Alternator output, battery life

[Mike Arman <armanmik@n-jcenter.com>]

>From: Todd Phenneger <phen9461@uidaho.edu>
>Subject: Re: 5ktq...batt keeps dying!
>
>On Fri, 30 Oct 1998, Pcarselle wrote:
>
>> Todd Phenneger wrote:
>> Todd:
>> I have not followed the thread completely but would comment as follows:
>
>	Well thanks anyhow.  Nice post.  Some of this I knew, some I did
>not.  I have a few questions though.
>
>> 2.- The 14.5 volts that are referenced to are "peak values", the
>> rectified sinusoidal wave from the alternator drops to zero between
>> cycles, and when a level below battery voltage is reached (in every
>> cycle) the battery discharges.
>
>	I thought alternators were DC meaning no Sine Wave pattern of
>peaks and dips in voltage.  Is this only from Battery you get true DC.
>

The output of the *complete* alternator assembly is DC - the output of the
alternator stator (part that doesn't move) is AC, which is rectified by the
diodes (six, nine, and sometimes one or two more for good luck) is DC - the
diodes are most usually mounted INSIDE the alternator housing, so that's
why the output of an alternator is said to be DC - this is mostly semantics.

The coils make AC, and the diodes take the AC and make it DC.






>> 7.- When adding load to the alt/batt combination it should be considered
>> that only when the alternator cycle is above 12v. the battery will
>> charge, but  below the 12v, (in every cycle)the battery discharges, if
>> the external load requires more current during that portion of the
>> cycle, than the current that entered the battery during the cycle time
>> above 12v. then the battery would slowly loose charge.
>
>	So a 90 amp Alternator "Averages" 90 amps but may peak at 180 and
>go as low as 0 inbetween Cycles?  Or is 90 amp the PEAK available at top
>of sine wave.
>

A 90 amp alternator can put out 90 amps. While internally, the output
varies, what is finally sent to the battery as "finished product" (DC) is
the rated output of the alternator. Kind of like your engine - if you pick
the correct microsecond, the engine is making NO power (like at TDC between
end of exhaust and start of intake stroke), and is simply coasting on
internal intertia (that's what the flywheel is for), but since we don't
drive on a microsecond-by-microsecond basis, we effectively "average" the
output over a period of time and then send it to the tires.



>> 8.- If the battery is severely discharged, and substantial loads are
>> connected, then Mr. alternator maybe strained and mr. coil o mr. diodes
>> can suffer. (Alt. belt should be in good shape !!!)

That can happen - a short (or near short) to ground can very easily fry an
alternator.



>
>	WHat diodes. 

Yes, they are in there - take an old alternator apart someday - you'll find
them.


 BTW, if Alternator is over strained then Alt.
>Voltage indicator whould show it wouldn't it.  Would it be up around 15v
>like it is if you disconnect Battery while motor is running.  (Mine came
>loose once and would pop off every time I accelerated.  Took about 10
>minutes untill I poped the hood to check Batt Connections.
>

The voltage indicator will NOT reliably show alternator overloads. This
requires an ammeter - there are two types - the usual kind is center zero,
which will show you how much charge or discharge you are getting at any
given moment - this is a useful indication, as extreme readings in either
direction indicate a problem - usually, the needle "cruises" at about zero
- center - in a stabilized system where load and draw are equal.

The other type is a "load only" or "output only" ammeter - shows you how
much is being drawn or supplied - surprisingly this extra information is
not particularly helpful - what we want to know is that supply and draw are
equal (plus a little extra to charge the battery) - exactly how many amps
we are using right now isn't terribly important. (This type is used in
electronics troubleshooting.)



>> 9.- Battery life is not chronological (time determined) it is rather
>> dependent on the number and intensity of discharge cycles (disregarding
>> other maintenance elements)
>> I don't know if these are some of the comments you were looking for
>
>	The the more I ask of it, the shorter its life?

Battery life is determined by a number of factors - the number of
charge-discharge cycles is one of them, and how discharged your battery got
before it was recharged - if you discharge a battery until it is totally
absolutely flat dead, it may not be able to be recharged at all - even a
new one! Cold temperatures retard chemical reactions (and the battery makes
electricity by chemistry), and overheating the battery also makes for a
short, albeit merry, life.

Batteries should be topped off with distilled water, but most often get a
dose of whatever tap water is available at the gas station or the owner's
home. If there is a high mineral content in the water (especially sulfur,
which you will smell), the battery has just been killed right now, with a
stake through its' heart.

Repeated charge and discharge cycles (and vibration) cause bits of lead and
lead sulphate to flake off the plates. These flakes are conductive - they
settle to the bottom of the battery, and if the sludge level rises to the
bottom of the plates, you then have a shorted cell, and it is new battery
time. Look at a motorcycle battery - clear plastic case - and you'll see
what I'm talking about.

This flake-off phenomenon also reduces the surface area of the plates - the
area available for chemical reactions to make electricity - so that's why
an older battery has less cranking ability than a new one, and often
doesn't hold a charge as well or as long either.

Hope this helps - end of ramble!

Best Regards,

Mike Arman