Hangtime
Posts: 346
Joined: 6/5/2002
From: Babylon,
NY, USA
Status: offline
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One of the interesting discoveries I ran into years ago when flying competition electric sailpanes was the high load performance of a low impedance sub-c Nicad pack in a high load enviornment was better immediately after as fast charge than it was immediately after a c/10 charge.
Of course, the current drain I was asking from the pack was 50 amps continuous for 45 seconds.
Now, as a data point, that's interesting; and i suppose that's what the charger mfg is refering too. However, it should be considered that the 3C fast charge rates and high current discharge patterns produced a duty cycle lifetime on that low impedance nicad pack that was only about 20 runs tops, and the pack was toast... and using the same 2C or 3C charge pattern and routines on a high impedance NiMH pack lead to total meltdown a lot sooner than that.
Using 'car pack' charging techniques on a pack used in Rx applications is mighty abusive to the pack. Since our nominal current draw in big bird Rx & servo applications at idle is about 1.2 to 2.0 amps and at max load about 20 amps peak (momentary) we get the benifit of the low impedance pack being able to shoulder those draws, and c/10 slow charge routines with an occasional fast charge at 1C are not abusive to the cells... a reasonable non-destructive appoach to charger support for packs in moderate draw environments.
Next we run into ultra high capacity NiMH AA cells as found in Raul's Tx pack.. VERY high impedance pack.. massive capacity, but the impedance rating is published as 25mOhms and I suspect it's actually a mite higher than that. That lil bugger should NEVER see a charge rate higher than 1 amp and even then it should be monitored very closely with a temp probe. Better to support it at or under the c/10 rate for best longevity. On the up side, it's in a Tx enviornment.. moderate temps, no vibration, constant current load under .5 amps in duty. Good for what it's intended for, but that lil cell in a Rx enviornment is a crash waiting to happen.. vibration, heat and current loads beyond it's ability to shoulder coupled with it's intolerance to fast charging make it a marginal cell even in a .40 sized 3-D plane as a Rx pack.. which is why we don't offer it in any configuration other than as Tx pack.
Different cell types & charge profiles for entirely different use profiles...
Hope this rambling is of some use!
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Steve Anthony
www.hangtimes.com
NoBS Batteries
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RE: Is this NIMH enough? - 
