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Alternators and Batteries

S

steppinthrax

Diamond Member
Ok this is the deal. I changed the alternator in my old 94 Nissan today. I had a friend jump start it for me and it worked great. However, I was told by someone else you should never use a new alternator to charge a drained battery. It's best to charge it fully first, I was told. My battery wasn?t completely dead, It was just dead enough to barley crank my starter motor.

So that gets me thinking of all the people who jump start their cars when leaving their lights on. Especially those who live the lights on to the point where the battery is completely dead????
 
Sounds like bull to me.

An alternator is responsible for keeping your battery charged for years, so I can't imagine it's not at least an "acceptable" choice for bringing your battery back to full charge from a low state.

I wouldn't sweat it for a second.
 
I agree with redgtxdi. The myth probably have something to do with people overloading their alternator and/or their alternator is about to die anyway. But a good alternator should not get damaged by charging a drained battery. However, I remember reading, from a credible source, that the car battery life is reduced when it's discharged to more than 20% of its storage capacity. It's having something to do with the thin lead plates gets dissolve. It is, however, okay to totally discharge a marine deep cycle battery because the lead plates are thicker, and, therefore, wont dissolve.
 
Originally posted by: Pacfanweb
Complete B.S. Don't worry about it.
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I was kind of thinking this.

I remember when I bought an alternator for my saturn years ago the guy at the shop told me "don't just put that alternator in there". You know autozone and advance auto does free battery charging for you. I left my battery there overnight. They had this special machine that charged the battery and at the end told you if it was good. It's free... However it kind of didn't make sense to me when I remember everyone jumpstarts all the time....
 
Originally posted by: cprince
I agree with redgtxdi. The myth probably have something to do with people overloading their alternator and/or their alternator is about to die anyway. But a good alternator should not get damaged by charging a drained battery. However, I remember reading, from a credible source, that the car battery life is reduced when it's discharged to more than 20% of its storage capacity. It's having something to do with the thin lead plates gets dissolve. It is, however, okay to totally discharge a marine deep cycle battery because the lead plates are thicker, and, therefore, wont dissolve.
Click to expand...

But, I remember reading though that lead-acid batteries are designed for "Deep discharge" conditions.
 
Your battery is designed to keep a battery topped off and run the electrical system. It was not designed to charge a dead battery PLUS run the full electrical system. That being said, I have done it many times and my alternator still works but exceeding the current rating on your alternator CAN AND WILL cause premature failure. Just ask anyone who has ever installed too much stereo in their car as i'm wure they've replaced a few alternators in their time.
 
Yes, it stresses out your components somewhat. But it's not going to fry anything that isn't already on its way out.
 
The alternator's current output is pretty low at idle, so if you want it to charge a dead battery, you need to go for a nice drive. 😀
 
Originally posted by: steppinthrax
Originally posted by: cprince
I agree with redgtxdi. The myth probably have something to do with people overloading their alternator and/or their alternator is about to die anyway. But a good alternator should not get damaged by charging a drained battery. However, I remember reading, from a credible source, that the car battery life is reduced when it's discharged to more than 20% of its storage capacity. It's having something to do with the thin lead plates gets dissolve. It is, however, okay to totally discharge a marine deep cycle battery because the lead plates are thicker, and, therefore, wont dissolve.
Click to expand...

But, I remember reading though that lead-acid batteries are designed for "Deep discharge" conditions.
Click to expand...

Deep cycle batteries are designed for deep discharge because their lead plates are thicker. Car batteries are not designed for deep discharge. Go here: http://auto.howstuffworks.com/question219.htm
 
It was said already but I'll say it too. Your alternator is not designed to charge your battery from zero and run the car's electrical systems at the same time. Your owner's manual will most likely have some information on this and warn that damage to the alternator will occur if you do this frequently.

Folks, this is why trickle charge battery chargers are available for automotive applications. Let it charge over night. If you have a dead battery and get a jump drive right home and hook up to the trickle charge to finish the job - your alternator will thank you.
 
Alternator is designed to keep the system charged, not to fully charge a dead battery.

It's not the alternator itself that will be hurt, but the diode and regulator that will burn up under that kind of stress and constant current draw (usually part of the alternator and thus requiring another expensive alternator purchase).

Even more of an issue on decked out luxury cars with lots of electronics straining the electrical system. Older cars you can get away with it, but on new cars with 100+ amp alternators and loaded with electronic gadgets and TVs and "power cup holders", not so much. You'll fry the diode.
 
Originally posted by: exdeath
Alternator is designed to keep the system charged, not to fully charge a dead battery.

It's not the alternator itself that will be hurt, but the diode and regulator that will burn up under that kind of stress and constant current draw (usually part of the alternator and thus requiring another expensive alternator purchase).

Even more of an issue on decked out luxury cars with lots of electronics straining the electrical system. Older cars you can get away with it, but on new cars with 100+ amp alternators and loaded with electronic gadgets and TVs and "power cup holders", not so much. You'll fry the diode.
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"may".

Considering that upteen bajillion people get away with it on a regular basis, I wouldn't go so far as to say that it will fry the diode.
 
It happens enough that the car makers recommend you not do it. What more do you need? I guess you need to actually fry it and then you will think it's a bad idea. Some people learn through failure I guess.
 
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