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Old 03-06-2005, 11:16 PM   #1 (permalink)
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When doing electrical work on the veh, we are reminded to disconnect the negative side of the battery. Why the neg and not the positive? Is this so that the preset radio codes will not be lost on digital radios?

Also, I have one of those twist-off battery disconnect devices, fits onto one batt post and the cable attaches to it, twisting a knob will sever the connection so you don't need to hassle with a wrench to disconnect a cable plus it can be used as an anti-theft device. Is it to go on the pos or neg side of the battery?

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Old 03-06-2005, 11:25 PM   #2 (permalink)
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With the neg. cable attached, the whole car becomes the ground, increasing the risk of a short from any 'hot' power source. Disconnect the neg cable, no risk of a short.
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Old 03-06-2005, 11:27 PM   #3 (permalink)
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Not sure why but when you disconnect the negative cable you don't get a small spark whereas when you undo the positive cable there will be a small spark. Something to do with electron flow i guess.
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Old 03-06-2005, 11:52 PM   #4 (permalink)
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The disconnect switch goes on the negative post.
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Old 03-07-2005, 02:13 AM   #5 (permalink)
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yes electrons flow negative to positive. tahnk you 2 years of college to tell me that...and i never use it...
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Old 03-07-2005, 02:22 AM   #6 (permalink)
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The car is a negative ground and the negative side of the battery is also a negative ground. This makes for a safter situation than accidently touching the + side of the battery and the car which makes sparks and potentially a fire. So by disconnecting the - side of the battery first you eliminate any chance of burning your car which is a good thing IMO
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Old 03-07-2005, 02:29 AM   #7 (permalink)
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A discharged battery that's been sitting a while will collect hydrogen gas around its top. Disconnecting the positive battery cable will cause sparks to occur that may ignite this gas and blow the battery. That's why they always say to disconnect the negative cable, no sparks.
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Old 03-07-2005, 02:32 AM   #8 (permalink)
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It really shouldn't matter.
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Old 03-07-2005, 03:11 AM   #9 (permalink)
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funny this topic came up. i just took my battery out to check the condition on my radiator support, didnt touch anything electrical. put the battery back in and POW...sparks everywhere and some wires smoked...lol oh well back to it i guess.
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Old 03-07-2005, 08:09 AM   #10 (permalink)
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Always, always, always remove the neg. cable first. My brother tried removing a battery from my car, he tried to take the positive cable off first and accidently touched the wrench to the inner fender while on the bolt, he got a nice poke and nearly welded the wrench to the inner fender! No worries if you remove the negative first.
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Old 03-07-2005, 08:32 AM   #11 (permalink)
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From a purely electrical point of view, it doesn't matter which side of the battery becomes disconnected first. Either side may elicit a spark upon reconnection, however, if sufficient current draw in the circuit is used.

From a practical standpoint, you should always remove the negative side first, which reduces the risk of completing a short circuit by a wrench, hammer, or screwdriver that accidentally touches both the chassis and the battery + terminal. There is no other reason to do it this way. It is simply a matter of risk reduction.
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Old 03-07-2005, 10:10 AM   #12 (permalink)
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I was told recently this an an alternator rebuild shop...it had to do with the alternator and its life expectancy. If you take off the positive first, it hurts the alternator.
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Old 03-07-2005, 02:27 PM   #13 (permalink)
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i put my battery disconnect switch on NEG terminal becausse its more out of sight and dthieves might not see it for what little help that is. otherwise as midlife says it makes no nevermind. with either cables off the total car is disconnected - electrons need complete path from/to POS to NEG or nothing works.
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Old 03-07-2005, 04:29 PM   #14 (permalink)
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What Mid said is exactly right.
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Old 03-07-2005, 07:34 PM   #15 (permalink)
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As others have said..disconnect the negative. When disconnecting both battery terminals, disconnect the negative first. When reconnecting both terminals, connect the negative last. Primarily, this procedure reduces the chance of the wrench touching some part of the car body and causing a dead short. This could cause all kinds of dramatic/dangerous happenings.
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