Powering car from a 13.8V power supply ok?

PURDooM · 05-24-2006, 01:19 PM

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.

Zebelkhan · 05-24-2006, 01:47 PM

M2 will take from 6v to 24v so that is not a problem. However it would not be advisable to do that unless you find a way to cutoff the car battery after it has reached full charge. Overcharging your car battery will damage it. It is better if you install a simple switch (or relay) so when the power supply is plugged in, M2 will feed off of the supply and revert back to the car battery when it is unplugged.

BassBinDevil · 05-24-2006, 02:01 PM

13.8 volts shouldn't overcharge your car battery. The great thing about lead-acid batteries is that when you apply a charging voltage of 13.8 to 14.4 volts, the battery only accepts a charge until it is full, then it just "floats" and draws next to no current.

Jeeve5 · 05-24-2006, 02:05 PM

I second that. Just think about what your alternator would do to your car's battery on a long drive

2k1Toaster · 05-24-2006, 02:05 PM

Quote: Originally Posted by BassBinDevil

13.8 volts shouldn't overcharge your car battery. The great thing about lead-acid batteries is that when you apply a charging voltage of 13.8 to 14.4 volts, the battery only accepts a charge until it is full, then it just "floats" and draws next to no current.

Then why the hell would anyone buy a trickle charger with a "smart" circuit to stop sending current? Is this really true?

Zebelkhan · 05-24-2006, 02:54 PM

On a long drive alternator does not damage the battery because it will cut power to the battery once it senses that its draw has been reduced. As far as the battery is concerned, because of the open cells in most lead-acid batteries, overcharging will tend to generate excessive oxygen and hydrogen gas reucing level of electrolyte running the risk of exposing the electrodes and causing degradation, which will permanently damage the battery.

Here is also an excerpt from the following site:

http://www.bigginhill.co.uk/batteries.htm

"A reasonable rule of thumb is that you should aim to charge the batteries only when they are between 70% and 40% discharged. If you charge them then they are only lightly discharged i.e. less than 40% you will end up boiling them unnecessarily which wastes energy in the form of heat and gassed off hydrogen and in turn shortens the life of the batteries. In effect the batteries are being overcharged which can cause degradation and buckling of the plates. In the process some active material is forced off the plates and drops down to the bottom of the battery. If this occurs frequently the eventual result is a build up of a bridge between the plates which in turn can cause a possible short across the plates. This situation leads to the destruction of a cell which then reduces the capacity of the battery."

Frederick · 05-24-2006, 11:35 PM

Quote: Originally Posted by Zebelkhan

M2 will take from 6v to 24v so that is not a problem. However it would not be advisable to do that unless you find a way to cutoff the car battery after it has reached full charge. Overcharging your car battery will damage it. It is better if you install a simple switch (or relay) so when the power supply is plugged in, M2 will feed off of the supply and revert back to the car battery when it is unplugged.

As I know, the M2 ATX have time off, it means you can turn on after start engine.

Frederick

BassBinDevil · 05-25-2006, 06:00 PM

I could be wrong, but the "smart" circuit is probably just a voltage regulator. Smarter chargers have an "equalize" mode where they deliberately overcharge the battery for a limited time to make sure all cells are equally charged, but after that they revert to "float" mode, where the battery sits around 13.8 volts and doesn't accept any current. Dumber chargers are unregulated and will keep on feeding current into the battery until it's warm and bubbling or you remember to disconnect it.

If the supply is regulated to 13.8 volts, it will not overcharge the battery. That's how your car's alternator does it. It can't tell whether the battery is drawing current (to charge itself) or that current is going to the headlights or electric seats.

S11D336B · 06-08-2006, 04:47 AM

Just put a rectifier diode on the power lead running to your battery from the M2 so that the house power supply can't charge your battery.

TruckinMP3 · 06-08-2006, 08:50 AM

Quote: Originally Posted by BassBinDevil

13.8 volts shouldn't overcharge your car battery. The great thing about lead-acid batteries is that when you apply a charging voltage of 13.8 to 14.4 volts, the battery only accepts a charge until it is full, then it just "floats" and draws next to no current.

BS... that is why float and trickle chargers are made.

Quote: Originally Posted by Jeeve5

I second that. Just think about what your alternator would do to your car's battery on a long drive

BS... Car charging systems use a voltage regulator to prevent damage to the battery.

Quote: Originally Posted by BassBinDevil

I could be wrong, but the "smart" circuit is probably just a voltage regulator. Smarter chargers have an "equalize" mode where they deliberately overcharge the battery for a limited time to make sure all cells are equally charged, but after that they revert to "float" mode, where the battery sits around 13.8 volts and doesn't accept any current. Dumber chargers are unregulated and will keep on feeding current into the battery until it's warm and bubbling or you remember to disconnect it.

If the supply is regulated to 13.8 volts, it will not overcharge the battery. That's how your car's alternator does it. It can't tell whether the battery is drawing current (to charge itself) or that current is going to the headlights or electric seats.

Again, BS... See above for car system.

zootjeff · 06-08-2006, 11:05 AM

Quote: Originally Posted by TruckinMP3

BS... that is why float and trickle chargers are made.

BS... Car charging systems use a voltage regulator to prevent damage to the battery.

Again, BS... See above for car system.

Truckin, You're flinging a lot of BS everwhere and it's piling up.

The other guys right, the Voltage Regulator in a car maintains a Constant Voltage of around 13.8 volts.

The only reason they have trickel chargers is to extend battery life as charging with large currents, while charging batteries fast, limits their life.

There are a lot of cars out there that don't try to limit the current on their charging systems, and since most car batteries are classed as starting batteries, they are not designed to get very depleted. This is not a problem for day to day life. Now if your using a deep cycle battery, that's another story. Most of the time when you bring out a car charger, you killed your battery and you want to charge it in a way that will preserve it's useful life. THATS why they have float and trickel chargers.

Jeff

Jahntassa · 06-08-2006, 11:23 AM

Moral of the story is you can probably power your CarPC off it, but put some sort of Diode or Relay between it and the car so you aren't trying to power the whole car or charge the battery while the power supply is on.

TruckinMP3 · 06-08-2006, 01:29 PM

http://www.carcare.org/Electrical/vo...egulator.shtml

"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.

It is not that "the battery only accepts the charge untill full" The charging system, in the car or an external charger, needs to protect the battery from overcharging that can damage the battery. Cheap charging systems need to be monitored, more complex chargers use circuts to protect from this over charge possibility. The Auto's voltage regulator performs this function as well as maintaining the correct voltage to run the accessories up to the limit of the alternator's output.

zootjeff · 06-08-2006, 03:33 PM

Quote: Originally Posted by TruckinMP3

http://www.carcare.org/Electrical/vo...egulator.shtml

"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.

It is not that "the battery only accepts the charge untill full" The charging system, in the car or an external charger, needs to protect the battery from overcharging that can damage the battery. Cheap charging systems need to be monitored, more complex chargers use circuts to protect from this over charge possibility. The Auto's voltage regulator performs this function as well as maintaining the correct voltage to run the accessories up to the limit of the alternator's output.

I still think your wrong here. The wikipedia says this about an alternator:

"Modern automotive alternators have a voltage regulator built into them. Typical car alternators generate the field using a DC current through slip rings. The field current is much smaller than the output current taken from the fixed stator windings, and so heavy duty slip rings are not required. For example, in an alternator rated to produce 70 amperes of DC, the field current will be less than 2 amperes. The voltage regulator operates by modulating the small field current in order to produce a constant voltage at the stator output."

It provides a constant voltage to the battery. The battery charges till it reaches that voltage and then current stops flowing. This is for Lead Acid Batteries. Not for Ni-cad or nickel metal hydride. Look at any gel cell 12 volt battery, it has a range for constant voltage charging. This is the same for wetcell batteries too.

The regulator reacts to changes in loads by providing more current, but the result is a constant voltage.

-Jeff

TruckinMP3 · 06-08-2006, 04:22 PM

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.

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