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Parasitic battery drain, is .4 amp too much?

 
Old 03-15-2011, 08:53 PM
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Jim Chambers
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Default Parasitic battery drain, is .4 amp too much?

Not sure I am reading my meter right but, used the 20 amp unfused setting and I get a reading of .4 amps from the wire feeding my radio. My Forum search finds that 40 to 50 milliamps is acceptable (.040 to .050 amps, right?). If that is true, my radio is taking 10 times too much.
I tried using the 200 milliamp fused setting on my meter and I think I blew the meter's fuse! Assuming .4 is a correct measurement, does that mean something is amiss in my radio and/or amp? I think the amp gets power when the radio is turned on, but maybe not. Haven't been to the amp with the meter yet.

Last edited by Jim Chambers; 03-15-2011 at 09:38 PM.
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Old 03-15-2011, 08:57 PM
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Leon Speed
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50 milliamps, not .5
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Old 03-15-2011, 09:12 PM
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Jim--

That's a lot of draw for the radio. Total drain (radio memory plus LH memory plus EZK memory) should be in the 25-40 mA range, 0.025 to 0.040 Amps. Measurements MUST be taken with the doors, hatch and hood closed, key out of the ignition, doors unlocked, and wait until the interior lights delay relay times out, allowing ALL the lights (including red door markers) to go out.
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Old 03-15-2011, 09:37 PM
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Jim Chambers
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I took the reading directly from the hot wire feeding the radio. Separated it at the inline fuse and connected the amp meter inline. Will I get a different reading if I do it as you suggest?
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Old 03-15-2011, 09:43 PM
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.4 A is 400 milliamps (mA.) If you still have the manuals for your radio/amp they might indicate what the standby current draw should be. Most modern radios I've tested have a standby draw of less than 150 mA. However, it all depends upon the unit and what's in it. If it is one of these new-fangled robotic satellite radio with DVD, GPS, and a beer cooler in it, then it might need 400 mA all the time.

For a normal 928, 400 mA is a bit high. My guess would be that the amp (if separate) is not wired correctly and is not shutting off the way it should. Just a guess though.
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Old 03-15-2011, 09:57 PM
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Yes I'd say 400mA is too high. Your radio is burning almost 5W, it's like leaving on a tail light bulb.
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Old 03-15-2011, 10:05 PM
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Jim--

No different reading. I thought yopu might be testing total parasitic draw in series with the battery ground cable. Didn't appreciate that this is for the radio only.

Q's: Are you sure you are on the red power lead for the radio itself? Does the current draw increase when you turn the radio on?

Remember that there is a lamp in the radio, and that they typically wire separately from the power circuit. Most I've seen go on with radio on, have a dimmer function built in for when the instrument lights are on.

My radio has two power leads. One requires power all the time (bus 30). The other sense line goes on and off with the key, on with X-bus powered. Lazy installers might connect both to all-the-time bus-30 power.

On power drain--

The battery is rated typically for somewhere around 50 amp-hours capacity new. 50/0.4 = 125 hours and the battery is functionally dead. With Dave's observed 0.150 Amp drain, 50/0.125 = 400 hours before the battery is functionally dead. Lots of parasitic current either way. Battery loses 10-20% of its -remaining- life each time it's drawn down and recharged.
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Old 03-15-2011, 10:06 PM
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Jim Chambers
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I am thinking the amp might be the problem. Is it correct that it should only get power when the head unit is turned on, thus sending current to the amp? If so, there should be no current to the amp when the radio is off.
How might the amp be wired wrong? And how do I check?

Edit:I know I am checking the main positive lead to the radio. I will try turning radio on while measuring.

I get about a week to 10 days before I have to recharge. Lately I am using a tender to keep the battery up.
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Old 03-15-2011, 10:32 PM
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My amp has a point for an input signal from the radio that kicks it on only when the radio is turned on. It does not supply drive current to the amp, it is a signal only and has only 600mA capacity. Drive power comes from separate fused connection directly to the battery.

My radio has three inputs, 12V constant, 12V switched and the dimmer/backlight. I did the install so I know they're all hooked up right.
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Old 03-16-2011, 12:05 AM
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Jim Chambers
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My concern that my problem is radio/amp related is based on the fact that at some point in its history the car had an aftermarket system. That was removed and a CD2 was installed in the car when I got it (however it would not play as I had no code). I installed another CD2 with a code and connected to a stock 6 channel amp. Between my work and that of the PO there is ample opportunity for error!
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Old 03-16-2011, 09:02 AM
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You may want to start over with wiring, I did!
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Old 03-16-2011, 09:27 AM
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Just reiterating 2 things said here:
1) The standby power and on power for radio are tied together (lazy install)
2) The amp is hardwired in to the battery or to an always on, hot lead.

My car's battery got knocked out by an aftermarket Alpine amp. Damn PO hard wired it to the battery.

Otherwise check your grounds for a short somewhere. That's a lot of draw if everything is off.
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Old 03-16-2011, 12:08 PM
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Jim Chambers
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I will do some serious checking of the wiring for both the head unit and the amp later today. I'll see what's hot and when, maybe take a pic or two, and be back with more questions. Thanks
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Old 03-16-2011, 01:31 PM
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Mike (and Jim):

Most serious aftermarket amps are wired directly to the battery. The current drawn by them is so high that patching them into existing car wiring is too risky; damage to both the amp and other systems in the car is possible with the typical high current draw. Consider that a good FET amplifier is maybe 90% power efficient. The rest is shed as heat from that big heat sink. So for simple math let's say you have a 400 Watt RMS rated amp. Basic Ohm's Law tells us that it will draw about 30 amps under that load (400 Watts/13 Volt), times the inefficiency 1.11 (100/90 efficiency), times 1.414 (estimated) for that peak/RMS part that assumes a sine wave audio signal. That works out to about 40 Amps, and the circuits are (or should be) cabled and fused at 50 Amps for a little safety margin. That's in the 6-8 gauge wire size range, tending larger if there's any more than a few feet.

Anyway, forget the math part, and consider that the amps typivcally have a 'trigger' lead that comes from the head unit. Blue on mine. I use it for the antenna amp, but it's there to supply current to external devices like amps so they can turn off when not needed. With amps running in the back near the battery and head units in the console, that means the installer will need to run a wire (hopefully blue) between them in the bundle of low-level audio. Wait, not a good idea to bundle that long noisy DC wire with the audio, so a good installer moves the blue wire away from the audio bundle. A noisy wire there or a lazy installer means that the trigger may not have been installed, and instead there's another possibility that the installer just put full-time power on the trigger lead.
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Old 03-16-2011, 02:00 PM
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Originally Posted by Jim Chambers View Post
I installed another CD2 with a code and connected to a stock 6 channel amp. Between my work and that of the PO there is ample opportunity for error!
If the stock stereo wiring is restored and everything else about the 'lectrical system is stick and good, then the static current draw measured at the battery should be on the order of 125 mA or less for the whole car after locking the door and waiting for 3 to 5 minutes.

I usually stop when I get to a static draw of 250 mA. To get that last ~125 ma out of it you really have to start taking stuff apart...
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