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Internal resistance in Nimh and Nicad

What is too high

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extra slim26/01/2018 21:42:34
465 forum posts
48 photos

hi chaps, to be honest, I mostly use Life packs for RX, but have some 2600 4cell Nimhs and a 600 nicad. I might need a bit of weight up front. My charger measures IR, and I understand it to a point, but I can't find any guidelines on how much is too much.. does anyone know of a source or rule of thumbs??

Dickw26/01/2018 23:02:19
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601 forum posts
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The data sheets on the Panasonic slte give some idea of what you could expect from NiMH cells in a reasonable condition. Just select the types and follow through to the relevant data sheets to get the IR values.

Too much is when the pack doesn't maintain the voltage you want under the load you are applying - not much help I know but it does depend to some extent on your application.

Dick

G194026/01/2018 23:37:53
3523 forum posts
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Not sure about NMh cells but my limit on LiPos (I have an iCharger and a Wayne Giles IR meter) is that once they go over 10 milliohms/cell they're usually well on the way to being useless for as power sources for electric flight even though they appear to store almost the same amount of energy as originally.

Those hight capacity NMh cells are very poor at delivering current for (say) an on board glow so I would guess their IR would be in the region of 50 to 100 milli ohms but it would be a wild guess. All you can do is monitor the terminal voltage under load. The IR is given by:

off load voltage - on load voltage/ load current

Geoff

onetenor27/01/2018 00:48:53
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1901 forum posts

The internal resistance is caused by the formation of Dendrites ( fingers ) which bridge out the Pos and Neg sides of the battery. These are literally shorting out the cell. A cure can be effected by FLASHING a higher voltage across the pack terminals. A welder can be used for this.No contact is maintained. literally just hold one contact in place and brush the other terminal with the other lead from the welder. Just the same way as you do when checking if a car battery has power .You know the blue flash trick. Under no circumstances hold the contacts in place ,and do not try this with Lipos or KABOOM Thousands of batteries have been recovered using this method and I have not heard of any accidents. Oh yes polarity is not important. I have FIXED some more than once but I don't know how often it can be done. The power source can be either direct or alternating current BTW

Dickw27/01/2018 09:50:37
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601 forum posts
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Posted by onetenor on 27/01/2018 00:48:53:

The internal resistance is caused by the formation of Dendrites ( fingers ) which bridge out the Pos and Neg sides of the battery. These are literally shorting out the cell. A cure can be effected by FLASHING a higher voltage across the pack terminals. ..............BTW

IR is inherent in the way cells work and is not caused by Dendrites.

Flashing was a way of lowering the internal resistance of Ni based cells in an attempt to get more performance.

Dick

Edited By Dickw on 27/01/2018 09:51:04

G194027/01/2018 11:24:54
3523 forum posts
1 photos

As Dickw says, all energy sources have an inherent IR. If they didn't then it would be possible to draw infinite current from one (perhaps only for a very short time ) I doubt if dendtrites increase IR hough that may be a side effect. They are more likely to be the cause of a battery not holding its charge as they effectively discharge it internally.

I also wouldn't advise flashing a lead acid car battery. Whilst you may get away with it - in fact usually get away with it - remember a lead acid battery gives off hydrogen when being charged and you know what happens when hydrogen is ignited.

Geoff

J D 827/01/2018 12:37:21
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1384 forum posts
79 photos

A couple of years ago I was charging a lead acid tractor battery in my workshop,having been on trickle charge over night I then uped the charger to boost function to try and breath some more life into the old beast.

I came back a few hours later to find the top of the battery blown clean off with bits of casing and acid all over the place. Glad I was not in there when it exploded. The acid did for some tools and other items.

extra slim27/01/2018 17:39:29
465 forum posts
48 photos

Thanks for the responses chaps. As I suspected, no definitive chart, but I guess the variables affect this. I'm surprised that this subject hasn't been nailed, as many modellers use these packs, cycle their packs, do their best to look after them but not really knowing if their efforts are worth it etc, but no one seems to know how high is too high.. I guess one of the trickiest factors is how the charger measures it.. and the application, but I would imagine someone would be able to give a IR recommendation for something like a 2000mah Nimh, driving a standard 5 servos.. at resting not directly after charge... but this figure even with caveats illudes the World Wide Web. Not an issue really for me as I use life which 'give/deliver' in a different I guess less susceptible way than Nimh.. the quest continues..

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