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car battery, alternator, charger question

mzeitlin3348

mzeitlin3348

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When a car is started, immediately after the car comes to life, the alternator begins to re-charge the battery. In my car, I notice that the battery will have a voltage of 14.5 or so volts across the terminals while idling, but I am curious what the current draw of the battery or what the alternator is supplying to bring it to full charge again while the car is running.

I had a car battery drain over a long time and when I put my charger on it, I always set it on a 2 amp charge instead of 10 amp and leave it that way for a couple of days. It does shut off automatically - but I was told/read that you should always charge at the slow rate when you can to preserve the battery. I'm not sure why.

Isn't the alternator charging at a much higher rate after the car is started?

... just curious.
 
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Yes, but if things are in good order, the battery is nearly full at startup on a regularly driven electronically-regulated car. Therefore, the charge rate is very light. It's very bad for both the battery and alternator if you have a very low battery, and jump-start the car for this reason. Some alternators put out 80 amps or so, but of course it will only deliver the current demanded by the battery. NOTE: In older cars with mechanical regulators, or worse yet in very early cars with magneto charging, overcharging the battery is a big problem. In fact, on some pre-WWII cars, the owners manual tells you to put the headlight on if on a long, daytime highway trip to avoid overcharging.

I put a trickle automatic charger on all of my collector cars, so as to keep the batterys topped off all the time. Since I started doing that a few years ago, I no longer had to replace batteries every 4th or 5th time I'd get one of the cars out. But there is a danger of fire, as evidenced by one of our own AK members who lost a huge building full of antique radios & TV sets due to a malfunctioning battery charger. Lay the charger on a cement floor, away from anthing that might catch fire if the charger has a meltdown, or if a cat pees into it (cause of that fire, I was told).

Charles
 
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The 14.5 volts only tells you the voltage the alternator is putting out, not the amperage. An amp meter at the alternator output and one at the return to the battery is the only way to really see what the system is doing.

When you first start your car, the battery takes a fairly big hit, but isn't dead. The regulator senses that and allows the alternator to put out enough power to both run the car and top off the battery. Once it sensed the battery is charged, it cuts the amps back to just run the vehicle, but at the same voltage. The vehicle should be running off the alternator, not the battery.

If you had the dual amp gauges, you could see both what the alternator is putting out in amps, and what is actually going into the battery.

Putting a high charge on a totally dead battery is a different thing. If there is sediment that settles to the bottom of the plates and you put a humongous charge to it, they can short/spark and go boom. A slow charge will allow the electrolyte to warm and bubble, sometimes breaking the crud up.

There are more technical reasons, too, but these are pretty basic reasons.
 
Thanks for the info! I definitely am going to have a shortened battery life on one of the car batteries.

When I checked it it was 1.7 volts (ouch).
The charger wouldn't even recognize it (i.e. did nothing for an hour).

But I put the charger into car start mode (55 amp / start) which put 13 volts across the battery terminals for a few seconds - I tried to start the car, but it still would not start, although it did click-click-click.

After doing this, I noticed the car battery without the charger was reading 12 volts again - but dropping. I put the charger back on at the 2 amp slow charge rate and the meter indicated it was drawing a charge this time.

I did this overnight and the next day, I was able to start the car on the battery alone.

Looks like I salvaged the battery for a bit longer - but I suspect it won't have the service life anymore.
 
mzeitlin3348 said:
Looks like I salvaged the battery for a bit longer - but I suspect it won't have the service life anymore.
Click to expand...

Maybe not. Sometimes if batteries are dead dead dead, they need a "jolt" to get them started, then a slow charge will bring them back properly.
 
The alternator isn't all that smart. Basically, tjhe regulator senses voltage and adjusts the alternator output to what ever amps necessary, within its capability, to try to maintain "x" voltage, typcally around 14V, give or take.

With all the electrical stuff on today's cars many alts are >100A.
 
bentpencil said:
Maybe not. Sometimes if batteries are dead dead dead, they need a "jolt" to get them started, then a slow charge will bring them back properly.
Click to expand...

The problem is that "automatic" battery chargers typically have to see some type of threshold voltage at the battery before they'll begin to try to charge.

That's where the "jolt" comes into play. By using a manual charge setting for a while, the battery's voltage comes up to the point where the automatic cycle will work properly.
 
mzeitlin3348 said:
Looks like I salvaged the battery for a bit longer - but I suspect it won't have the service life anymore.
Click to expand...

Yes, but I would not worry, it might go on for some time. Worse of course if car is an automatic and u can't bump start.

Quite a good basic test of battery (fully charged) put on headlights and measure across battery when starting engine. Less than 10.5V when cranking engine is a bit low.
 
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mzeitlin3348 said:
Thanks for the info! I definitely am going to have a shortened battery life on one of the car batteries.

When I checked it it was 1.7 volts (ouch).
The charger wouldn't even recognize it (i.e. did nothing for an hour).

But I put the charger into car start mode (55 amp / start) which put 13 volts across the battery terminals for a few seconds - I tried to start the car, but it still would not start, although it did click-click-click.

After doing this, I noticed the car battery without the charger was reading 12 volts again - but dropping. I put the charger back on at the 2 amp slow charge rate and the meter indicated it was drawing a charge this time.

I did this overnight and the next day, I was able to start the car on the battery alone.

Looks like I salvaged the battery for a bit longer - but I suspect it won't have the service life anymore.
Click to expand...

The life of a battery is 7 years at BEST. From a good friend of mine and a very good mechanic to boot, 4 or 5 years and get yourself a new one or you run the risk of getting yourself a nice new alternator.
Cheers
 
whoaru99 said:
The problem is that "automatic" battery chargers typically have to see some type of threshold voltage at the battery before they'll begin to try to charge.

That's where the "jolt" comes into play. By using a manual charge setting for a while, the battery's voltage comes up to the point where the automatic cycle will work properly.
Click to expand...

I picked up a fancy auto/reforming type charger a while back. If the battery is dead, it won't do a thing. It doesn't have a manual charge setting, so I was using my old charger to get a charge on it and then switching to the new charger. Eventually I wised up and just used the old charger. Not sure what the point of a charger is that needs a half charged battery to work.
 
moinau said:
The life of a battery is 7 years at BEST. From a good friend of mine and a very good mechanic to boot, 4 or 5 years and get yourself a new one or you run the risk of getting yourself a nice new alternator.
Cheers
Click to expand...

Then I guess our two Chevy Impalas did all right, I got 8 years on mine, my wife's got 8-1/2.
 
VinylHanger said:
I picked up a fancy auto/reforming type charger a while back. If the battery is dead, it won't do a thing. It doesn't have a manual charge setting, so I was using my old charger to get a charge on it and then switching to the new charger. Eventually I wised up and just used the old charger. Not sure what the point of a charger is that needs a half charged battery to work.
Click to expand...

The point of some, at least the one like in my boat, is to keep the batteries constantly charged with minimal supervision, and to restore a partially discharged battery to full capacity in relatively short order. I don't believe they're intended to revive a stone dead battery.
 
moinau said:
What is the function of an alternator?
Click to expand...

An alternator (Automobile) is a generator, belt driven from the engine - it's function is to recharge the battery after starting the engine and keep it topped up with charge, and to run all the electrical devices in the car while the engine is running. If all is correct with the electrical system of the car, no power is used from the battery while the engine is running.
 
I've moved this thread over to the Wheels, Wings, Mud, and Water forum. It's a lot more likely that the car folks will see it here, and the OP might get even more help in this forum than General Off Topic.
 
moinau said:
What is the function of an alternator?
Click to expand...

"To maintain the battery's state of charge while the engine is running"... or to keep the battery charged when heavy electrical loads are present while the engine is running..lights, rear window defog, a/c blower, power windows, etc. It is not a battery charger.

007
 
merrylander said:
Then I guess our two Chevy Impalas did all right, I got 8 years on mine, my wife's got 8-1/2.
Click to expand...

At one time I was keeping batteries for as long as possible, 10 plus years until my friend explained the make up of a battery and the function of an alternator. A battery that can not keep its charge any longer will strain the alternator also, a battery that is older than 6 to 7 years will also strain the alternator. A battery only cost $35.00 to about $75.00 in most cases. So why bother buying a charger and all kinds of gimmick that sometime cost more then a battery and on top of it strain the alternator. For my part, every five years the battery goes costing me a grand total of $65.00 or so and the assurance that I wont be caught short at the Mall or early one cold morning.:D:D
 
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