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should nimh get warm when charging

Old 08-06-2008, 10:48 PM
  #1  
btyler
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Default should nimh get warm when charging

I keep hearing that a nimh battery should be warm when fully charged, well i have a wing dragon 4 and the wall wart charger that came with it. I have 2 batteries now and if i were to follow the manuals instruction i would charge them for 4 hours. Problem is i get at best 3 mins of flight and no power to climb form this and the battery is not warm in the slightest. The other day i forgot i left it plugged in for about 20 hours it was slightly warm is this what i am looking for i know 20 hours is pretty crazy but it was only slightly warm not hot not even really all that warm but more than usual when it charges for 4 hours does anyone have any idea what up with this ?
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Old 08-06-2008, 10:57 PM
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Lieutenant Loughead
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Hmmm -- lots of things here...

1) Yes, NiMh batteries should get warm when "peak charged". ("Peak charged" means the battery is "topped off", and completely charged. NiMh batteries actually LIKE this, becuase they have a high "shelf discharge" rate.)

2) Yes, NiMh batteries should get warm when discharged. This is normal.

3) A NiMh battery left on a charger for 20 hours will be given time to cool -- it will NOT be warm. This is because the charger has shut off, and is no longer "peak charging".

4) My best guess is that your "wall wart" charger is defective -- what is the number of cells, and the battery voltage immediately after charging?
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Old 08-06-2008, 10:59 PM
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btyler
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8 cell 9.6 1000mah and a 11000mah
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Old 08-06-2008, 11:09 PM
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after charging its somwherea around 11.6ish
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Old 08-06-2008, 11:14 PM
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Moved to Batteries and Chargers.
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Old 08-06-2008, 11:25 PM
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flydiver
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First-check the output of the wall wart. If it's dead there will be no voltage reading or no where near what the label says. Yeah, you need a voltmeter for that. If you are in this sport and don't have one, you need one.

Is that a STOCK battery and charger? If an aftermarket 8-cell and the charger is only good for 7-cells you have a no go situation. You need a new charger.

If the charger checks out, what is the mAh rating (on the label). Do the math:
Say it puts out 500mA (.5A)

1000mAh-battery divided by 500mA-(charger) = 2 hours. You should add about 20% if it is a 'dumb' charger-does not stop until unplugged.
Or 1000(battery) divided by 200mA-(charger) = 5 hours. For much lower charger rate add a 50% fudge factor.

If it's a peak charger it should turn itself off when finished. Usually these charge much quicker, like an hour.
Most wall wart chargers are dumb or 'timed'. Check your manual. There should be info.
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Old 08-06-2008, 11:27 PM
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btyler
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its dumb and it sounds like its a total pos
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Old 08-07-2008, 03:14 AM
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Originally Posted by btyler View Post
8 cell 9.6 1000mah and a 11000mah
I'm going to assume you meant to say it's:
  • 8 cells
  • 9.6 volts (nominal = 1.2v * 8 cells = 9.6 volts)
  • 1000 mAh and 1100 mAh (not 11000mAh!!!)
Originally Posted by btyler View Post
after charging its somwherea around 11.6ish
Okay -- that's actually really good, though I would have expected it to be closer to 10.5 or 11.0 volts... However, the point is, the charger IS charging the battery.

I wonder if the batteries have been over charged, or over DIScharged... They might just be a cheap (and inexpensive) batteries, and they might just be at the end of their life-cycle... :o

Originally Posted by btyler View Post
its dumb and it sounds like its a total pos
Maybe -- but this might be a good opportunity to make the jump into LiPo batteries! The battery prices have really fallen recently, and the chargers can be bought for as little as $20.

If you are interested in LiPo technology, send me a PM.
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Old 08-07-2008, 06:21 AM
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Larry3215
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Most wall warts are just dumb trickle chargers. Lok on the charger and see if there is an output ma rating and a voltage.

With trickle chargers, the charge rate may be low enough that the pack will never get really warm.
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Old 08-07-2008, 03:58 PM
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everydayflyer
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First off most new battery packs are not formed by the manufacture as it takes to much time. There is very good reasons why a preminum Zapped / matched racing pack cost a lot more than a bargin priced run of the mill pack.

In most cases it will take a mininum of 10 cycles to get anything near best performance from a new NiMH battery. NiMh do perform best when warm. During charge them start to warm when they are fully charged and in fact are being overcharged. Using a slow charge this is how they are balanced. Heating using a peak charger depends on the charging rate , quality of the cells in the battery,the quality of the charger,the peak sensitivity setting of the charger, quality of the charging leads,connectors and many other variables.


Charles
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Old 08-07-2008, 09:18 PM
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Rodneh
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Heat in itself is a sign of overcharge and is not desireable. Heat can come from two primary but different sources. When charging, there is always some heat created due to the internal resistance elements of the charging cycle; this is usually very small and inconsequential unless you are using excessive charge rates. The second cause which occurs near the end of charge is the recombination of oxygen with other elements at the positive terminal (in short combustion or fire internally) as at the end of charge the oxygen can not be used to charge any further. This form of heat is not good, in fact detrimental to the life of the cell as it accelerates the deteriation of the insulation material between cell plates and, if extreme, builds up internal pressure and can blow the seal at the positive end of the case. Short story, if getting hot at end of charge, you have exceeded the limit of chemical change and are now slowly destroying your battery.
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