Safer To Lift Battery POS Or NEG First?

[nigelmercier]

Originally Posted by Symphony

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...

Your point about a lot of metal surfaces is a valid one, but I'm afraid the reference to earth ground is totally irrelevant.

[TurninTurtle]

You generally will see the fuses on the + leads.

Pulling the - you still have the fuses in case of something going wrong (if you leave the + connected)

Pull just the + and somehow while working you arc the + to metal and make a complete circuit the result becomes a question of what fails first: insulation making a fire, battery exploding or something else... but nothing good.

Pulling both cables means you have to have something really rare happen to create a problem other than the potential of dropping a wrench that contacts both battery terminals.
Not much can protect from that one other than using caution.

Disconnecting both is possibly paranoid... but with things that can create fires and explosions is being slightly paranoid being paranoid enough?

*************

Should be really rare now... but if its a + ground system, pull the + first.
I've worked on some old stuff.

[basssears]

Was taught (in fire academy) to disconnect negative first: no arcing as you move the wire away from the battery, if you disconnect positive first you will get an arc if there is any live load, arcing can cause damage that leads to corrosion and is theoretically dangerous if explosive gases are present.

[mikecambrai]

You must always disconnect the negative side of a wet battery first to avoid sparking which can cause the battery to explode!
Across the top of the battery unseen will be traces of hydrogen which will explode if there is a spark. In addition, if you disconnect the negative and touch anything else nothing will happen. There is a risk of flashing if you touch the positive to any earth metal in the area.
Best practice is to blow across the top of the batteries first or wave your hand across the tops to create a draught. Then disconnect the negative terminals.

[boeing1]

remove neg First, and install neg Last...
also done on aircraft......

[Cadence]

Originally Posted by Symphony

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?

Just a good practice. Why not?

[Scorpius]

As previously said, disconnecting EITHER terminal takes the battery right out of the picture and results in EVERYTHING being de-energized. However, because in the act of undoing the bolt on the positive terminal it is FAR more likely the wrench will touch a ground (thus causing a dead short and a weld or burn [you will NEVER get a shock from 12v]) than a live wire, ALWAYS disconnect the ground (negative) side first.

[guyrj33]

disconnecting only the positive terminal leaves the battery grounded and any contact between the positive terminal of the battery and any ground on the boat (or car) will result in a shock. With only the negative terminal disconnected contact between the positive terminal and a ground will not cause current flow.
The negative terminal MUST be disconnected, do whatever you please with the positive terminal.

[Panacea2183]

Originally Posted by MarkSF

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.

Agree 100% as an automotive mechanic all my life have ALWAYS disconnected NEG first no ground, no circuit, no problem.
Touch the POS with your wrench chance of touching ground somewhere is high.
PS: you can get a serious shock from DC, ask me how I know.
Don't believe me? Wet your hands and grab POS & NEG terms on your batt.
I'll wait for your reply as you type with bandaged hands

[JD-MDR]

I always wonder about this also . I read that years ago that the positive cable should be disconnected first because it is less likely to arc when disconnecting at the battery near the gases from the batteries that could explode. Oh now I'm confused because it seems so obvious to disconnect the neg first but I remember being confused thinking that the instructions seemed opposite.

[ramblinrod]

Always connect/disconnect negative cable first. There is really only one thing you can touch a negative cable to that will creat Sparks ( the positive termina)l. There are all kinds of things you can accidentally touch a positive terminal to and make sparks.

Professional Sparky

[tkeithlu]

Nigel is the pro, and yesterday I replaced a five year old starter battery. For the first time, I followed his advice and disconnected the negative first. He's right - down there between steel frames, wiring, drive staff, and a bunch of other potential grounds, I had lots of opportunities to touch a wrench to a ground. It felt so much better disconnecting the positive, and installing the new battery, without having to worry about what I touched.

[Symphony]

Thanks everyone. The arcing that can occur pulling a live POS is surely reason enough to explain why to pull the NEG first

[leftbrainstuff]

Negative connect last and disconnect first is standard practice for negative ground systems.

It avoids the potential of accidentally dropping or striking the positive terminal on grounded components like the engine and transmission.

If you ground a wrench on the positive terminal, with the negative still connected, you will allow full battery current to flow through the wrench. You never want to experience that.

As a technical trainer in the 80s I would fail any apprentice in any practical assessment who didn't follow the negative terminal off first and on last teaching.

Sent from my SM-N900T using Cruisers Sailing Forum mobile app

[smac999]

Originally Posted by basssears

Was taught (in fire academy) to disconnect negative first: no arcing as you move the wire away from the battery, if you disconnect positive first you will get an arc if there is any live load, arcing can cause damage that leads to corrosion and is theoretically dangerous if explosive gases are present.

]

the current on the neg and pos will be the exact same. and either would spark.