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Old Nov 05, 2006, 01:18 AM
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Larry3215's Avatar
Gig Harbor, Washington, United States
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Salt water baths DO NOT work - more proof

I had a 3S 1320 pack that had suffered some minor crash damage, then got over discharged and it finally puffed very slightly.

I decided to do one more test of the "salt water bath" urban ledgend that keeps croping up over and over and over again.

This pack was about 1/2 charged when I dumped it into a 1 quart mason jar 3/4 filled with salt water. Its been in there for the entire summer because I forgot about it untill tonight.

Its been so long all the salt water evaporated.

I just desected the pack and ALL THREE of the positive aluminum tabs are completely erroded from the cells.

Tomorrow I will take the pack outside and puncture it to see how much stored energy is still in the cells.

DONT use salt water to discharge cells or packs for disposal!!

It does not work.

Larry
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Old Nov 05, 2006, 01:32 AM
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Lidingo, Sweden
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I think its been mentioned several times before that salt water is NOT for discharging the cells, I dont remember incorrectly, you should submerge them in saltwater AND puncture the cells.

Im no chemist, but I was at least under the belief that if you do that, the salt in the water prevents the battery to burn which it does in normal water.
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Old Nov 05, 2006, 02:03 AM
rogue_medic
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I hate to disagree, but the reason for the saltwater bath, is indeed for discharging damaged cells

of course the tabs are going to corrode if you leave them in there for weeks <months?> on end , as salt water will corrode most things.


did you check the voltages <well you couldnt in this case , because the tabs were eroded,> but i can almost garentee that they would have been at zero within a couple of days < that is after all the point, to kill them so they dont short/catch fire/self incinerate, so tyhey can be safley disposed of.

regular water is non conductive , at the voltages in question< well negligibly so >that is why the salt is added, it then becomes conductive<shown by the electrolysis>

i can add from personal experience that once burning, a lipo will not be put out by water even if it is salty

.... flame suit on ....
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Old Nov 05, 2006, 02:21 AM
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<sigh>

See what I mean? It just wont die!

Larry
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Old Nov 05, 2006, 02:36 AM
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Larry,

What exactly do you mean when you say it doesn't work? Do the cells still show voltage? If not, it worked perfectly.
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Old Nov 05, 2006, 03:42 AM
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I started another thread on this some time back.

http://www.rcgroups.com/forums/showt...t=510204&pp=53

In it there is much discussion of the principles behind whats happening as well as many photos showing the process starting on about post 53. I also did measurements of the current flows and timed how long it takes for large chunks of aluminum to errode away to nothing.

Basically, when ever you run a current thru a salt water bath, the electrodes in the water will be subject to a process called galvanic corrosion.

Do a Google search and you will get hundreds of pages of explanations.

What it boils down to is that the aluminum tabs on our lithium cells will errode away long before the batery can discharge to a safe level.

I have left packs in salt water for weeks and they still had the majority of the charge in them even tho they read zero volts at the main leads. They were still very very dangerous!

When you put a charged pack in salt water, the aluminum tabs errode away and the circuit is then broken. If you measure the pack with a meter it reads zero volts - but thats because the curcuit is no longer complete.

Once the tabs are gone, the discharging essentially stops and the cells still have significant and dangerous charge!!!

The reason for this thread was to show that the positive tabs were completely gone on each cell in the pack. None of the three cells were still conected together.

Tomorrow, when I puncture the pack, I fully expect to get some heat and sparks if not actual flames.

I'll post more pics then.

Larry
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Old Nov 05, 2006, 05:30 AM
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The ONLY reason for the salt bath is to reduce the Lithium to a safe compound. The correct procedure to neutralise danaged Lipo cells is to first discharge them to zero (or close) with a resistive load, puncture them in a lot of places THEN put them in the salt water for a few days. The cells will then be safe to throw in the trash. I did this to my damaged packs a few months ago but I forgot about discharging them first. I got some violent activity when I punctured my fully charged packs. The salt bath bubbled and spewed a lot of smoke for a while but eventually settled down. Of course, I learned another thing that day. If you are going to puncture charged packs like I did, don't use a plastic bucket to contain the salt water. Mine burnt through the bottom before the cells stopped cooking.
Charles
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Old Nov 05, 2006, 05:40 AM
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*Sigh* I thought we'd been through this before.....

The briny bath should not be used to discharge Lipos. The order of operation here is to:

*Use a small light bulb or resistor to discharge the pack completely

*Carefully slice open the skin of each cell (use caution here - make sure the cells are completely dead before you do this)

*Toss them into the salt water bath to neutralize the electrolyte in the cells. Let them bathe for a few hours.

*Dispose of the now dead pack into the garbage.


Brad
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Old Nov 05, 2006, 06:48 AM
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everydayflyer's Avatar
Haralson County GA. USA
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But someone once posted that the salt water would discharge them so it must be correct. If it worked before it will still work now correct?.

Someone once posted that 2.4 volts per cell was a good safe LVC and that is still true ,correct?

I have a hard enought time learning the right way to do something the right way once. I sure wish the LiPoly rules would stop changing!!!!


Charles
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Old Nov 05, 2006, 07:07 AM
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Lidingo, Sweden
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Quote:
I sure wish the LiPoly rules would stop changing!!!!
Sooo true
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Old Nov 05, 2006, 07:46 AM
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Quote:
Originally Posted by everydayflyer
But someone once posted that the salt water would discharge them so it must be correct. If it worked before it will still work now correct?.

Someone once posted that 2.4 volts per cell was a good safe LVC and that is still true ,correct?

I have a hard enought time learning the right way to do something the right way once. I sure wish the LiPoly rules would stop changing!!!!


Charles
Someone once also posted that Lipo's would not burn -
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Old Nov 05, 2006, 07:54 AM
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Quote:
Originally Posted by hoppy
Someone once also posted that Lipo's would not burn -

You mean they will burn? That is news to me. Perhaps I had better stop charging them in my garage next to those two 5 gallon cans of gasoline and my gas water heater.

Charles
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Old Nov 05, 2006, 08:39 AM
turn, turn, turn.
Athol, Massachusetts
Joined Oct 2005
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Well, lets see if the salt bathed packs have any charge left....I'm betting they are dead.
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Old Nov 05, 2006, 02:03 PM
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LiPo's are only different in their chemistry

An aluminum can tossed into salt water will be gone after a few days because it will be completey oxidized to aluminum salts. This is exactly why the Al electrodes on a LiPo are gone as noted in previous posts to this thread.

There is nothing mysterious about Lipo's. All batteries, including LiPo's are electrochemical cells consisting of an anode, cathode, and a non-electron conducting seperator containing an electrolyte that physically seperates the anode from the cathode. Only the chemistry is different between the batteries.

As a battery operates, electrons flow from the anode through an external circuit to the cathode. The act of giving up an electron at the anode is called oxidiation and this leaves behind cations (positively charged ions). The flow of electrons to the cathode results in reduction of cations to elemental atoms (i.e., Cu(II) --> Cu metal, elemental Cu has no charge), leaving behind the negatively charged counterions or anions present before the reduction occured). If this was to continue positive charge would build up at the anode and negative charge at the cathode. Since the anode and cathode are physically separated in space, this would result in charge separation. Charge separation is something nature abhors without an input of energy (i.e., photosynthesis, neurons, heart, etc.). Hence, in order for the battery to continue to operate the build up of charge separation at the anode and cathode is prevented by the flow of ions through the separator...positive ions flow to the cathode where anions are accumulating and negative ions flow to the anode. The key point to remember in a battery is that electrons flow from anode to cathode (to perform work) and ions flow between cathode and anode to prevent charge separation, completing the circuit.

To completely discharge a battery, the chemical potential between anode and cathode has to become zero (or as close as practical). This can only be accomplished by allowing the chemical reaction between anode and cathode to go to completion. This can be done by passing current from the battery through a resistive circuit such as a bank of bulbs, etc, as noted before. Or a direct, ELECTRON conductive path between the anode and cathode can be created. By puncturing the battery walls and submersing into a conductive solution such as salt water such a direct electron conductive path between anode and cathode is established. If this is done with a battery still having a significant amount of charge remaining, this can become pretty exciting because elemental Li (or some alloy) will be present. The reductive potental of Li is high enough to reduce water to lithium hydroxide and hydrogen (which burns very effectively). Initially, distilled water won't work as effectively because its conductivity is much lower than salt water. However, eventually the ions in the separator would diffuse/leach in to the water, increasing the waters conductivity and eventually completing the short circuit.

We shouldn't be surprised about the hazards of the "new generation" of batteries. An increase in battery cell voltage from 1.2V (NiCd, NiMH) to 3.7V for LiPos should be looked at from the underlying chemical reason it is occuring...an increase in chemical potential. The greater the chemical potential the more unintended pathways (i.e., fire) mother nature will permit the energy to be released. Therefore, it can just about be gauranteed that if a battery with a cell voltage higher than a LiPo can be developed, it will have more or a higher level of hazards than todays state of the art LiPo's. Should we be fearful of them? No, not at all. Just respectful of the underlying chemistry, armed with the knowledge that the more we demand from chemical systems, the more sensitive the systems will be to operate within.

Mike
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Old Nov 05, 2006, 02:26 PM
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First of all - aluminum cans sitting in salt water are not gone after a few days. Its a simple experiment to do. Try it and see how long it takes your can to disolve

It takes a current flow to accelerate the process - Google galvanic corrosion for a full explanation.

Boats with aluminum outdrives and props sit in salt water for months and years on end and as long as there is no current flow from onboard batteries or dissimilar metals, there is only surface corosion.

How many aluminum hulled boats do you supose there are sitting in your local marina? They dont disolve in a few days - or years.

Its the combination of salt water, aluminum and a current flow that disolves the tabs on lipos.

OK, I just cut open the pack with a shovel and it still had a significant amount of energy in it. It was hard to tell, but I think one cell was mostly drained and the other two were not and did some sparking.

In the pics below you cant see it well at all, but there was smoke comming off the pack and it was quite warm.

I tossed it back in some salt water after cutting and it bubbled strongly for a few seconds then tapered off. I didnt think to get the temp gun untill several minutes had passed. The water started at about 70 deg - room temp. So there was enough energy to raise 1/2 a quart of water about 30 deg in a few minutes.

So - after more than 3 MONTHS in salt water the pack still had a fair amount of energy in it.

In other words - the salt water bath did NOT discharge the pack to a safe level even after months of submersion.

Once the aluminum tabs are gone the discharge essentially stops.

Try the experiment for your self if you still doubt it

Larry
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