You are here:

Automotive/Jumping a dead battery

Advertisement

Expert: - 5/15/2010


Question
Dear Expert,

When I am jump starting the dead car, I was told to step on the gas pedal of the source car, revving it up to high rpms in order to give the charge a little more "juice".  Is there any logic in this?  Does it actually do anything?  Thanks in advance.

Michael

Answer
If you have a battery condition meter (effectively an rms voltmeter), then you may notice that the voltage may climb a volt or two above a base thirteen volts or so if you take the engine up just a couple of hundred RPM, ie: to fast idle.  If such happens, then just applying enough throttle to bring up the idle and the voltage to the point where it stabilises at its regulation point would be fine, but going beyond that point will only waste fuel.  

The alternator on most cars is a very small auxiliary load, and is upper limited by how much iron is in its stator and rotor and by the design voltage established by its winding count, along with how much current those windings can provide.  Alternators do produce AC really, ahead of the their rectifier stack, and the upper limit of its output and thus the power it will demand from the engine will be limited by its own reactance - synchronous reactance - which goes up with current and frequency (which varies with speed).

Your best bet is just to make sure your own battery is up to the task of starting the other car.  Keeping your own engine running, even at idle only, will allow that battery to recharge after the boost without having to worry about starting your own car.

As always, be sure to pay attention to jumper cable polarity, and connect the negative cable ends last.

Regards ... EGK  

Automotive

All Answers

Answers by Expert:

Ask Experts

Volunteer

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

©2012 About.com, a part of The New York Times Company. All rights reserved.