternator charges more at faster speeds
and higher rpm. Are all alternators similar in this
regard or are there alternators available that operate
just as well at low speed and low rpm.
You can run your battery flat if you are stuck in
a three or four hour traffic jam at night with your
lights on and barely moving forward. This gets worse
in cold weather.
I was just wondering if there were alternators available
that charge well at low rpm.
Thanks in advance
My '94 Crown Vic does this, too. When there's a load on the
system, as with lights and blower and rear defrost on, the voltage
level will go down 10 to 15%. I had my mechanic check this last
week. He reported that the alternator is only charging 65 amps at
idle, though the alternator is rated to put out 95 amps. He
figures it's a bad diode in the alternator, since it only happens
under load at idle. The charging is just fine when there are RPMs
on the engine. Since a new alternator is $350 installed, we agreed
that I'd just keep an eye on it for now and watch to make sure the
battery light isn't coming on at any point. (BTW: I trust this
guy totally. He's never once steered me wrong in the 10+ years
I've been going to him) Once the spring comes to the forsaken
lands here and the blower and rear defrost aren't being used all
the time and I don't need lights to drive to and from work, the
load on the alternator will be much reduced and I can probably
drive through the summer with it. I'll likely get a new one in the
Hope this helps.
most alternators drop slightly in output voltage when idling, however this
should really be an insignificant amount on a correctly operating alternator
and sitting at idle for a lengthy period of time even with your lights on
should still charge your battery, so if it doesn't, then either you have too
much load on your alternator or its probably got a burnt diode or similar
alternators with certain faults may indeed continue to charge but not as
efficiently. and of course once this happens your alternator will have to
work twice as hard to keep the battery voltage topped up so its life-span
from this point onwards is probably very short.
have it properly checked and replace as necessary, you might get stuck
I would have considered this to be normal operation. Every vehicle I've ever
had charges better when off idle, including the Chrysler (sorry, I do still
have the Granada) that's now only two weeks old. The headlights get
marginally brighter when all the switches are on and the revs are increased.
If you're stuck in traffic turn your headlights off and sit there with just
the sidelights on, switch 'em back on when you move at any sort of speed.
Three to four hour traffic jam!!! find a different way home from work or get
a train man! It'd be cheaper to get a hotel than idle the way thru' all that
That reminds me of the power crisis in California. They said that if
everyone would just shut off his lights on the cars, they would save enough
energy to prevent the power outages. Unfortunately, cars don't usually come
with power cords.
It is kind of like a water pump. The faster it goes, the more juice it can
Sounds like a problem either with the battery or the alternator or maybe
voltage regulator. While an alternator should produce more electricity at
higher RPM, it should produce enough electricity at low RPM to run the
engine electronics, ignition, CD player, lights and charge the battery.
Now, if you have one of those truck with the rack of lights at the top and
another at the bottom, and you're using all of them, well, that is why you
can't keep the battery charged.
water pump huh? actually your wrong, years ago before alternators were
invented or used cars made use of dynamo's, which did this exact thing,
provide more power with faster speed, however with modern electronics which
demand an acurate regulated voltage and current flow this would only serve
to eventually burn out the components it is powering, which is why we use
alternators designed to provide a steady voltage regardless of engine speed,
true the voltage can sometimes drop at a slow idle speed and when under load
from headlamps etc but as mentioned before this should be a signifficant
The electronics limit the voltage, of course. However, you can't reach the
voltage (and current output) if not enough energy is coming into the
alternator (i.e., the alternator is turning fast enough). The electronics
themselves don't generate electricity, they can only regulate what's there.
So the faster the alternator is turning, the more electricity it can
generate (which really means that at the output voltage, it can generate
more current going faster, because the output voltage is regulated by the
your just guessing arn't you jeff, do you know how an alternator works? or
did you get a starters guide to home electronics (ages 3 - 5 yrs) and
figgure that qualifies you to argue your point.?????
alternators have a cut-in point, IE they need to turn a minimum speed before
the regulator will transfer the voltage to the battery, this is designed
because at an extremely low engine speed the alternator may only kick out 10
or less volts and this fed into a battery holding 13.5 volts and above will
have an opposite effect of charging, so therefore it needs to supply in the
region of 13 - 15 volts and a regulated feed (IE amps) in order to maintain
a healthy charge.
at the other end of the scope, if it wasnt regulated then you may find
anything up to and over 16-18 volts which will not do todays modern
electronics any good whatsoever.
so therefore alternators are designed to provide the same voltage and
current flow once the alternator has kicked in. if yours isnt doing this, or
the fluctuations are over + - 1 volt then i would say its faulty.
steve "pissed and can't be arsed"uk
This is normal. As long as it's moving the voltage up at idle, you're ok
Often, people will get a replacement alternator, and not notice the
pulley is different. If you put on a replacement alternator and the
pulley is larger than original, you will have problems. Some rebuilders
don't worry about the difference, as long as the amp rating is right.
Had this problem on an 82, they gave me a v8 alternator instead of a
six. Alternatively, you can look at other alternators for your
application and find one with a smaller pulley if you do a lot of
idling. Many of these high capacity alternators the kids are putting on
their cars to power their sounds systems use a small pulley.
The smaller pulley simply speeds up the rotation of the alternator, which is
exactly what happens when you come off idle. The real need here is to fix
Either you have an excessive load or you have something wrong in the
electrical system. First, check all the electrical connections at the
alternator, battery and grounds. A loose connection will cause the charging
system to lose efficiency. If that all checks ok, have the alternator load
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.