Hi Guys,

I'm still trying to diagnose a power leak that causes whatever battery I connect up to drain and die. I was going to connect an ammeter between the negative power cable and the negative battery terminal to check the current draw.

I was advised by a mechanic not to do that and to use a "passive load switch" that would not allow current spikes to damage any electrical components.

Does this make sense? What was the guy talking about? Is this thing available somewhere? I looked at the snap on site and there wasn't anything like that. All I could find was battery testers etc.

Anyone know what this thing is?

Thanks
Lee

 

The guy is right - but I would think it would depend on how "fast" (current use) your battery is draining / being drained. To me if you have a "dead short" ("big" arc occurs when disconnecting the battery cable) yes - use a passive load switch type implement.

A "passive load switch" usually indicates that there is a minimum voltage/amp requirement that must be met across the switches or cable contacts before the "switch" engages/completes the circuit.....producing less of an arc.


When you "close" a loaded electrical connection - such as reconnecting your disconnected battery cables to your battery - you get an "arc" when you connect/disconnect the last terminal to complete/open the voltage loop...this is normal. The reason for the arc is because there is some type of load on the battery cirucit (PCM/ECM, dash cluster...stuff like that which draws power to retain its memory).

An electric "arc" has a non-linear relationship between current and voltage. Once the arc is established or by momentarily touching the electrodes then separating them, increased current results in a lower voltage between the arc terminals. This negative resistance effect requires that some positive form of impedance to be placed in the circuit. This property is the reason repative uncontrolled electrical arcs in apparatus become so destructive, since once initiated an arc will draw more and more current from a fixed-voltage supply until the apparatus is destroyed. When an inductive circuit is switched off the current cannot instantaneously jump to zero; a transient arc will be formed across the separating contacts. Switching devices susceptible to arcing are normally designed to contain and extinguish an arc, and snubber circuits can supply a path for transient currents, preventing arcing. If a circuit has enough current and voltage to sustain an arc formed outside of a switching device, the arc can cause damage to equipment such as melting of conductors, destruction of insulation, and fire.

You can place a "passive" load such as a 12v test lamp across the unconnected battery terminal and cable to reduce the arcing/current ramp up upon connecting battery cable or a meter for testing. Remove the lamp/load once the connection has been made......you will still see an arc but it should be to a lesser degree.

You should be able to use your current meter the way you intened without a real problem....as many before you have done. You could also pickup one of these clamp on style meters: http://www.sears.com/shc/s/p_10153_1...2369000P?mv=rr

During circuit testing the use of an low amp in-line fuse is also highly recommend.

 

engle1147... outstanding technical reply, and well written. It is members like you that contribute to the success of this forum.

 

Thanks a lot Richard. That was a doozie of an explanation :-) I orered the sears clamp on ammeter. My battery drains completely in 4 days. I tried 2 different batteries and they both do the same thing.

If A battery holds 400 amps and it drains in 4 days, that literally means that I'm losing about 100 amps a day or rough;y 4 amps an hour. I should be able to register that much current with that sears ammeter even though the variance is around 3% or so.

I still would like to find that passive load switch thing and looked around some more for it but can't find anything even remotely close. This has to be the most esoteric thing I've ever looked for.

Thanks again for the writeup. Great Job! -Lee

 

It is safe to disconnect a battery cable and connect an ammeter to measure the leakage current. Every time your starter motor is switched off after cranking, a large voltage pulse occurs due to the inductance and high current in the starter motor. Automobile electrical systems are designed to withstand this and disconnecting a battery cable cannot generate a pulse anywheres near what a starter motor can! Your mechanic is incorrect. Also, I do not know what he means by a , "passive load switch", I suspect he is talking about a battery disconnect switch and it isn't necessary to use one!
First, at night, look for underhood lights, vanity mirror lights, door map lights, console light. Do you have a GPS, CB, radar detector, aftermarket alarm or radio or audio amplifier? Check em. Do not leave your ign key in the car because this keeps the antitheft circuit on and will discharge your battery.
Pull the courtesy light fuse so you can keep the door open and pull fuses and connect your ammeter and watch the ammeter and pull fuses one at a time. A fuse that drops the current dramatically is the circuit you will have to investigate further. Feel the seat adjust motors and if they are warm, you have a stuck seat adjust switch. GM says leakage current should not exceed 50 milliamps and my 87 vette measures 27 ma. and I have no problems. It would help if you posted what year vette you have as they are not all wired the same way. Early C4's have a bolt behind the battery with 8 red wires and these should be disconnected one at a time while you watch the ammeter. If one is the cause of leakage current, you will have to trace that wire to see what it is connected to. C4's also have some circuit breakers if you cannot find the offending fused circuit.
I am an electronic engineer!

 

Car batteries don't ,"hold 400 amps", their capacity is measured in amp-hours and they are rated in reserve time, usually around 90 minutes. Reserve time is the time a battery can deliver 20 amps until the battery voltage drops to 11.9 volts which is considered discharged. Very roughly, a 1 amp leakage current will discharge a battery in about 24 hours.

 

Another gem by jfb. Been reading his posts since '02, and you can bet the bank on his information. Thanks for staying interested, jfb.