Connecting AC Power Source to Car Battery

**SolusCado** · 01-08-2007, 09:46 AM

Hello everyone. I have a CarPC implementation using the CarNetix P2140 power supply to great effect. However, there are time when I want to leave the PC running overnight to synchronize files over the wireless network, etc. and of course the PC draws far too much power to run off the battery all night long.

I also have 15 amp, 13.8V AC/DC 'converter/power supply'. It is a device I picked up at Radio Shack that plugs into an AC outlet, and provides the above spec'd DC power via two terminals (Red/Black, +/-). I have managed to mount this device next to my car battery and routed the AC cord through the grill so that I can just pull out the cable and plug it in to my garage without even opening my hood.

I have successfully used this device in the past to power my CarPC by removing the terminals from the car battery and instead wiring them to this device. The 15 amps isn't enough to power headlights as well, so I have to be sure to turn them (and all other ancillary electrical devices in the car) off, but it is sufficient power for the CarPC itself.

I have however been hesitant to connect the terminals to between the Power Supply and the Car Battery because I don't know what would happen... Would the AC device actually charge the Battery? Would the battery's electricity somehow short out the AC device? If the AC device DID charge the Battery, what would happen when the Battery reached full capacity? Does anyone either know or at least have some educated input?

**evandude** · 01-08-2007, 11:04 AM

If you're worried about the car battery dying while your carPC is running from it, it seems to me it might make more sense to just buy a car battery charger. For not too much money you should be able to find one that is smart enough to switch between full charge and trickle mode so it automatically charges the battery properly all the time.

Car batteries are pretty rugged, and generally should charge and be just fine with a 13.8V supply hooked up. However, without actually trying it, it's difficult to say if your supply would be OK with it - if the battery was discharged a bit, it could try to draw a huge current from the power supply at first - the power supply voltage might just droop a bit under the load, or it might just shut off, or it might even damage it, depending on how the supply is designed.

**SolusCado** · 01-08-2007, 02:16 PM

I do actually have a car battery charger that is a pretty slick device, but its trickle charge is not enough to power my PC all night long, and in any case my DC-DC power supply/inverter/regulator (whatever the hell it is) monitors my battery power and shuts down the PC when it gets too low. So draining my battery isn't really a concern - it's not going to happen.

However, I DO want to be able to keep that PC on for as long as I want - by plugging in the vehicle. And since the power supply wasn't designed to be hooked up to a battery - it's just supposed to be a source of DC power for whatever bench work you may be doing - I'm not sure how it would interact with the battery in terms of drawing current. What stops the battery from just drawing as much power at the device outputs? Also, is there any likelihood that the battery itself would actually output power to the device?

**SolusCado** · 01-09-2007, 08:08 AM

Well, I went ahead and connected the terminals to try it out - and nothing caught on fire, so that's good.

However, the PC left running still drained the battery. As noted previously, I have in the past successfully run the PC off JUST the power output from the Power Supply (though it was a different DC-DC regulator powering the PC then, one that produced fewer watts). I think I just need a more powerful power supply, and Radio Shack has a 25-amp version, so I think I will give it a shot next. Anyone want to buy a 15 amp 12V DC power supply?

**Ralphh85** · 01-09-2007, 11:55 AM

a car battery charger would stop it from draining the battery.

it would realise the voltage was dropping and would up the power it was putting in, should keep the batter on 13.5 volts ish.

Ralph

**Suzari** · 08-04-2008, 11:49 PM

Hey, my name's Aaron. The main thing I want to know is how you can create a portable supply (via outlet) through source such as a car battery. I mainly want to know this because I am in the process of a 360 laptop, and the only thing about it is...you still have to plug it in to a outlet for it to run.

Answers please!

**Bugbyte** · 08-05-2008, 03:37 PM

Threadjacking

**nobb** · 08-05-2008, 04:42 PM

I always wanted to build an AC power source that can be connected directly to the car too. A car battery charger isnt going to keep the battery charged while its being drained because the carpc will probably draw more current than most battery chargers can provide.

Strange that your battery still drains with the voltage supply connected. Did you try measuring across the terminals what the voltage is when it is connected and the carpc is running? Maybe there is too much voltage drop when its being loaded. 13.8v is just enough to trickle charge the battery. Most alternators should put out between 14.0 - 14.5v, so maybe a high current power supply at around that voltage would work better?

**rickd** · 08-06-2008, 01:23 AM

Originally Posted by SolusCado

I also have 15 amp, 13.8V AC/DC 'converter/power supply'. It is a device I picked up at Radio Shack that plugs into an AC outlet, and provides the above spec'd DC power via two terminals (Red/Black, +/-).

The 15-amp converter should not have had any trouble keeping the battery charged, assuming it was reasonably charged when you started. It might not charge a battery very quickly, but there are lot of 6-amp and 10-amp battery chargers out there that do just fine.

13.8 volts isn't really high enough to charge a battery quickly, but then it doesn't provide enough current, either.

15 amps at 13.8 volts is 207 watts, and that's more power than is used even by regular desktop computers.

I can't explain why the battery would go dead.

Here's food for thought: My motorhome has a 40-amp converter. It charges a Group 27 deep-cycle battery, plus the engine starting battery, and also keeps all running 12-volt devices alive. The only time a battery has ever gone dead under its care is when a cell went dry...

...okay, here's a theory...

...Check the water levels in your battery. At 13.8 volts, a fully charged battery might get a bit toasty if the converter has no way to back down the voltage. You might have boiled a cell dry, and that would definitely result in demise. I would think it would have to have been low, because 13.8 volts isn't really high enough to cause real boilage. You might also check the converter's output to make sure it isn't running too high--anything above 14 volts will boil a fully charged flooded-cell battery.

If the water level is very low, refill it with distilled water and try to charge it.

If you are going to invest in a new AC power supply, consider getting a multi-stage converter/charger intended for use in RV's. It will take much better care of your battery. Something like this:

http://cgi.ebay.com/ebaymotors/WFCO-...QQcmdZViewItem

RickD, with no experience with the WFCO brand