Powering car from a 13.8V power supply ok?

**PURDooM** · 05-24-2006, 12:19 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, 12:47 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, 01:01 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, 01:05 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, 01:05 PM

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, 01:54 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, 10:35 PM

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, 05:00 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, 03:47 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, 07:50 AM

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.

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.

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.