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Originally Posted by fuji
Thanks again Bob,
I would agree with your explanation.
So the failed cells had no electrolyte solution left whatsoever? Or enough of the separator was exposed that it was not cooled.
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The latter is the case. We're seeing current flows of 50-75 amps through the NHW11 batteries. As the electrolyte 'dries out', the area this current flows through becomes smaller and smaller, eventually forming point resistors that get hot enough to melt the plastic film separators. At that point the cell shorts and is unrecoverable. It has to be disassembled and rebuilt with a new separator.
Quote:
Originally Posted by fuji
. . . The poor efficiency of the used cells 30-60% shows that there is a non-conductive/high resistance film . . .
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It is the reduced area as the electrolyte 'dries out'. It really isn't drying in the conventional sense but rather the inevitable electrolysis to H_{2} and especially O_{2}.
Quote:
Originally Posted by fuji
. . . present on the electrodes which will heat the electrolyte solution leading to water blow off from the pressure release valves. Keeping the electrolyte topped up should prevent a "hard" failure. I think you would still need periodic reconditioning/restore by the method you showed to extend the life of the battery. . . .
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After the KOH electrolyte loses the hydrogen and oxygen, you have potassium left behind. Adding distilled water works because it forms KOH and releases H_{2} that out-gasses afterwards. I'm planning to measure how much to get an idea of the quantities involved. But also, there is something called electrode 'activation' that occurs during NiMH manufacture.
I don't have enough information about electrode activation but I suspect there is a layer of gas, probably oxygen, on the surface that has to be 'purged' so the KOH can reach the metal hydride. This layer would prevent the hydrogen molecules being absorbed into the hydride matrix. I suspect electrode activation applies enough charge to displace and/or generate enough hydrogen to convert the surface oxygen into hydride ions.
I am not convenced that battery reconditioning, the use of a .1C or lower charge, is as effective as claimed. The reason is I have some AA NiMH batteries that had excellent capacity, 2,000 mAhr, that after using in my digital camera now show barely 800 mAhr. I suspect excessive currents have led to premature, out-gassing and loss of capacity. Over in Prius Technical Stuff, I'm planning to do the test as well as putting in some probes to measure individual cell characteristics.
Bob Wilson