Safer To Lift Battery POS Or NEG First?

[leftbrainstuff]

Originally Posted by smac999

]

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

+1.

Arcing will only occur when a load is present. That's why we fit battery isolators.

In fact arcing is unlikely on low voltage DC. Air is extremely resistive. Thousands of volts are needed to arc. This is how spark plugs spark.

If you see arcing its because the last point of contact is small and melts locally which can release a globule of metal. Arc welding is an example of this where the effect is intentional. Voltages here (open circuit voltage) are 10s of volts and the current flow is 10s of amps.

Corrosion is not an issue for properly coated terminals. A light application of vaseline or special battery grease will protect the terminals from corrosion. The dull color is lead oxide. You should clean the mating surfaces of the terminals just prior to assembly exposing shiny lead. Which is much less resistive than lead oxide. Checking voltage drop across the terminals under load will highlit this issue.

There is no danger of explosion unless your battery is vigorously outgassing releasing hydrogen. The academics love to present the scary chemical reaction which shows this risk. In practice it rarely happens except in large battery banks in enclosed areas. The small amount of hydrogen liberated is dissipated quickly.

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

[nigelmercier]

Originally Posted by Symphony

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

Yes, if it were true. It isn't.

You are just as likely to get a spark from either terminal, and an arc is unlikely unless the load is inductive.

[JD-MDR]

I remember now the lesson i read about battery cables. It wasn't the cables at all it was for jumper cables that said connect and disconnect the positive first Still I don't know what is correct. But I sure do appreciate these discussions. a friend of mine was blinded for three days when a battery blew up in his face. and just a few months ago my neighbor did something so stupid . He thought he'd be cute by putting beer in his 8d battery that was low on water,The next day i went into his engine room and saw the battery all puffed up ready to blow.unbelievable. I wrapped myself in a tarp and hardhat and goggles and reached a cap with 24" channel locks. I didn't get hurt but It was really scarry

[quandary]

It is all about the risk of creating a short circuit and getting burnt or starting a fire.
Electrically, it makes no difference which one is disconnected.

Practically it does make a difference because much of the boat metal is exposed, uninsulated and connected to the negative.
Virtually everything connected to the positive is insulated except the battery terminal itself.
It is therefore more likely that one would cause a short circuit with a spanner if working on the positive terminal and touching some arbitrary metal of the boat than working on the negative terminal and touching the same metal with no consequences. The chance of touching the actual positive terminal itself is low because it is relatively so small.
So lift negative first!

[smac999]

Originally Posted by JD-MDR

I remember now the lesson i read about battery cables. It wasn't the cables at all it was for jumper cables that said connect and disconnect the positive first Still I don't know what is correct.

for jumper cables they say connect neg last and to the car chassis not the battery. that way the last connection will spark away from the battery instead of over it. if you connect pos last you really have no choice but to connect to the pos post of battery. over the battery

[Redpcx]

The confusion about EVER connecting or disconnecting negative or ground or earth last is with jump starting a dead car.
1. Connect positive to positive, directly on the battery. No complete circuit, no current flow, no spark.
2. Connect negative to negative first to the dead car, AT A GOOD GROUND BUT NOT ON THE BATTERY. After all, you know something is wrong on this one.
With the the engine running, attach the negative to the ground of the good car, but preferably not on the battery.
Why? The bad battery could be gassing, could have an internal fault, and other Bad things. If you at the other other car when the battery pops a little bit, just hose out the engine compartment and replace the battery.
If you are clamping onto the bad battery when it ruptures, probably require a new mechanic and a new battery.

Disconnect ground first. Period.


.

[Zil]

I also received shocks from 12 volt batteries. I have witness three wet cell batteries explode. I have seen a man burn himself severely when his metal wristwatch band touched the positive terminal and the battery hold down bolt, while working around the battery without disconnecting the negative first.

[MarkSF]

Originally Posted by Zil

I also received shocks from 12 volt batteries. I have witness three wet cell batteries explode. I have seen a man burn himself severely when his metal wristwatch band touched the positive terminal and the battery hold down bolt, while working around the battery without disconnecting the negative first.

I'm sufficiently paranoid that, considering the amount of work I do on cars and boats, I won't own a watch with a metal strap. Always leather or plastic.

[MarkSF]

"There is no danger of explosion unless your battery is vigorously outgassing releasing hydrogen. "

And how do we guarantee that it isn't? I personally blew the entire top inch off a single 12V battery, not in a confined space, when a wrench shorted the two terminals out. The wrench started to glow red, and a few seconds later, boom. It had been on charge a few minutes before.

Is it possible that the internal parts of the battery were also glowing red, and thus set off the hydrogen inside?

I suspect that the possibility is not as low as you think it is.

[Symphony]

Great conversation in this thread. Thanks to all. We all worry about the hurricane storms and 50' rogue waves, but the situation that is likely to end your cruise is a simple fall on deck that breaks your leg and ankle, or a burn from fuel or batteries.

[rwidman]

Originally Posted by weavis

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.

If you disconnect either terminal, there is no circuit. If you disconnect the negative battery lead, it doesn't matter if you short the positive lead to ground because the battery has no connection to ground and there is not a complete circuit.

[rwidman]

Originally Posted by leftbrainstuff

In fact arcing is unlikely on low voltage DC. Air is extremely resistive. Thousands of volts are needed to arc. This is how spark plugs spark...............

A great example of how complete and potentially dangerous wrong information gets passed around on the Internet. I don't know where you heard this but it's absolutely wrong.

Take a small screwdriver and a nine volt "transistor radio" battery. Go into a semi-dark room and watch while you short the terminals with a screwdriver. There will be an arc when you make the connection and another when you break it.

Arcing is something that has to be taken into account when designing switches, especially for DC current and it's why there are voltage ratings on switches.

[rwidman]

I don't know if anyone mentioned this, but the best plan is to turn off all loads before disconnecting the battery cable. Just like you turn off all loads before connecting or disconnecting your boat's shorepower cable.

[thinwater]

Originally Posted by Symphony

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

I'm not sure why folks think this is true. Perhaps they shouldn't work on power systems. Same current break on both sides. How could it be otherwise? Where are those electrons going? It will arc on either side if there is load.

A better reason is that code requires positive connection to be guarded but not negatives. Therefor break the negative.

[JD-MDR]

I guess I didn't mean explosion like a fire bomb .I think the compression get so high it pops the caps off and spews acid all over.