Parasitic Drain - Bulb Trick

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Just wanted to ask:
Is the "light bulb" test (putting a 12v bulb in series with the battery circuit) an accurate test for a parasitic drain? My ammeter only goes
up to 0.25A, and I don't want to risk overloading the meter.
How bright should the bulb light up, if it should at all, for a "normal" drain? (clock, ECU, radio memory)
Thanks!
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See the relevant page at Bill Darden's excellent Battery FAQ: http://www.uuhome.de/william.darden/carfaq10.htm
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The Unofficial Honda/Acura FAQ
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sharx333 wrote:

sure, but it depends on the bulb and the drain!!! if you describe your problem, maybe we can address that question better?
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Sure, jim. And thanks.
Last week, the battery drained overnight on my '95 Civic 1.6L. I took the battery to be recharged at a shop, and we found that the water level was low. But I suspected a "ground" somewhere, because I had just sprayed the engine with a degreaser and rinsed it (low pressure garden hose).
Since my ammeter only goes up to 0.25A (250 mA), and I read on some posts that the current could go over 1A, I tried the bulb trick first. I don't know the exact rating of the bulb (aftermarket dome/trunk light), but it glowed brightly. (All doors/trunk was closed, key off)
I tried pulling underhood fuses and even disconnecting the alternator, with no result. Then I tried pulling the cabin fuses, and I found that it was the Pioneer head unit that was pulling most of the power (for the preset memory, I guess).
When I disconnected the radio, the bulb was finally very dim. I tried the ammeter and expected less than 80 mA (from the battery FAQ link, thanks to TeGGer). But I still got 170 mA or so. I've checked all the possible bulbs, and I think a bulb would pull more than that.
I finally concluded that there was no parasitic drain at all, but I'm puzzled as to what could be making that extra load.
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sharx333 wrote:

well, first, you have to eliminate the battery as a problem - if it's draining itself, it's no good.
after that, 170mA is a 2W bulb at 12V [hence bulb glow]. that drain seems high for the ecu memory [what you'd have left after disconnecting the audio.] i'd continue looking for leakage. check both sets of fuses pulling each one individually and connect the meter probes between the slots.
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Ok. Thanks for the heads-up.
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That might be a low wattage lamp, and glow brightly at a low current draw. A stoplight bulb is a better choice, perhaps. I could look at autozone.com to find the dome light bulb number, and then sylvania.com for the bulb specs, but I think you found your culprit.

I think you are indicating that 170mA made the bulb glow dimly, and the Pioneer made it bright. That's not good. The pioneer should only draw a few milliamps in standby. There should be a figure in the manual. There should also be a standby control on the Pioneer.
A voltage supplied to the Pioneer that is always on is expected to be a heavy current feed for the amplifier, that also draws a tiny amount of power in standby. There should be another switched control that comes on with the radio or ignition that cause the Pioneer to turn on its amplifiers. I think yours is switched on all the time.
Or maybe not... ;-) Check the manual for some specs on the Pioneer.
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snipped-for-privacy@XReXXParas.usenet.us.com wrote:

but the 170mA is /after/ the audio is disconnected! yes, the audio needs to be addressed, but there's still unusually high residual drain.
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jim beam wrote:

The obvious question is - can you see the Pioneer front panel lit up?
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Yes, jim's right, there is 170mA *after* the Pioneer's fuse is disconnected. With it connected, the bulb glows brightly and it's too high for my ammeter.
Strangely though, the battery has not discharged yet. I've left it connected for two days now without charging/starting it.
And of course, all the measurements were taken with the Pioneer off, front panel dark, "demo" mode off..
Hard to see how the wiring could be wrong, but yes, the standby draw seems too high. I'll look into the manual for the specs. Will post again ASAP.
Many thanks.
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I'm not sure exactly what it should be, but 170 ma besides the audio system is way high. I'd expect more like 20-30 ma, mostly for the alarm system.
Mike
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Even more strange, the Pioneer manuals doesn't mention standby power draw. It mentions allowable voltage range, max power draw, db levels etc, but no standby draw.
And also, this model does not have an alarm system. There's a digital clock, though.
I also tried my "test bulb" on a '96 Civic (also with clock, no alarm), and the bulb didn't light up at all. Puzzling.
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Sorry, I meant "..the Pioneer manuals *don't* mention.." Please excuse me.
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I could be wrong about the head unit. I was thinking of high power units that can draw a lot of power. If this is is a lower power unit, it might not have the high current lead. It would probably have a "pink" wire for the low current memory lead, and the current required might be tiny, and not mentioned in the consumer level manuals. I see a rating of 5mA for some head units.
What about the light bulb in series with just the power for the Pioneer? You could connect it across the fuse holder with the fuse out. (The draw of the Pioneer is interesting to me. I don't know what the draw for the rest of the car ought to be.)
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Just re-measured the standby drain today... Now it's at around 60 mA. I haven't really changed anything, so I'm really baffled now.
Maybe it was a "ground" of some kind caused by water from the engine wash (alternator got wet)?
The water could have dried out by now, resulting in the more normal reading.
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BTW,if you want to extend the DC amps range of your inexpensive DMM,use a 1 ohm resistor in series with what you want to measure(use short heavy leads soldered to the 1 OhmR),and the DMM measures across the 1 ohm R. Since I=E/R,1 volt across 1 ohm= 1 amp. If your DMM has a 2V range,that's 2 amps full scale.
I'd use a 10W 1 ohm WW resistor.
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I forgot to add "volts" so it would read "and the DMM measures VOLTS across the 1 ohm resistor."
< Since I=E/R,1 volt across 1 ohm= 1 amp.

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...and be careful. If you by some mischance get it across the battery it will try to dissipate I^2*R W. One ohm at 12V passes 12A, so thats 144W, well in excess of the 10W its rated for. It will get red-hot and fail in some interesting way, quite suddenly...
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wrote:

So,if you're that inept,put a 3A fuse in series with the resistor,like many DMMs that are fused for the current ranges.
Of course,any ammeter would also be a short across the 12V.
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Hello again, I was the original poster of this thread. I'm posting an update to the problem, for those who are interested.
After disappearing mysteriously, the parasitic load was back again last night. Today I've finally found the problem. It's the *alternator*. I found it by disconnecting the alternator ground. It was very hard to find before because it was intermittent, but it's definitely the alternator.
Bad alternator diodes..? I recall the problem started just after an engine wash..
Thanks.
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