EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: engineheat on December 20, 2018, 03:46:07 am
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Say, a NiMh AA battery. What it short circuit and explode? If you place an ammeter in the water, would you measure a large current everywhere?
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Salt water might conduct electricity, but it's not a great conductor. A single AA NiMH battery is not going to be able to push out a lot of current at all.
If it's terminal voltage is above 1.23V you might get some electrolysis, but since the nominal voltage is 1.2V, you are only going to get that happening at the start of discharge of a fully charged cell.
It would be difficult to directly measure any current unless you took special measures, because you are in a 3D conductive environment - but you could waggle meter probes all around for voltage measurements.
Bottom line - it's going to be pretty tame to look at.
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you can easily measure it by connecting a battery to wires and putting them in water or by gluing the battery into a extension with a inline ammeter with a plate exposed to the water.
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I'll do an experiment...
but honestly, the reason I asked is kinda lame and paranoid. I'm trying to design a battery operated device to be attached to the body. The body can sweat and I'm a bit worried that the sweat will get into the battery compartment and cause a short circuit even if the rest of the circuit is well designed. Obviously, this will require a LOT of sweat and the compartment will have to be poorly designed. But just in case, even in the worst scenario if the battery is submerged or covered in an ionized fluid like sweat, how dangerous is it? I'm talking a AA Nimh battery.
I googled sea water conductance and it's about 5 S/m, much worse conductor than most metals, so perhaps just like someone mentioned, having the battery covered in sweat is NOT equivalent to shorting it with a wire. It might lead to a relatively large current and heat up a bit but that's about it?
Thanks
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I'll do an experiment...
but honestly, the reason I asked is kinda lame and paranoid. I'm trying to design a battery operated device to be attached to the body. The body can sweat and I'm a bit worried that the sweat will get into the battery compartment and cause a short circuit even if the rest of the circuit is well designed. Obviously, this will require a LOT of sweat and the compartment will have to be poorly designed. But just in case, even in the worst scenario if the battery is submerged or covered in an ionized fluid like sweat, how dangerous is it? I'm talking a AA Nimh battery.
I googled sea water conductance and it's about 5 S/m, much worse conductor than most metals, so perhaps just like someone mentioned, having the battery covered in sweat is NOT equivalent to shorting it with a wire. It might lead to a relatively large current and heat up a bit but that's about it?
The battery contacts will corrode, rendering the thing unusable and leave some (disgusting to some people) remains - that's all what will happen. No heat, explosions, whatever.
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Have you checked out fitness watches, garmin's etc.
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you can easily measure it by connecting a battery to wires and putting them in water or by gluing the battery into a extension with a inline ammeter with a plate exposed to the water.
That's an example of what I meant by:
.. unless you took special measures
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As someone who submerged a Motorola bag phone way back in the day in sea water (It is a very very long story :palm: ). They ran an external 6V SLA battery no smoke came out I wasn't electrocuted in the boat that was full of seawater (part of the story ;) ) and there was no heat in the pack when it got lifted out but the battery was toast due to internal corrosion.
My heart rate monitor for riding btw runs an o'ring seal where the battery goes in with a screw down hatch. No signs of any corrosion or sweat related issues internally apart from the elastic band which reeks after a long ride :phew:
So don't worry about the safety of the wearer but the life expectancy of the battery if you don't seal it in.
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Say, a NiMh AA battery. What it short circuit and explode? If you place an ammeter in the water, would you measure a large current everywhere?
To answer your heading, but not your question, the battery would get wet!
Apart from that, very little would happen.
An AA battery would be battling to supply a "large" current to a short circuit, let alone the several thousand \$\Omega\$ of salty water, due to its own fairly high internal resistance.
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keep in mind in water the battery will also be well cooled
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keep in mind in water the battery will also be well cooled
As it was not specified by the initial poster that conclusion is purely speculative on your part as no temperature was mentioned unlike the reply of vk6zgo stating a definate fact that the battery would be wet.
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so something defined as a bucket of salt water is slightly damp only?
i figure its not that bad of a description.
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keep in mind in water the battery will also be well cooled
Your statement is what is undefined and as such is speculative and based on information never stated by the original poster in any post.
First Law of Thermodynamics applies to your statement but can not be defined due to that lack of information.
'Water is wet' is defined no matter the salt concentration or not so vk6zgo is correct to point this out.
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The answer is "nothing much." Keep in mind that water also has a very high specific heat.
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I once dropped a Fluke 77 in a bucket of salt water.
A friend had suggested that you could calculate the % of salinity of the water by measuring the resistance between two identically sized pieces of Copper clad board (PCB), then using a suitable correction factor.
I was interested in this because I had a saltwater chlorinator in my pool.
Anyway, I fumbled it, & dropped the meter in the bucket of water.
I removed it immediately, cleaned the outside, then opened it up----strangely, very little water had got in.
A quick wash with distilled water, followed by methylated spirits, then left it in the Sun for several hours.
The only thing that failed was the continuity beeper.
It was a work meter, gave no other problems.
After some time, it was due to go to Philips for a routine fixed price checkup & calibration check.
It came back fitted with a new beeper.
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Electrolysis, of course.
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Say, a NiMh AA battery. What it short circuit and explode? If you place an ammeter in the water, would you measure a large current everywhere?
:palm:
:palm:
:palm:
:palm:
:palm:
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Saltwater WILL conduct electricity.
There will be electrolysis with Hydrogen appearing at the - Electrode and Oxygen appearing at the + electrode. If these gasses are contained in some way, they can recombine explosively. With a single battery, there would not be enough to do more than offer a "Pop" sound. With a larger battery it can be significant as an explosion.
The battery itself will be discharging at a pretty significant rate. Usually the entire case is the anode and thus only a fraction of an inch from the cathode (+). In salt water, this would be a pretty high current, low resistance discharge. The battery could burst but that would depend on its construction and how much the surrounding water was cooling it.
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You won't get jack diddly squat out of a 1.5v AA battery.
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Try it, it will even work with a nearly dead battery. You'll see bubbles and such. You could also electroplate things (or remove rust).
Don't inhale the fumes, as there is also chlorine in there. (very bad for your lungs)
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I've put 12v lipo batteries into buckets of salt water and not much happened. Few bubbles and over a couple of days they fully discharged with no heat, smoke, or puffing.
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Electrolysis will happen. It will generate hydrogen and oxygen (and maybe other gases such as chlorine if salt?) and also produce heat due to the resistance. Basically the water will act as a load. The battery will eventually deplete.
You can put 120v ac into water and do the same thing, you'll see more bubbles the closer the probes are. (use a GFCI for your safety if you're going to do that)
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Electrolysis will happen, but with the more normal metals use there will be no or very little oxygen. Instead the would be extra corrosion dissolution of the positive side metal. This way there is also a possible current and gas formation at low voltage: the oxidation of something like iron of copper gives the extra energy to produce some hydrogen even below 1.2 V.
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The only thing that failed was the continuity beeper.
Many moons ago, we produced a product that used water-washable flux AND a beeper. It was one of those self oscillating ones.
We would get a very low occurrence, but daily beeper failures. And yes, the seal was not removed until after the board had passed thru the dryer.
It turns out that the drier, in an attempt to increase throughput, would still leave a not-quite-dry board, and minuscule droplets could still ingress into the beeper after the seal was removed.
Increasing the drying time solved the problem. I was surprised how little humidity is required to damage one of those beepers.
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Electrolysis will happen. It will generate hydrogen and oxygen (and maybe other gases such as chlorine if salt?) and also produce heat due to the resistance. Basically the water will act as a load. The battery will eventually deplete.
You can put 120v ac into water and do the same thing, you'll see more bubbles the closer the probes are. (use a GFCI for your safety if you're going to do that)
Many years ago, I was talking to one of the guys in the Power group of the Telco where I worked. He was quite sure that if water flooded one of our offices, it would blow the breaker if the water got into an outlet. I took a power cord, cut the end off it and stripped back the insulation so there was exposed copper. Then I dropped it into about a one liter bowl of tap water. The current was something like a hundred milliamps at 110 Vac. There is no significant electrolysis when you're using AC because the polarity reverses too fast for the reaction to get going.
When I was a kid (back when dinosaurs roamed the earth), we had a vaporizer at home to put steam into the air when you were sick and having trouble breathing. It stopped working so I took it apart and found that it was nothing but two large metal plates directly connected to the mains. I cleaned the hard water deposits off the plates and it worked fine. The large plates were needed to get enough current flow to heat the water so it would turn into steam. The instructions said to add a pinch of salt if the unit didn't produce lots of steam. The nice thing about that design is that when you ran out of water, the steam and the current flow stopped. There was no danger of overheating.
Ed
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While I am sure that all of the above answers are technically correct, I would trust only the ones that report actual experience.
My experience is that it is usually hard to tell that anything is happening with short (seconds to minutes) exposure to fresh water, brackish water or sweat. I am sure that some current is being drawn which will reduce battery life, but no bubbles of gas, no heat, no swelling of battery.
Can't really report any personal experience with exposure in the minutes to hours duration range.
Longer exposures (days to weeks) do result in corrosion of exposed metal parts, but no noticeable heating or swelling of the battery. Battery life is definitely impacted, and often ended.
Very long exposures (months to years) turns the battery into a nasty corroded pile of gunk.
All of these observations are subject to change under conditions other than what I have observed. For example the nasty corroded pile of gunk stage can probably be achieved in only a few days or weeks under some conditions, say a hot ocean beach for example.
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One thing I find interesting is that pure distilled water is an excellent insulator and also has a fairly high dielectric constant. Enough that one could make useful capacitors perhaps using it.
the only problem is even if you start out with distilled water salts are present in so many places and things they degrade that perfect nonconductance in almost all real life situations.
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Many years ago, I was talking to one of the guys in the Power group of the Telco where I worked. He was quite sure that if water flooded one of our offices, it would blow the breaker if the water got into an outlet. I took a power cord, cut the end off it and stripped back the insulation so there was exposed copper. Then I dropped it into about a one liter bowl of tap water. The current was something like a hundred milliamps at 110 Vac. There is no significant electrolysis when you're using AC because the polarity reverses too fast for the reaction to get going.
When I was a kid (back when dinosaurs roamed the earth), we had a vaporizer at home to put steam into the air when you were sick and having trouble breathing. It stopped working so I took it apart and found that it was nothing but two large metal plates directly connected to the mains. I cleaned the hard water deposits off the plates and it worked fine. The large plates were needed to get enough current flow to heat the water so it would turn into steam. The instructions said to add a pinch of salt if the unit didn't produce lots of steam. The nice thing about that design is that when you ran out of water, the steam and the current flow stopped. There was no danger of overheating.
Ed
Relevant videos:
https://www.youtube.com/watch?v=dcrY59nGxBg (https://www.youtube.com/watch?v=dcrY59nGxBg)
https://www.youtube.com/watch?v=qQssSAf6DPA (https://www.youtube.com/watch?v=qQssSAf6DPA)
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Saltwater WILL conduct electricity.
The battery itself will be discharging at a pretty significant rate. Usually the entire case is the anode and thus only a fraction of an inch from the cathode (+). In salt water, this would be a pretty high current, low resistance discharge. The battery could burst but that would depend on its construction and how much the surrounding water was cooling it.
I understand salt water will conduct, but according to Google the conductance of sea water is only 5 Siemen/m, this is orders of magnitudes lower than most metals. I'm primarily concerned with "short circuit" current and heat, not so much about corrosion or battery damage.
I think by putting the battery in salt water, the current out of the battery is way lower than if one short circuited with a wire. For an AA battery, a "rough" calculation shows it's akin to connecting the ends of the battery with a 30 ohm or 50 ohm resistor. Yes it'll drain the battery but it won't explode or burn someone, I think...
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I dunked a device in the ocean once by accident, it corroded the hell out of contacts, component leads and PCB traces in the few minutes it took me to retrieve it and get the battery out but it wasn't very exciting at all. If you were to drop a AA in seawater it would drain the battery flat over a period of a few hours while simultaneously rusting the ends, but it's not going to be exciting. Salt water is a conductor, but not an exceptionally good one. It used to be common to use barrels of salt water as rheostats for low budget theatrical lighting.