Thread: Draining Battery

My multimeter is autosensing - and I even set it manually to DC, but still reading zero - which I know isn't right...

I'm assuming there are 3 connectors on the multimeter... 1 com (common ground) and two possible places for the positive wire. Try the positive wire in the other spot.

Yep, you're right. One is Unfused 10amp, the other a fused 0.5amp. Neither work.

Perhaps my multimeter is busted, as far as the ammeter side goes? I'll try to find someone else around that has one I can borrow.

I ordered a CNX-P5V and plan on covering the +5V pin on the USB cable so USB devices only draw from the P1900.

With this set up though, once the computer goes to sleep - won't the loss of power to the P5V (and the USB hub/devices) cause the Mini to wake up again?

Originally Posted by PaulF

To test current draw, you have to hookup the equipment through the multimeter. So hookup the equipment's positive wire to the positive on the power source and then hook up the equipment's negative wire to the positive on the multimeter and then the multimeter's negative back to the negative on the power source.

Check how many amps the multimeter is rated for. I have one from radio shack that is fused at .5 amps so you really can't do much with that you might be able to take out the fuse and put a piece of metal across the fuse contacts temporarily to measure this, but don't do this for more than a couple of minutes or else you risk fire.

Not sure if understand the logic behind the ammeter and its fuse.
You will need to connect the circuit and ammeter in series. If you want to do it from the car battery, disconnect one of the terminals, then use your ammeter like a piece of wire to connect the battery terminal and the lead.

Basically, the ammeter has negligible resistance, so it will not affect your circuit at all. If you have tried to measure the current by doing it in parallel, meaning like how you measure voltage, alot of current will flow through, as I = V / R, since V = 12V (car battery) and R is very very small, I is very very large, and it will most likely fry the fuse in the ammeter. Subsequent correct measurements will come out to be zero as the fuse is gone and there is infinite resistance, so no current will flow through.

As for the AC and DC, some multimeters will only have signs to say, a ~ sign means AC, DC is an unbroken line above a dotted line (you need the DC one).

How about disconnecting your entire setup for 1 day, and see if it is really the cause. A simple short or near short somewhere could be the problem too.

Originally Posted by fone

Not sure if understand the logic behind the ammeter and its fuse.

The logic is that on Ammeters that have fuses, they are A) meant for in-circuit work and B) their electronics and/or leads are not rated to handle a current higher than the fuse.

What you really need for this kind of work is one that clamps over the power line and isn't in-circuit. I have one from Craftsman that I just picked up to deal with my own power issues.

-dave

Just to make sure: not all clamp ammeters can handle DC! The cheaper ones only handle AC (a normal induction I guess), while the more expensive ones can handle both AC and DC (using a Hall-effect sensor).

They are also nowhere close to the regular ammeters when it comes to precision of measurement - most of the Hall-effect-based ammeters need to be calibrated more or less before each measurement to compensate for Earth's magnetic field.

If your battery lasts for a few hours while parking, use a clamp meter. If you are hunting down some tens or hunderds of mA, use a regular multimeter instead.

Originally Posted by TTeuer

I ordered a CNX-P5V and plan on covering the +5V pin on the USB cable so USB devices only draw from the P1900.

With this set up though, once the computer goes to sleep - won't the loss of power to the P5V (and the USB hub/devices) cause the Mini to wake up again?

It probably uses the same sequencing for the shutoff that it does with the +12V. The P1900 first shuts down the +12V (i.e. touchscreen with USB) and then the +18V for the computer, so that USB transients from the screen will not wake the computer. Check the manual, but I imagine it uses the same for +5V.

Originally Posted by Bugbyte

Jirka's method of covering up the +5 volt pin is the slickest method I've seen. You just cut a very small sliver of tape and place it over the +5v contact on the connector. Then, plug it in and it won't draw power from the Mini.

I've been using a special USB hub that mp3car is developing for solving this exact problem. Not available yet, but I've been testing it. Until then, I used an unpowered hub.

Again, you can use Jirka's tape trick, or open up and actually cut the 5V line on a spare USB cable. If you want to preserve the possibility to draw from the computer, put a switch on that line and enclose it in one of those radio shack project boxes.

What model of USB hub are you using to accomplish this? I tried the 'covering the 5v lead' method but it would cause my hub not to be recognized after standby, and cause the computer to crash if then pulled to reset. so I had to revert back.

Any idea when this hub will be available? I have bought a few relayed and I am debating making my own usb switch to cut the power off when in standby, but would rather have it all into one built in unit.

Thanks