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RE: Is this NIMH enough? - 5/20/2005 4:45 AM   
JoeAirPort



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Status: offline 		Rajul, I know that this is a quote you posted here and it's not your words but I have never heard that before and it doesn't make sense (but maybe I am not a battery expert either). How would a battery know what rate it's charged at? If it's fully charged it's fully charged. I do agree that a fast high current pulse will drop the voltage on a high interal impedence pack. But it's hard to believe that a 2C charged battery discharges at a higher rate than a C/10. If this is true then I have learned something new (and will charge my packs differently from now on).

quote:

ORIGINAL: rajul

I received this statement from a charger manufacturer:

"It is recommended charging NiCd/NiH with higher current then C/10 if
you use the battery with higher discharging pulses. rx batteries
charged by C/10 failed at the voltage when higher discharging pulses
are needed. If the voltage is too low for a short moment, the rx fail-
safe takes over flying your model.
If you need high discharging current then you have to charge with higher current. e.g. the
other extreme is using NiCd/NiH batteries in electric flying models you
have no power in using a C/10 charged battery, but you can operate at
full capacity with a >2C charged battery"

FWIW...............




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RE: Is this NIMH enough? - 5/20/2005 5:02 AM   
rajul



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Status: offline 		Joe, you're right. I'm still trying to figure out that statement myself.

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RE: Is this NIMH enough? - 5/20/2005 4:23 PM   
Hangtime



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Status: offline 		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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RE: Is this NIMH enough? - 5/20/2005 10:14 PM   
JoeAirPort



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Status: offline 		I'm looking for reliability in my rx packs so I always charge slow now. I only do fast charging at the field in case I run low on capacity, and that's after a LOT of flying.

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RE: Is this NIMH enough? - 5/20/2005 10:30 PM   
Rodney



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Status: offline 		Joeairport, I have to agree with everything you said above as it jibes with my experiences over the last couple of decades. A batteries chemistry stays the same, whether you charge slow or fast. As long as you watch temperatures and keep them in bounds, the charge rate is somewhat a secondary consideration.

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RE: Is this NIMH enough? - 5/23/2005 7:09 PM   
zope_pope



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Status: offline 		Just an update. I charged a 6v 600mah nicad and threw it in the airplane and the voltage indicator didn't even flicker. put my fully charged 2700mah nimh on the airplane and it wouldn't stay at green for the life of it. WOW! What an eye opener!

I got the battery from NoBS and what a quality pack. Even the shrink wrap is good feeling. It comes with many tips on installation and a chart to keep track of the condition of the battery. Just something you have to see. The difference is huge. I will give you guys an update after the formation charges are done. I am doing three slow charges at .1c and discharging each at 500ma. I can't wait to try these out.

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RE: Is this NIMH enough? - 5/23/2005 7:31 PM   
famousdave


 

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Status: offline 		You have just discovered what many of us have known for a long time... if you are going to run NiMH NoBS is the only way to go!!

DP

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RE: Is this NIMH enough? - 5/26/2005 7:22 PM   
zope_pope



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Status: offline 		Hey guys,
I have completed my formation charging. I have ordered the 1950, and I assumed this was 1950mah, but when I cycle it, I can get a little more than 1750 each time. Did i miss something, or is pack malfunctioning or what. I formation charged at .20amps and discharged at .5amps. Did this 3 times and all I can get is 1750. Let me know what ya think. Thanks.
Adam

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RE: Is this NIMH enough? - 5/26/2005 9:43 PM   
JoeAirPort



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Status: offline 		This is a common confusion with discharging batteries and then charging them (cycling). What voltage per cell did your charger stop discharging at? It can vary between 0.9 and 1.1 volts and changes how much capacity the battery will take to reach peak. So your battery is probably perfectly fine but they advertise 1950 mah at a 0.9 per cell starting voltage for the charge cycle. For example if you use a 1.0 volt per cell discharge voltage your battery will only take 1700 mah to reach full charge. If you use a 1.1 volt per cell discharge voltage your battery may only take 1500 mah to reach peak.

I see these posts all the time and I think the batteries are in most cases fine.


quote:

ORIGINAL: zope_pope

Hey guys,
I have completed my formation charging. I have ordered the 1950, and I assumed this was 1950mah, but when I cycle it, I can get a little more than 1750 each time. Did i miss something, or is pack malfunctioning or what. I formation charged at .20amps and discharged at .5amps. Did this 3 times and all I can get is 1750. Let me know what ya think. Thanks.
Adam



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RE: Is this NIMH enough? - 5/26/2005 9:47 PM   
zope_pope



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Status: offline 		Cool. never thought about it like that. I saw the setting just thought that the batts were tested at 1.1v cuttoff. Thats what mine is set to. Thanks joe, I will try .9 tonight, and hopefully fly the batt this weekend.

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RE: Is this NIMH enough? - 5/27/2005 4:50 AM   
zope_pope



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Status: offline 		For those of you who read the original message. My Mistake. I thought I had run my batt on the charge cycle. Instead I had just discharged it. BATTERY ROCKS!!!

Adam

< Message edited by zope_pope -- 5/27/2005 5:52 AM >

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RE: Is this NIMH enough? - 5/27/2005 6:26 AM   
RCAddiction



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quote:

ORIGINAL: zope_pope

For those of you who read the original message. My Mistake. I thought I had run my batt on the charge cycle. Instead I had just discharged it. BATTERY ROCKS!!!

Adam


Do you mean the 2700 is okay?

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RE: Is this NIMH enough? - 5/27/2005 7:15 AM   
zope_pope



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Status: offline 		Oh no, that one is crap. The NoBS 1950mah is awesome. It works pretty much like a nicad. Unbelieveable!

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RE: Is this NIMH enough? - 5/28/2005 6:44 AM   
JoeAirPort



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Status: offline 		Zope,

Did you slow charge the pack once and then do fast peak charges after that? 1C?

quote:

ORIGINAL: zope_pope

Oh no, that one is crap. The NoBS 1950mah is awesome. It works pretty much like a nicad. Unbelieveable!



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RE: Is this NIMH enough? - 5/28/2005 9:12 AM   
zope_pope



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Status: offline 		no, i did three slow charges and discharges and then one fast and a discharge to check capacity. Flew the battery today, after 3 long flights, the voltage indicator still didn't budge from the green indication. I am one happy camper!

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RE: Is this NIMH enough? - 5/29/2005 12:27 AM   
JoeAirPort



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Status: offline 		Mine was 4.5 V at 250 ma load when I recieved it. I slow charged it at 200 ma (0.20 amps) for 14 hours. I discharged it to 5.5 volts and it had 1667 mah. If I had discharged it down to 5.0 volts or 4.5 volts, I'm sure it would have had the rated capacity of 1950 mah.

I'm slow charging it again right now. I will use this battery for the Maiden of my DPM 27% Edge this weekend (tomorrow or Monday depending on the darn wind).

Joe

quote:

ORIGINAL: zope_pope

no, i did three slow charges and discharges and then one fast and a discharge to check capacity. Flew the battery today, after 3 long flights, the voltage indicator still didn't budge from the green indication. I am one happy camper!



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