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| Weird test results with battery cut-off installed. https://slantsix.org/forum/viewtopic.php?t=58303 |
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| Author: | Sam Powell [ Mon Sep 07, 2015 4:36 pm ] |
| Post subject: | Weird test results with battery cut-off installed. |
When I installed a new battery, I also installed a cut-off on the ground cable. Turning the green knob counter clockwise does disable all battery functions operationally. The lights don't work, the fuel pump is dead, the radio will not play, starter motor does not crank. The cut off appears to do its job. And yet, in this state my VOM still shows 12.6 volts between the positive post and the chasis. I do not see how this could possibly be. Also, the VOM shows continuity between the chassis and the battery ground post! And yet, all appears to be operationally dead. When I wired this car I followed the most basic electrical rules: black to ground and red to positive. Nothing is blowing fuses, or catching on fire. And yet it did eat a battery. Any ideas? This is above my paygrade at this point. Should I put a cut-off on the positive battery post? Are there some components in the system which will show a continuity from b+ to ground internally such as light bulb filaments? But there are no lightbulbs lit. I do have a dead dome light. The bulb is seized in the socket, and I have ignored it for years. It is not blowing fuses however. I am afraid I have created a ghost in this system that haunts it. Thanks as always. Sam |
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| Author: | Pierre [ Mon Sep 07, 2015 4:47 pm ] |
| Post subject: | |
I've seen some battery cutoff doohickeys have a very small gauge wire bypass. It's a small wire that always connects the post to the cable. This is intentional so that enough juice will still flow to keep the radio/computer/etc alive to keep time or other various super low current memory functions, but prevents anything else from working as enough current can't flow. Meant to be more of a anti-theft then a battery saver. You can verify this by taking it back out of the car and measuring. As for measuring battery voltage between the positive post and chassis - if you have any modern electronics (megasquirt doesn't count since that's probably on a relay that physically disconnects it, right?) such as a car stereo receiver or amplifier, they have capacitors in them. You could be measuring the voltage in them until they get discharged. |
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| Author: | Sam Powell [ Mon Sep 07, 2015 5:12 pm ] |
| Post subject: | |
Pierre, I think you hit the nail on the head. I went back and checked it again, and the voltage did drop off and go away after a bit. I walked away when it hit 1 volt. There is no supplementary wire on this cut off, although I did see those that had that feature. Thanks. Sam |
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| Author: | emsvitil [ Mon Sep 07, 2015 8:31 pm ] |
| Post subject: | |
There's probably a capacitor somewhere giving you the voltage. So you're not measuring battery voltage, but capacitor voltage. Leave the volt meter connected long enough, and you can watch the voltage drop. |
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| Author: | Sam Powell [ Tue Sep 08, 2015 4:14 am ] |
| Post subject: | |
Is the capacitor discharge taking place through the meter? In order to discharge, does it not need a path to ground, which is now disconnected via the cut-off switch? The meter must become the path to ground. Like I said, this gets above my paygrade real fast. Everything I know about electricity, I learned in my early model railroading. Now even that is above my paygrade with carrier control and computers running things. Pierre, Yes, the Mega Squirt is on a relay with a separate dash mounted toggle as the coast down of the electric fan (turned generator)would keep it running when the regular ignition key was turned off. Not elegant, but it works. Sam |
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| Author: | Pierre [ Thu Sep 10, 2015 9:23 pm ] |
| Post subject: | |
The meter, during the act of measuring, consumes power. It's very small with digital meters, a bit more with analog needle style. Depending on the size of the cap (since they store a finite amount of energy when the system as a whole is powered down) it may take fractions of a second or hours to discharge through the meter. |
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