How to meter power drain? (dead battery issue)

**XxAndyxX** · 05-19-2006, 10:40 PM

If I leave my car off for a few days, the battery dies. I have a lot of aftermarket equipment in my car and I guess I could just unplug each one and see if it stops the power drain, but I'd like to do it a better way. Is there a way I can measure a drain like with a multimeter or something? I wonder if I used a MM to test for amperage, with the power unpluged on the device, it would give me a negative value, thus telling me how much amperage the device is draining. Any other thoughts? I'm sure someone has done this before for standby battery drain. BTW, I use hibernation on my computer, so it's completely off.

**turbocad6** · 05-19-2006, 10:58 PM

you need a shunt..... sears or a snap-on dealer should be able to get you one.... it'll go between the battery terminal & the terminal connector.... then you hook the multimeter to it..... this will give you a precise reading... clamp on's are not acurate enough for mv...

**JimmyFitz** · 05-19-2006, 11:02 PM

Yes, use a multimeter to measure amps for suspect devices. You will have to put it in series with the power feed to each device to get meaningful readings. Anything drawing more than about 250mA (while supposedly off) could be your problem. Your concept of negative current is incorrect but it doesn't matter. Don't be surprised if your PC draws current while off. If you want more help with any of this, you'll have to give us a clue about your hardware. All we know now is that you have a CAR (Firebird), a COMPUTER, and some OTHER STUFF.

**XxAndyxX** · 05-19-2006, 11:32 PM

I'll try using the MM first because I actually have one of those handy.

Potential hardware culprits:
(2) eD NINe.1
eD NINe.2X

For the computer: OPUS 150w, and I don't remember what mother board I'm using in conjunction with my athlon mobile processor. I don't see any other component causing power drain issues on the computer...

**turbocad6** · 05-20-2006, 01:14 AM

some meters can read up to 10 amps through the meter... it's protected inside the meter with a fuse, but a shunt is much safer... if your careful though to not overload it you can put the meter in line with each accessory & read the drain of each...

the better way to find a draw is to put the meter on the whole car buy putting it inline to a battery terminal.... this shows the overall total... you can then one by one disconnect & reconnect until you find the one that makes the meter drop.... you need a shunt for this because the shunt can run 50 amps continuous..it can even handle cranking & starting intermitantly. you may have to open a door or turn on the ign.... if you do this through the meter it will pop... the tiny fuses are hard to find

**eCar™** · 05-20-2006, 03:15 AM

Turbocad is right. A shunt would be the best way to do it. With the shunt, you measure the volage drop at the shunt, and calculate the current. This means there is zero risk of overloading your meter.

But...

I have used my multi-meter many times to check current drain on batteries. I put it in series to the batt (disconnect the neg cable and complete the circuit with the meter).

My meter has a max of 10A, and is protected by a fuse. But I have never popped it. You need to take care to not turn on things that would exceed 10A though. Cranking the engine would do it, obviously. Higher powered radios can easily draw more than 10A. Most carputers draw more than 10A. Headlights also. But an interior light or two won't.

Even if I did pop the fuse, mine uses a cheap 8AG fuse (normal glass automotive fuse), so I'm not too worried about it.

**Jahntassa** · 05-20-2006, 03:48 AM

I've used my multimeter, on the 20A setting (NOT the 250ma)

Typically what i'll do is start working back from the battery. Go in series with the main positive connector (with the ignition OFF). See how much current is moving with everything turned 'off' as it would be if the car was just sitting overnight. Note the value, as said, if it's less than about .2 amps, you should be fine and probably need a new battery.

What i'll then do is reconnect the positive to the battery and start pulling fuses in the under-hood fuse box. Typically starting with the IGN, aux, or dimmer fuses. Basically track down which circuit is pulling power. You can just pull the fuse and gently connect the leads of the multimeter into the empty fuse slot, it'll then tell you how much that circuit is pulling.

**JimmyFitz** · 05-20-2006, 01:43 PM

The shunt idea is great if you need to measure for the whole system under load. What XxAndyxX said his problem is most likely is caused by one of the aftermarket add ons. Instead of the top-down approach, in this case a bottom-up approach might be easier to identify the culprit. Since he is looking for current draw when things are supposedly off, high current measurement is not necessary. Since he already has a multimeter and sounds confident that he can figure out how to use it, that is the logical way to start the investigation.