DC - DC ATX current drain?

**l9nux** · 08-27-2004, 02:52 AM

I've had my carputer installed for 2 weeks now, and when I woke up this morning I couldn't start my car because the battery was drained. Now I'm sure that the computer was turned off, but my USB GPS was still powered by the computer, and the Liliput was completely powered off although it was connected to the USB port too.

My question is, although the computer is switched off the power supply is still powering my GPS receiver and there is still power to the motherboard so how much current is this draining?

My DC - DC power supply is a 250w ATX, and has three connections; 12v +, Ground, and ground on/off. I've already got a start button to turn my computer on, so the power supply is on all the time but I'm now thinking it would be better to have power to the power supply only when my ignition is on?

Anyone have any similar problems or thoughts?

**cheerio** · 08-27-2004, 03:38 AM

example from another fourm testing a amplifier

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This is the only true way to test. Remember that power equals volts by amps. so you will need a non-inductive test load (like a high wattage power resistor) and a true RMS digital VOM. A true RMS amp meter. A constant test signal will be required like a 1khz tone at 0db on a cd. (If you dont have a audio test cd i have a ISO i can link you to if needed). Then all you need to do is hook up the resistor to the amp output like a normal speaker but with the amp meter in seres with it. then play the tone and turn the amp up, read the voltage across the load resistor and keep turning it up untill the voltage reaches the max. read the max voltage and the current write it down. turn off equipment. multiply voltage by current to get the power output rating.

as for your computer, you could set it up to not recieve any power at all once the igniton is killed(but you would have to make sure you power it down before u turn off your car) by using a relay.

**l9nux** · 08-27-2004, 06:42 AM

The funny thing is I've had this powered this way for two weeks and only this morning I noticed my battery was flat.

What I really need to know is would a 250w DC-DC power supply drain the battery when it's got power (but the PC is off).

**Laidback** · 08-27-2004, 06:49 AM

Originally Posted by l9nux

The funny thing is I've had this powered this way for two weeks and only this morning I noticed my battery was flat.

You mean you didn't notice the other mornings??

**l9nux** · 08-27-2004, 06:53 AM

No every morning the battery was fine and the car started with no problem. Also, every lunch I watch a movie with the engine off and it's still been fine! This is why I'm so confused to why it drained last night. Unless I have another problem, but it is a brand new battery.

**BeamRider** · 08-27-2004, 06:55 AM

Well a digital multimeter is good enough to test for current drain.... put it in current mode and check the current drained before DC-DC (try also removing some usb components) then reconnect all as usual and check voltage.
Power is V*I as usual

The best way to avoid drained batteries if you plan to leave the car for a long time is to unplug the 'puter power line fuse. You'll loose the GPS fix but I don't think this will be a great problem after 4 weeks.

BTW nice car!

**Confused** · 08-27-2004, 06:59 AM

If you're watching a film every lunch time for two weeks with the engine off, and not charging it well enough to/from work/school (ie you're not driving long enough) then you've been gradually taking power from your battery, so when you went to start it this morning, it had probably gone below that threshold, so it wouldn't start.

Garry

**l9nux** · 08-27-2004, 08:04 AM

When I drive home everyday it's 35 miles of charging so I'm confident that the alternator (70amp) is charging the battery but it's a good thought.

I think this weekend, I'm going to install a relay to turn the power supply on with the car ignition. I didn't want this because I often start the car after the PC is switched on, but I'll have to remember not to do that!