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Dead Cell?

Old 02-25-2013, 07:39 PM
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impulse09
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Default Dead Cell?

Is it possible for a battery pack to function with a dead cell? It's an 8-cell NiMH, and after charging at .1C for 15-16hrs the voltage is around 9.6-9.5V. My other pack is ~11V immediately after charging. Capacity isn't as stated either when the pack is cycled.
Old 02-25-2013, 10:07 PM
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ALWAYSPDG
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Default RE: Dead Cell?

Hey Don.

When you charged the battery, did it feel warm to the touch when you pulled it off the charger?


Mike
Old 02-26-2013, 05:40 AM
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Rodney
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Default RE: Dead Cell?

Yes, this is normal when a cell shorts out. You have a dead cell in the pack.
Old 02-26-2013, 11:57 AM
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Default RE: Dead Cell?

Yes, this is normal when a cell shorts out. You have a dead cell in the pack. 

If a Nimh 'shorts out' (very rare), they can vent and even explode, worse than a Lipo. 

NiMh cells fail open, zero volts, and render a pack useless. I suspect you have a cell going bad - won't take a charge due to high internal resistance. You can poke a T-pin through the shrink wrap and check each cell individually. 
Old 02-27-2013, 06:26 AM
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Default RE: Dead Cell?

It is not warm off the charger. What will poking with a T-pin do?
Old 02-27-2013, 08:20 AM
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Default RE: Dead Cell?

 What will poking with a T-pin do? 

A T-pin at each end of a cell in a pack is where you can measure the cell voltage individually. You need to be careful not to touch both cells and cause a short. I measure off each negative side of adjoining cells. It's easy if you think about it. 
Old 02-27-2013, 08:26 AM
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Rodney
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Default RE: Dead Cell?


ORIGINAL: eddieC

Yes, this is normal when a cell shorts out. You have a dead cell in the pack.

If a Nimh 'shorts out' (very rare), they can vent and even explode, worse than a Lipo.

NiMh cells fail open, zero volts, and render a pack useless. I suspect you have a cell going bad - won't take a charge due to high internal resistance. You can poke a T-pin through the shrink wrap and check each cell individually.
I find this totally wrong. The most common failure for both NiCad and NiMh is to have a shorted cell. Anytime that you can measure some voltage from the pack and it is significantly less than normal, the cell is shorted. If it were open, you would not measure any voltage, a very rare type of failure for these types of batteries. These types of shorts are usually caused by dendrites forming and piercing the membrane separating the two plates which shorts out the cell. Now, an external short, like connecting the two power leads together, will usually just burn the wires in two or burn through and leave an open circuit which is external to the battery itself. A Nixx battery with a vent will never explode. However, some of the hard shell batteries constructed without a vent (rarely seen) can build up a high internal pressure if shorted and blow the end cap off with considerable force. By the way, LiPo's do not explode, they burn which is quite different from exploding.
Old 02-27-2013, 09:22 AM
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Default RE: Dead Cell?

How about I carefully disassemble and post pics? I think I'm going to switch to LiFe anyway.
Old 02-27-2013, 09:35 AM
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Default RE: Dead Cell?

I find this totally wrong.

Your description is not based on facts. Larry Sribnick of SR Batteries details the differences in nicad and nimh in detail. He's supplied the gov't for decades. Maybe you should call him and relate your unique experiences. 
One big diff is: NiMhs fail open, Nicads fail closed. A nicad 5-cell rx pack with a failed cell may get you home, but a NiMh 5-cell will have you walking to the crash site. 
I didn't mean to infer Lipos would explode, but that a NiMh can detonate like a shotgun shell; loud, and lots of hot shrapnel. Pretty rare, unlike Lipo 'thermal events'. 

I've experienced a failed NiMh, pack voltage 0.0, located and replaced the bad cell. With a bad cell, the pack won't charge either. 
Using temperature to determine level of charge is beyond flawed. A pack under anything above an overnight charge will warm up and stay there. Pack voltage and a cycle is the only way to determine pack capacity. 
A NiMh pack may show fully charged voltage, but have little capacity. It's called a false peak. Also have experienced that, even after overnight charge. 

Old 03-06-2013, 06:43 AM
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Default RE: Dead Cell?

Ummmm, 7 of 8 cells read about 1.3V.

1 cell is 0.0V.

I can still get 9.15V total on the pack.
Old 03-06-2013, 07:51 AM
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Default RE: Dead Cell?

Ummmm, 7 of 8 cells read about 1.3V. 
1 cell is 0.0V. 
I can still get 9.15V total on the pack. 

Odd. I've had a pack with an open cell, 0.0v on the pack. 
 I've had a solder bridge on a nimh pack that gave me fits. Would overnight charge fine, voltage fine, but no capacity. Found some tiny balls of solder at the outer edges. Was a no-name pack, tossed it. 
Old 03-06-2013, 07:55 AM
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Default RE: Dead Cell?

Checked it a few times, a few different ways. I'm not interested in using the pack now, but for fun what is the best way to remove and replace the cell, since it has a welded plate?
Old 03-06-2013, 08:05 AM
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Default RE: Dead Cell?

If it's a flat pack you cut the straps on both sides of the "weld" on the positive and negative ends of the cell and remove it. You can carefully do this using a dremel with a cut-off wheel.
After you can rough-up the straps with sandpaper or the dremel, get a piece of wire , some silver solder and flux and bridge the cut straps with the wire.DO NOT hold the iron on too long and overheat the tabs/cells!
Then you will have a 7 cell pack.
Old 03-06-2013, 08:09 AM
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Default RE: Dead Cell?

Don't wiggle it, as it will tear out the bottom of the adjoining cell. 

A fine round diamond saw, the kind that fits in a small hacksaw frame, spendy but the saw will be handy for other projects. An old fine-tooth hacksaw you don't mind tearing up. Take it to a jeweler or instrument/watch repair shop. 

Initial attempts with an Xacto or other small blade will only result in some sparks and a hot blade & handle. 
Old 03-06-2013, 09:44 AM
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Rodney
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Default RE: Dead Cell?

Any time that you can read a voltage from a pack indicating lower than expected voltage by connecting to the normal output leads, YOU DO NOT HAVE AN OPEN CELL, you have a shorted cell. I have had numerous NiMh battery packs fail with a shorted cell but NEVER one with an open cell. This is over several decades of working with NiCad and NiMh cells, I have yet to see an open NiMh cell and I've worked with hundreds if not thousands of NiMh packs. The only way you will get an open (when measuring at the output terminals) is if a break has occurred in the interconnecting leads or a shorted output has burned a connecting lead in two. Either those who say a NiMh fails open has a new undisclosed definition of "OPEN" or or just not cognizant of how things work.
Old 03-06-2013, 10:05 AM
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Default RE: Dead Cell?

Rodney,
Maybe it's my terminology, but a cell that has 0.0v and no continuity is, to me, failed open. Also, SR Batteries warns about NiMhs failing open. I haven't had hundreds, but dozens of nimh packs, and have had one fail in such a manner, also a AA cell for a camera filed the same, no continuity. 
Old 03-13-2013, 10:57 PM
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Default RE: Dead Cell?

Maybe it's my terminology, but a cell that has 0.0v and no continuity is, to me, failed open.
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