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Old 26-07-2016, 09:40   #1
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Safer To Lift Battery POS Or NEG First?

I just read a section of a Don Casey boat book. The book is fantastic in all regards, but he says: when working on the DC system - you should always disconnect the negative/ ground side of the battery bank first.

I always figured I should disconnect the positive battery pole first, on the idea that there are lots of possible earth grounds on the boat to mistakenly touch to a +12v source, but if the the battery bank positive post is completely out of the loop then there is no possible danger of shorting the batteries or welding your screw driver or other metal bits is removed as I probe around circuits and connections.

I respect Don Casey's experience. am i missing something?
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Old 26-07-2016, 10:11   #2
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Re: safer to lift battery POS or NEG first?

If in the process of disconnecting the positive clamp with the negative still connected, and say you inadvertently touch the wrench to something that is grounded, you will get the dead short.
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Old 26-07-2016, 10:17   #3
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Re: safer to lift battery POS or NEG first?

I always remove the ground first...

Too easy for a short..

Oh, I see Nigel already commented..

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Old 26-07-2016, 10:18   #4
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Re: safer to lift battery POS or NEG first?

Quote:
Originally Posted by nigel1 View Post
If in the process of disconnecting the positive clamp with the negative still connected, and say you inadvertently touch the wrench to something that is grounded, you will get the dead short.
That's what I always learned too. The chances of touching something ground with a live wrench are much greater than the chances of touching live with a grounded wrench, as (particularly on a metal car) the battery is often surrounded by ground.

I'm not convinced it makes much difference inside a boat's battery box, though, as they're usually made of an insulator. Still, stick to best practice as a matter of habit.
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Old 26-07-2016, 10:19   #5
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Re: safer to lift battery POS or NEG first?

Quote:
Originally Posted by nigel1 View Post
If in the process of disconnecting the positive clamp with the negative still connected, and say you inadvertently touch the wrench to something that is grounded, you will get the dead short.
thanks, Nigel. good point.
If I were to safely remove the POS connection and even cover the POS terminals on the batteries, then would it not be possible to safely leave the NEG terminal still connected while working on the DC wiring?
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Old 26-07-2016, 10:21   #6
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Re: safer to lift battery POS or NEG first?

Quote:
Originally Posted by Symphony View Post
thanks, Nigel. good point.
If I were to safely remove the POS connection and even cover the POS terminals on the batteries, then would it not be possible to safely leave the NEG terminal still connected while working on the DC wiring?
Yes
But why would you?
If you are working on on the wiring then remove ANY chance of a problem. Disconnect.
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Old 26-07-2016, 10:27   #7
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Re: safer to lift battery POS or NEG first?

I work on cars all the time with just the negative lead disconnected. Is that wrong?
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Old 26-07-2016, 10:31   #8
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Re: safer to lift battery POS or NEG first?

Quote:
Originally Posted by MarkSF View Post
I work on cars all the time with just the negative lead disconnected. Is that wrong?
Nothing is right or wrong until there is a problem.
I remove all power sources just to be sure... I watched a guy drill a hole into the chassis of a car once and the bit burned through the live feed to the starter motor.... the live feed was connected...Something went terribly wrong... still not figured it out 20 some years later...

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Old 26-07-2016, 10:32   #9
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Re: safer to lift battery POS or NEG first?

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Originally Posted by Symphony View Post
thanks, Nigel. good point.
If I were to safely remove the POS connection and even cover the POS terminals on the batteries, then would it not be possible to safely leave the NEG terminal still connected while working on the DC wiring?

I cannot see why not, if the positive terminal is well covered.
Out of interest, why bis there a need to leave the negative terminal connected?
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Old 26-07-2016, 10:33   #10
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Re: safer to lift battery POS or NEG first?

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Nothing is right or wrong until there is a problem.
I remove all power sources just to be sure... I watched a guy drill a hole into the chassis of a car once and the bit burned through the live feed to the starter motor.... the live feed was connected...

Yes, but presumably, so was the ground. If the negative had been disconnected, there would have been no circuit.

I'd be more than happy to disconnect the live terminal as well, if there is a hazard, but I can't think of any conceivable one.
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Old 26-07-2016, 10:36   #11
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Re: safer to lift battery POS or NEG first?

I always disconnect the negative first.

I bet in a battery box the biggest danger when disconnecting the positive first is for multi-battery banks as the removed cable could end up Hanging out and ground a still connected battery.
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Old 26-07-2016, 10:57   #12
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Re: safer to lift battery POS or NEG first?

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Originally Posted by MarkSF View Post
Yes, but presumably, so was the ground. If the negative had been disconnected, there would have been no circuit.

I'd be more than happy to disconnect the live terminal as well, if there is a hazard, but I can't think of any conceivable one.
I rest my case.

there was an issue that would have not occurred if the battery was disconnected completely.
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Old 26-07-2016, 11:25   #13
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Re: safer to lift battery POS or NEG first?

I can see lifting the NEG in an automobile because that is a system without earth ground (rubber tires) and where a lot of surfaces are metal.

I can see lifting the NEG in a steel or aluminum boat because there are a lot of metal (earth ground) surfaces lurking.

But an FRP boat does not have many metal surfaces, plus any metal surfaces are likely connected to Earth Ground all the time through the bonding system or prop shaft. If the batteries are not connected to the earth ground or NEG bus then they couldn't possibly create a dead short to anything on the boat. right?
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Old 26-07-2016, 11:26   #14
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Re: safer to lift battery POS or NEG first?

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I rest my case.

there was an issue that would have not occurred if the battery was disconnected completely.
You haven't made any case and you've completely missed my point.

If you disconnect the negative only, the accident described cannot happen as there is no circuit. The battery is no longer a source of power. If you connect the body ground to any live cable, nothing will happen.

To make a case for your argument, you need to describe an accident that can happen with the positive cable connected, but the negative disconnected.
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Old 26-07-2016, 11:37   #15
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Re: safer to lift battery POS or NEG first?

Quote:
Originally Posted by MarkSF View Post
You haven't made any case and you've completely missed my point.

If you disconnect the negative only, the accident described cannot happen as there is no circuit. The battery is no longer a source of power. If you connect the body ground to any live cable, nothing will happen.

To make a case for your argument, you need to describe an accident that can happen with the positive cable connected, but the negative disconnected.
Whatever.

If you are happy having half a circuit connected then go for it. I dont like the idea of someone else being 'helpful' and connecting the earth, or something falling on the earth terminal and completing the circuit.

I prefer to disconnect completely when soldering or jointing or working on electrical circuits. I prefer isolating the power source. That way I am sure.

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