alternator facts

[johnceggleston]

for those of you who are 12v auto electrical gurus this will be no surprise, but i have just learned something.

 

my battery has been "leaking" on and corroding my hold down a little bit all the time, apparently for years. recently it did it so much it oozed down onto my ATF lines, the metal ones bolted to the frame under the battery. now they are corroding. i'll have to clean and or replace them. (what do you think?)

 

so i concluded my alt was overcharging. i put my stethoscope on it and it is making some noise. not a lot but more than my GT. so i pull it off and swap in another one i had. i located a rebuild shop and off i go. well the alt i pulled is fine, 14.3v without a load and 100 amps when needed.

 

but i was told if my battery is bad or slightly bad , maybe a cell going bad, the alt will over charge to compensate. news to me but it makes sense. also a good working alt will only produce the amount of juice the system actually needs. the greater the need the more it produces. therefore a bad battery causes it to put out more juice. turn on the headlights, it makes more juice.

 

i also learned that if the alt doesn't have a good ground it will / can over charge, or if the block ground is missing, or the trans ground.

all the wires / plugs on the alt are positives, the ground is through the housing and mounting bracket.

 

also a charging alt can make noise, a low whining.

 

who knew?

 

i don't remember if i bought this battery or if it came with the 97 OBW when i bought in 12/05, but whatever it's problem, it has been around a long time. i'm surprised it has not yet failed completely.

 

so any way, learning about alts.

 

BTW, the price for a rebuild is $28 labor plus parts. this sounds reasonable unless the parts are 100$. but even then, less risk than a reman from the local parts store.

Edited October 29, 2010 by johnceggleston

[OB99W]

Thanks for sharing your experiences, John.

 

[...]but i was told if my battery is bad or slightly bad , maybe a cell going bad, the alt will over charge to compensate.[...]

A typical "12 volt" automotive battery has six cells in series, each at 2.1 volts when charged, for a total of 12.6 volts. Due to various factors (improper charging, excessive vibration, etc.), as the battery ages some of the plate material finds it's way to the bottom of the cells, and forms a conductive "sludge" or "mud". If that sludge level gets high enough, it can partially short out the plates in the cell. Such a cell can no longer be charged to 2.1 volts.

 

The alternator will attempt to keep the voltage at whatever level its regulator is set to -- let's say that's 14.4 volts. If all's okay with the battery, that voltage will be about evenly shared across each of the cells, 2.4 volts apiece. Now, let's assume that one cell has too much sludge (but the other cells are okay), and only 1.3 volts appears across the bad one during charging. That leaves 14.4 minus 1.1, or 13.5 volts to be shared by the remaining five cells. Those five will each see about 2.7 volts, instead of the usual 2.4. That's enough to overcharge those five cells (it's the equivalent of the alternator outputting 16.2 volts into six healthy cells), and electrolyte venting would not be unexpected. A more complete short in one cell, or a partial short in more than one, would make matters worse.

 

For anyone interested, I found a link to some good battery info: http://www.landiss.com/battery.htm

[Allpar Mod]

Great facts, John.

 

Many people underestimate how the problems that batteries can, and do, cause can vary from the simple to the unexpected. There is always a balance being held in the charging system of any car and the battery's condition has a big role to play in maintaining the balance. That's why it's so important to...

 

1) Buy a good quality battery, period. I'm of the mindset, by experience to some degree, that when you cheap out, you pay for it twice.

 

2) Maintain it. That includes the posts and terminals and cables along with any associated wires in the circuit.

 

3) Replace it when it shows signs of going bad.

 

As to "good quality batteries", another reason for keeping vigilent of them is even an expensive battery can go bad. I recently had a two year old Optima battery I had in my truck go bad. I couldn't figure out why I had issues with it going dead. I don't drive the truck every day, sometimes for a week or so between drives. I had checked the charging system, grounds, relays, etc. Turned out that it had a defect. Good thing Optima red tops have a three year free replacement guarantee.

[Olnick]

Great thread, gentlemen! Good information to know. Thanks johnceggleston, OB99W and Allpar Mod.

[OB99W]

Great thread, gentlemen! Good information to know. Thanks johnceggleston, OB99W and Allpar Mod.

Thank you, Olnick. I like to contribute because it keeps my mind from wandering and confusing MTs with 4EATs, if you know what I mean.

[idosubaru]

i've seen quite a few alternator replacements within a year of battery replacements and the other way around too. which implies that when the old one started to fail it caused excessive "wear" on the other. so yeah, replace sooner than later keeps all charging components lasting longer.

 

you've gotten nearly 5 years out of that battery, that's pretty good so i'm not surprised at all it's time to move on.

 

subaru has a recall special from around 97 legacy's i believe so you can buy brand new alternators for like $60 or $70 from Subaru. mine isn't even failing in my legacy but since i want to keep it for awhile and it's so cheap i bought one this summer, though i've yet to install it.

[Fairtax4me]

Due to various factors (improper charging, excessive vibration, etc.), as the battery ages some of the plate material finds it's way to the bottom of the cells, and forms a conductive "sludge" or "mud". If that sludge level gets high enough, it can partially short out the plates in the cell. Such a cell can no longer be charged to 2.1 volts.

Exactly why the 5 year replacement interval is recommended, even if the battery appears to be in good working order.

On the flip side, just because a battery is new doesn't mean it can't have a bad cell. It's just not likely to be for the same reason as an old battery.

[Tahoe Subaru]

I ran across this. I thought I knew every thing about batteries. not. I'm going to wear gloves when I clean the terminals from now on.

 

http://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery