Charging battery with alternator

JDgreen227 · Jan 7, 2012

Guys, I left the key switch on my JD on and ran the battery flat, thankfully it has been unseasonably warm recently so the fully discharged battery never froze. I have a charger on it right now, and am going to let it charge that way instead of starting it up immediately with the boost feature on the charger.

In the past I have read that the quickest way to ruin an alternator is to use it to recharge a dead battery, but what about the motorists who run their battery down and get a jump start, and then use the alternator to recharge the battery? Does a jump start actually put enough current into the run down battery so the alternator isn't damaged?

Willl · Jan 7, 2012

JDgreen227 said:
In the past I have read that the quickest way to ruin an alternator is to use it to recharge a dead battery

I have never heard of this. Links please.

irwin · Jan 7, 2012

Willl said:
I have never heard of this. Links please.

Hiya Willl.. It's something I've heard about for many years. The typical alternator is designed to keep a battery fully charged, not to charge a dead battery..this puts a load on the diodes and can overheat them (at least that's what I've been told by the electrical motor sales representative from NAPA..(not the counter person, I run a auto repair shop and we get visits from representatives of part supplier main office)

Many alternator boxes from parts store state "do not use an alternator to charge a dead battery, use a battery charger"

This doesn't mean you can't get away with doing it, people do it all the time, but it's not recommended by the manufacturers.

I don't have time to do any real research.. but there is anecdotal info that's easy to find.

Will I damage my alternator by using it to charge a flat battery?

Fixemall · Jan 7, 2012

I agree with Tim, today's electronics suck out a lot of juice and usually the manufacturers put in just big enough alternators to barely cover, the exceptions being Police pkg and heavy service systems in trucks. If I were in your situation I would charge the battery then take it in to be tested, if it fails I'd be spending the money for a new one, no sense in making a small problem bigger.

JDgreen227 · Jan 7, 2012

irwin said:
Hiya Willl.. It's something I've heard about for many years. The typical alternator is designed to keep a battery fully charged, not to charge a dead battery..this puts a load on the diodes and can overheat them (at least that's what I've been told by the electrical motor sales representative from NAPA..(not the counter person, I run a auto repair shop and we get visits from representatives of part supplier main office)

Many alternator boxes from parts store state "do not use an alternator to charge a dead battery, use a battery charger"

This doesn't mean you can't get away with doing it, people do it all the time, but it's not recommended by the manufacturers.

I don't have time to do any real research.. but there is anecdotal info that's easy to find.

Will I damage my alternator by using it to charge a flat battery?

Thanks, I was searching for a good link and saw that one you supplied. I am rather skeptical of what searches turn up, just wondering if anybody here on TBN had real life experiences with the situation. The part that most interests me about the issue is just how much charge a jump start puts back into a low battery.

Cat_Driver · Jan 7, 2012

I even saw a set up where a guy used an electric motor connected with a fan belt to an alternator all bolted to a plank of wood as a battery charger.

JDgreen227 · Jan 7, 2012

Cat_Driver said:
I even saw a set up where a guy used an electric motor connected with a fan belt to an alternator all bolted to a plank of wood as a battery charger.

My biggest charger puts out a maximum 10 amps, most alternators put out about 60-70 amps, would be interesting to compare the two methods side by side and find out the difference in charging times...!!! Also have to wonder just how fast the electric motor on that jury rigged setup is turning the alternator as compared to the OEM belt driven speed.

Soundguy · Jan 7, 2012

IMHO.. jumpstarting a car with a truly dead batter is a bad idea.. 'BOOSTING' a car with a battery just too weak to start is quite a bit different.

remember.. it's not just the charging that the alt has to do.. but all the onboard power hungry electrics. lamps, etc.. etc...

soundguy

boomer1025 · Jan 7, 2012

I've seen it, what happens is the alternator burns up trying to charge a dead battery. Alternators are not made to operate for long periods at max output like charging a dead or very low battery. Another thing, an alternator won't fully charge a very low battery, it will being up the voltage to a point and the battery will work, but won't be fully charged. So if you have to jump start a battery to get a vehicle running fine, after you get home or done using the vehicle, put a charge on the battery and top it off or there is a good chance the voltage won't stay up.

MF 1532 · Jan 7, 2012

I have always been told , an alternator will never fully charge a battery . The last place I heard it was from my Interstate Battery rep . and in their magazine .

Soundguy · Jan 7, 2012

it's made to carry the electric load of the auto when driving and replenish the storage battery, which is used to start the vehicle, and supply power when not running..e tc.

it is good at it's job.

no need to abuse it when bat chargers are so cheap....

BobRip · Jan 7, 2012

I am not sure that the alternator will be damaged, but it is a lot cheaper on fuel cost to use a charger run from the power mains than to idle an engine or drive just for the purpose of charging.

Cat_Driver · Jan 7, 2012

Here's a picture of some Jerry rigs.

Bob's Homeade Battery Charger

Soundguy · Jan 8, 2012

at a tractor show a guy has a push lawnmower with the blade removed and am alt mounted face down into the deck with a belt/pulley on the mower shaft. it terminates in jump cables.. makes a push jumpstart/charger..

JDgreen227 · Jan 8, 2012

I remembered earlier today about the alternator issue I had on my Olds 98 that had a digital dash, I had replaced the OEM alternator at about 100,000 miles, the rebuilt replacement had a lifetime warranty and after about 3 years, I was on the way to work and the digital gauge readout began flashing "Battery 14.9 Volts Overcharging" and the normal readout was supposed to be "Battery 13.7 Volts". Well when I got home I removed the alternator and returned it to the seller and asked for a replacement they gave me a hard time about that saying their tester showed no overcharging but the replacement I got went another four years and never read over the normal 13.7 volts. I always wondered just how many batteries get ruined when alternators overcharge batteries and the cars have only a useless idiot light that furnishes no real useful information.

JDgreen227 · Jan 8, 2012

Cat_Driver said:
Here's a picture of some Jerry rigs.

Bob's Homeade Battery Charger

Thanks for the link, very neat setup !! You could probably make something like it with a vertical shaft engine if you wanted to.

ctgoldwing · Jan 8, 2012

irwin said:
Hiya Willl.. It's something I've heard about for many years. The typical alternator is designed to keep a battery fully charged, not to charge a dead battery..this puts a load on the diodes and can overheat them (at least that's what I've been told by the electrical motor sales representative from NAPA..(not the counter person, I run a auto repair shop and we get visits from representatives of part supplier main office)

Many alternator boxes from parts store state "do not use an alternator to charge a dead battery, use a battery charger"

This doesn't mean you can't get away with doing it, people do it all the time, but it's not recommended by the manufacturers.

I don't have time to do any real research.. but there is anecdotal info that's easy to find.

Will I damage my alternator by using it to charge a flat battery?

Hey if on the internet it must be true, right?

BertZegers · Jan 8, 2012

The best way to charge a battery is with the alternator in your car, it stops charging when the battery is full. The alternator in my Jeep interacts with battery temperature! Cheap chargers ruin your battery by overcooking. I think using the jump start function from a charger is realy bad for your battery.

If you need to jumpstart a car, let the helping car run for a while so that the dead battery gets charged thru the full battery and alternator of the helping car.

Then start and charge the battery in your vehicle. The alternator is your best charger. Your battery life is longest if you use your vehicle frequently.

This is my experience.

MHarryE · Jan 8, 2012

OMG I'm glad I learned about this now. I guess I have been super lucky because in the past 43 years since I got my first car I must have completely drained at least 43 batteries (avg 1 screw up per year), jumped them, and let the car/truck/tractor run to charge the battery and have never replaced an alternator. It's a miracle! Of course I have had lower life on a couple batteries that got abused too much.