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Old Mar 18, 2003, 12:41 AM
BEC
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Another case of premature HECell death

As I noted in my MiniFlash thread in Parkflyers, I was suspecting one of my two 8-cell packs of HECell 1100s was bad. Now I'm sure. One of the cells in that pack has discolored the solder on the positive end and there is a greasy substance on that end as well. Obviously there's been some leakage.

This pack is one of two I soldered up when the HECell 1100s first became available and it is possible that I overheated the cell when I built the pack, I suppose. But it was working fine last fall.

I only got 11 flights out of the pack. The cell died after the pack was used once after four months of inactivity and giving one shorter than normal flight (that 11th one). It acted as if it were discharged this morning even though I'd peaked it on the way to the park - less power than the 720 packs I've also used.

Has anyone else seen this failure mode?

Even though I'm moving towards LiPolys, perhaps it's time to try a pack or two of KAN 950s for my SB and MF while I wait for the next generation of LiPolys (which will be very useful in these two planes) to become available.

Oh - maximum charge rate has been around 1.5A and max discharge rate is 12A or so after the initial peak (16T miniAC in a GWS "B" ratio gearbox in my Switchback Sport RG.
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Old Mar 18, 2003, 01:35 AM
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Bernard, I have seen damaged seals on the Hecells due to overheating and the symptoms are as you described. I believe that is why David at Edogfight recommended welded connections to me. I have used my Hecells for a long time now while charging them at 1.5-1.6A in Reflex mode and drawing 12+ Amps from them. They are still going pretty strong.

Kasra
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Old Mar 18, 2003, 10:07 AM
BEC
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I've built a bunch of battery packs over the years, but if these are more fragile than some, or if I just goofed on that one cell (certainly possible), then it could be. I'm a little surprised it took so long to appear if I did, though, and with no "triggering event" other than trying to put the pack back into use after four months of disuse.

Thanks for the thoughts.
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Old Mar 18, 2003, 12:03 PM
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San Mateo, Ca , USA
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Bernard,

Are you sure the "greasy" stuff is not flux? It will discolor the solder, as it would seem.

The leaking cell will produce a clear liquid(almost like water) and turn into white-ish crystal as it dry. Just like the vented cell has the white fuzzy stuff on the positive terminal.

Overheated a cell will cause it to vent and produce a whiteish crystal.

Brian, an EAJ
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Old Mar 18, 2003, 01:02 PM
BEC
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Fairly sure, yes. First, I didn't use any separate flux when I built these two packs. Second, of 16 cells between the two packs, this one cell is the only one discolored and with some sort of fluid near the positive terminal.

I have now peeled back the heat shrink from that cell so it may dry and turn to the powder I've seen many times in my 20 years + of E-flight.....
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Old Mar 18, 2003, 11:49 PM
BEC
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Exposed to the air and allowed to dry it's white powder all right. Definitely leakage.
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Old Mar 19, 2003, 01:20 AM
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Bernard,

OK, it is leaking then, bad seal probably. It will dry up and die prematurely.

Brian, an EAJ
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Old Mar 19, 2003, 01:56 PM
BEC
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Based on the pack's recent performance, the cell is already dead.

Time for some KANs or maybe I should just wait for a Thunder Power pack that is tailored for this application.
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Old Mar 19, 2003, 07:46 PM
Giddy up!
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Woodridge, Illinois
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I killed a pack running it way too hot. I try not to use them in apps drawing more than 14-15 amps. Have plenty of cooling too. If they stay cool they last a long time...
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Old Mar 20, 2003, 01:12 AM
BEC
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Sal,

The peak load "hot off the charger" is about 14A and it settles to 11-12 in the first 30 seconds or so as I was using this pack (for 10 flights) in the SB.

Ordered two 8 cell KAN 950 packs from Doug at Mountain Models tonight. I expect these will be the last NiMHs I buy for a long long time unless I do a big airplane that needs lots of current (meaning 16-20 cell Sub-C type power).
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Old Mar 20, 2003, 02:59 PM
I hole jumped him!!!
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Amarillo Intl, Texas, United States
Joined Dec 2001
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Replacement HEcells

I do happen to have two new HEcells that I never got into packs. They are all I have left, and they're over in the classifieds area if someone's interested.
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