# Thread: Powering car from a 13.8V power supply ok?

1. Jeff,

I read that from the original site. It does not say the battery stops accepting a charge when it is full. (bottle theory)

I do understand chargers/charging systems on cars, not a formal education granted. But I do not see that part of the article supporting either side of the discussion. It covers rate of charge not the control.

The discription of the article does mention "car and motive deep cycle batteries" and does not deliniate what parts of the text apply to which battery types. I do understand and never disputed the various types of batteries.

BassBinDevil stated "the battery only accepts a charge until it is full, then it just "floats" and draws next to no current." that is a function of the regulator, whether the car version of a charging system does that or not... it is not controled by the battery. Quoted from the article "Three stage "smart" chargers usually have the bulk, absorption and float stages."

Cheers
David

2. Originally Posted by TruckinMP3
Jeff,

I read that from the original site. It does not say the battery stops accepting a charge when it is full. (bottle theory)
From above:

"The ABSORPTION stage is where the charger voltage, depending on the battery type, is constant between 14.1 VDC and 14.8 VDC at 80° F (26.7° C) and the current decreases until the battery is fully charged, which is typically the last 20% of the recharge."

I guess you just have to trust me on the point that with a constant voltage, the battery will draw less and less current until it's internal voltage matches the constant voltage and no current is flowing. They don't mean that the 14.1 and 14.8 is varying, you choose a voltage between that and keep it constant.

Batteries have what is called an "internal resistance." If you model a battery as a true voltage source and a resistor, you can model how much current is going to be flowing. If the batteries 80% full and it's internal voltage is 13 volts, and you hook up 14 volts to it, it is going to draw an ammount of current based on it's internal resistance. If that resistance is 0.1 ohms, then 10 amps are going to be flowing in to charge the battery. V/R=I. (14v-13v)/0.1 = 10A Once the battery is fully charged it's internal voltage will be 14 volts and the internal resistance will still be 0.1 ohms and V/R=I tells us that (14v-14v)/0.1 = 0 amps. Thus the battery stopped drawing current. You can force more current into the battery by raising the voltage, but that would be bad for a lead-acid battery, hence the allowable range ~14.1 to 14.8

Ni-cad chargers work by putting in a constant current until they measure a certian voltage or temperature, or putting in a trickel charge current that you can leave on forever. If you keep Lead Acid battery on trickel charge you will start to kill it if you don't limit the voltage.

-Jeff

3. @ zootjeff

If the batterys in control of the charging when the regulator in an alternator goes down it will put out around 18v and the battery will boil dry so if the batterys in charge of the charging how does this happen?

4. Originally Posted by Paul C
@ zootjeff

If the batterys in control of the charging when the regulator in an alternator goes down it will put out around 18v and the battery will boil dry so if the batterys in charge of the charging how does this happen?
I never said the battery is "in charge" of charging, I said: "If you put a constant voltage on a lead acid battery, then it will charge untill full and the battery will stop drawing current."

Now that only holds true if you're putting the "correct constant voltage" on the battery. Putting 18 volts on a 12 volt lead acid battery is going to destroy it.

Take a Battery, Put 13.8 volts on it, leave it for a day or so. You have a charged battery.

5. Originally Posted by zootjeff
But that's wrong. The Battery is in control of the flow of current

Deep cycle battery chargers are smarter and limit the current which inturn limits the voltage, but this is Not what they have in cars.

-Jeff

in charge of charging or in charge of flow of current tell me the difference please

If deep cycle batterys have smart chargers why are they fitted to cars as auxillary batterys and charged from the alternator?

6. Originally Posted by Paul C
in charge of charging or in charge of flow of current tell me the difference please

If deep cycle batterys have smart chargers why are they fitted to cars as auxillary batterys and charged from the alternator?

Hmm, lets see. If you have an ideal voltage source, it can put out a fixed voltage with infinite current. If you have an ideal current source, the current is held constant, but the way it keeps the current constant is by moving around the voltage until the right ammount of current is drawn.

A smart charger is a current source. An alternator coupled with a car charging system is a voltage source.

When you hook up a dead battery to an ideal voltage source, 1000s of amps will be drawn initially. This is very bad for the completly dead battery. Basically what all the stuff in that charging article said was the bulk stage is when the battery is really low, IDEALLY you want to start out with a current source. Since a car doesn't have a "current source" type of output, It just applies a constant voltage. This is good for starter batterys (that are not dead) but bad for deepcycle batteries (that are dead). Most people who parallel them up use a battery isolator, and this acts as a poor mans current limiter. The only thing you want to avoid is going over the manufacturers maximum initial current.

Man you guys just don't want to trust me on this. Here, I will show you a typciall Lead Acid battery fricken Lable.

Ok,

See that first line there: "Constant Voltage Charge" ? Thats what I've been trying to say. You charge a Lead-Acid battery with a constant votlage.
See the second line: "Cycle Use" That is the voltage for the Absorbsion phase of charging. 14.5 14.9V Basically you can charge at this voltage till the battery is full, but don't keep the battery at this voltage forever or you'll boil it over.

See where it says "initial current below 2.6 amps"? That is where you don't want to blow up the battery during the Bulk stage. (This is the stage Car battery charging systems skip. They are bad on batteries that are fully discharged.) This stage is basically a constant current source, and the stage Smart Chargers use.

Standby Use: This is what they were calling the "float stage". You can keep it at this "constant voltage" forever.

So, moral of the story, Car alternator based chargers don't charge batteries at 14.9 volts, they charge them around 13.8 to 14 or so. This keeps them charged and is a tradeoff between float and the absorbsion phase.

What's interesting is when your car is running at 500 rpms, the alternator is not capable of putting out a ton of current. Basically this will have the effect of limiting the available current delivered by the alternator. (And this is the closest you can get to having a non damaging "Bulk" stage for your battery) If you have a completly dead battery, you should not jump your car and then press on the gas. It will deliver more current to your battery, but you will violate that initial current rating in the bulk stage that your battery is currently going through. You should let the car idle for a while and then drive around..

I hope you guys will trust me on this, I design oscilliscopes for a living. I am an Electrical Engineer. I've studied this a lot..

Here is what an engine starts looks like if your interested, notice the 14 volts right away? This measurement is taken right at the battery with a standard Starter Battery available at GI Joes or Mo's Auto Parts.

-Jeff

7. Originally Posted by zootjeff
I never said the battery is "in charge" of charging, I said: "If you put a constant voltage on a lead acid battery, then it will charge untill full and the battery will stop drawing current."

Now that only holds true if you're putting the "correct constant voltage" on the battery. Putting 18 volts on a 12 volt lead acid battery is going to destroy it.

Take a Battery, Put 13.8 volts on it, leave it for a day or so. You have a charged battery.
I'm gonna agree with zootjeff. To truckin and some of you others, I am not sure if you are misinformed, or if you are simply unknowingly arguing semantics because of the way things are explained/worded on some of these informational sites. And, I think you're arguing about two completely different methods of battery charging, some without realizing it.

FIXED VOLTAGE CHARGING
The voltage regulator in a car does just that, regulates the voltage to that nominal 13.8v or so. Where I think some of the confusion is coming in is in statements like:
Originally Posted by TruckinMP3
"Description: The voltage regulator is an electronic device that regulates alternator output according to the battery’s state of charge and accessory loads. "

Not constant but Load dependant, part of the load is the battery's state of charge.
That description in quotes is rather misleading. I think you kind of have the right idea, but thinking that the regulator 'determines' the amount of current that it outputs is not the right way to look at it. It is a voltage source, its output voltage is fixed, even as the output current changes (within its operating current range). The output current is a function of the load; with an arbitrary and varying load, it's not possible for the voltage regulator to control both voltage and current simultaneously; V=IR... with V fixed, I depends on R. A car isn't a purely resistive load, but the relationship holds.

As zootjeff says, if you apply 13.8v to the battery, it will draw more currrent when it's dead, and less and less current as it charges, eventually being fully charged and drawing very little current - when the battery voltage is less than the voltage regulator output, then it draws current and gets charged. As it charges, the voltage rises, and that voltage difference gets smaller, until eventually the battery is at essentially the same voltage as the regulator output, and would draw very little current. The battery is a varying load; the state of charge of the battery is in control of how much current it is getting from the fixed voltage of the regulator.

FIXED CURRENT CHARGING
For the regulator to have control over the current going into the battery, it would have to vary its output voltage; that would make it a current source, a different way of charging the battery, which is the kind of thing you'd see in many 'smart' chargers.

Originally Posted by 2k1Toaster
Then why the hell would anyone buy a trickle charger with a "smart" circuit to stop sending current? Is this really true?
current sources are a great way to quickly and effectively charge a battery, because it makes the battery absorb energy at a relatively constant rate throughout the charge cycle, which can be used to 'gently' charge a battery for prolonged life, or to 'fast' charge a battery at a high current without damaging it... (as compared to voltage charging, where a completely dead battery could draw a LOT of current when first hooked up, possibly damaging it, but drawing very little toward the end of charging, making it take longer) a current source would be something it would NOT be okay to continuously charge the battery with at a rate above the trickle charge rate, which is why those types of chargers have to monitor battery voltage and change or shut off the current source at the right times during the charging cycle.

Originally Posted by TruckinMP3
And is controled by the volatage regulator... the battery has no control over the flow of energy in volts or amps.

The post I called BS on indicated the battery just stoped being charged.... the voltage regulator (whether internal or external to the alternator) is in control not the battery.

We may be saying the same thing... the battery does not decide to 'accept' or 'decline' the energy if it is provided.
If you are charging with a fixed current source, or even with a fixed voltage source that is too high for the battery (like 18v) then that last statement is true, and if you leave either of those connected indefinitely, it will destroy the battery. However, if you are charging with a fixed voltage source of the proper voltage for the battery, then yes, it does "decide to 'accept' or 'decline' the energy" as you say. It's important to recognize this distinction between constant voltage and constant current charging methods, which seems to be one of the biggest areas of confusion in this thread... and the car itself uses a constant voltage method.

8. Originally Posted by PURDooM
For a ham radio my grandpa gave me an old power supply he found at a flea market or something, it says it is an astron rs-20M (http://www.labx.com/v2/spiderdealer2...m?LVid=3058432) and puts out 13.8 volts DC at 500 watts, and has a 5 AMP fuse.

I (will) have a m2-atx power supply powering my compy in the car soon, and I was wondering if I tied this into the car if I could power the computer for extended periods off of house power rather than off of the car battery. Will it dammage anything? I would imagine it would just charge the car battery and power everything, but I am no expert at these kind of things.
Ignoring all the other stuff that is going on, you are going to be fine using this power supply to run your devices. I have the same one, and it will work fine.

HOWEVER....I wouldn't trust it for vehicular charging. Not because of all the science that is being thrown around. But because while using this, it has a tendency to kick off even while under amperage (while still providing 13+ volts) It could just be the one I have, but I have never been able to find out why it does this.

It is a nice supply, but every once in a while, CLICK, off it goes. I wouldn't want to find out that overnight, it died, and your car killed its battery.

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