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Automotive/Head Unit Battery Drain Part 2

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Expert: - 8/8/2007


Question
Thanks so much for the quick reply.  After going back and testing again I can now tell you what I have done and what I know.

Firstly, I disconnected the negative battery terminal and tested the current draw with a multimeter on the 10A setting.  The result was a 0.95 readin which I would say is 950mA and way too high a draw for just the clock, etc.  I then started pulling fuses one at a time until the draw dropped off.  It was the dome fuse that was pulling most of the current so I started to trace the components on this system.  None of the lights were staying on, I took the amp out entirely and the stereo as well.  In testing each of the wires I again found that the constant +12v  wire was drawing much more current than anything else.  I actually fried the fuse in my multimeter (oops) but the reading was around 720mA.  Any idea what could cause this kind of draw?  Thanks so much again for your time.

Answer
Thanks, that does clear things up nicely!

Yes, I would say that a constant 950 mA from a battery really intended for automotive starting service would cause the battery to lose enough charge as to not be able to start the car within a day or two, especially if the battery itself was nearing its amortisation point (which is a euphemistic way of saying "going west" or <gulp> dying).

There could be a problem with your stereo unit, causing its to draw more than its design "stand-by" current.  Or, again, it could instead be that an external contactor was intended as a supply control, or it could even be that this "constant" power lead was intended to be connected to the accessory bus, and hence powered only when the ignition switch is in the "accessory" or "on" condition.  Check the documentation.  A contactor would be specified or at least recommended if the current on this lead goes high (several tens of amps or ??? depending on the size of your amplifier), in which case the contactor coil should be energised only from the accessory bus.  The accessory bus itself would be okay if the current drawn remains low, at 950 mA or not exceeding a few amps.  Either of the above would be an easy work-around fix, whether specified or not.

While you are at it, check your battery anyway.  Give it a load test, using one of the standard 100A load bank testers; a friendly garage may let you use one, or do the simple check for free, definitely if they figure they can sell you a battery!  Whether you do the load test or not, how old is the battery anyway?  It is quite possible you need a new one, and your stereo system has taken the role of being an early-warning messenger.

Yeah, one sometimes forgets to switch the DMM leads from the 400 mA jack to the 10A jack ... it happens.

You'll solve the problem.

Carry on ... EGK

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Ernest (Ernie) Kenward

Expertise

The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!

Experience

A key skill in my work and hobby pursuits both is STRATEGIC TROUBLESHOOTING. I am a senior instructor in Electrical Engineering Technology at a leading Canadian polytechnic, my areas being Electrical Power and Industrial Control, electrical and electronics design and manufacturing, and AutoCAD and related CAD/CAE software - plus equipment problem-solving and new equipment design and prototyping. Hobby-wise, I have 30-plus years of experience in auto restoration, mostly in electrical and mechanical systems. Ongoing projects include a 1959 Edsel Corsair, my 1978 Ford E250 class-B motorhome conversion, and the care and upkeep of my Mercedes 300CD. My vehicles become engineering test beds for electrical and mechanical upgrades as ideas present themselves. This includes the design and production of circuit boards to restore or enhance features for which no OEM replacement parts are obtainable, or where better specifications or reliability can be had via newer concepts. Regarding the E250 RV conversion, I designed and continue to revise a custom power distribution system, managed by a Programmable Controller (PLC); this has made most revisions as easy as uploading new firmware as I develop it. The "mini" PLC is a powerful device for custom automotive control systems. One good example (there are many) would be the Moeller "Easy Relay"; these offer a wealth of control, monitoring and variable-and-status display options for such projects. A good example project which has worked well is that one for my RV noted above, which has been on the job - revised in firmware only - for a decade now. It is a load management and charging control system to avoid the sulfation-induced early failure that often befalls deep-cycle batteries used in RV power applications. The battery installed in 2003 lasted long enough to more tnan pay for the PLC that contributed to its longer life ... and the PLC will be there for the next battery as well!

Organizations
IEEE - senior member ... past WCC Student Activities; SME - senior member ... past chair, greater Vancouver chapter chair 318; Edsel Owners' Club - have served in various capacities on chapter executive during seventies; have been Power and Driveline resource on the Edsel Owners' Club "E-team" for more than a decade.

Education/Credentials
Graduate of UBC

Awards and Honors
Certificates of appreciation from IEEE and SME for work in student and chapter activities

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