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Sick NiMh Cells Burn During Charge

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Old 01-26-2005, 01:39 PM
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E-Challenged
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Default Sick NiMh Cells Burn During Charge

I had a sick HE-1100 8-cell NiMh pack that had lost power and duration and had been running hot. I noticed that after charging, some of the cells had lower voltage than others. I charged the individual cells to get them up to the same level as the others then set the pack aside for a day. Next, I cycled the pack on my new Hobbico Elite Accu Cycle charger setting the charge rate at 2.0 amps ( 2C). I watched the display for a while, everything seemed ok, later there was acrid smoke coming from one or two cells. I quickly disconnected everything. The cells were now very hot, two adjacent cells had brown scorch marks on the ends but were not shorted together. The workbench was charred where the two cells were sitting. It may be that one cell vented very hot gas, can't see any evidence of shorting.

Lesson learned: Don't leave the scene, keep an eye on what's happening when charging a tired pack(??)
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Old 01-26-2005, 02:30 PM
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Default RE: Sick NiMh Cells Burn During Charge

Post Mortem: [:@]The charred mark on the work bench was in the shape of a T-pin. I found a T pin nearby with charring on it. During emergency, Hobbico Elite was beeping and error message "battery voltage too low" was displayed. Cell ends were uninsulated/uncovered for testing. I must have set the pack on a T-pin which didn't make contact right away. Later, two cells ends shorted via the T-pin causing intense heat and smoke external to the cells. I am not a beginner which makes this all the more embarassing. Lesson learned: Charge exposed cells with extreme care in area clear of debris.
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Old 01-26-2005, 02:35 PM
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Default RE: Sick NiMh Cells Burn During Charge

In the first place, you were charging at a much to high a rate. On NiMh, you should limit charge current to 1C, in your case 1.1 amps.
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Old 01-27-2005, 01:14 AM
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Default RE: Sick NiMh Cells Burn During Charge

You are right that 2C is too high of a charge rate for maximum Nimh longevity but you'll find that many e-flyers charge NiMh packs in this capacity range at a 2C rate especially when field charging. I do plan to reduce charge rates to 1C on any future Nimh packs that I buy.
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