Electronics > Beginners
Overly sensitive NiMH battery chargers
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ogden:

--- Quote from: james_s on July 31, 2019, 05:31:48 pm ---I have never seen any noticeable damage from draining a NiMH battery down to near 0V, I have lots of them that have done that repeatedly and they still test at greater than min rated capacity and show no signs of degradation.

--- End quote ---

Again - it depends. Near 0V is fine up-to few days until you find mains plug for your charger, yet you would not want to store overdischarged NiMh's for weeks because it degrades usable capacity much faster compared to charged cell.


--- Quote from: Rick Law on July 31, 2019, 06:51:35 pm ---But I did consider going-negative and going low-voltage as the same thing in what I wrote.  There is a difference between the two, so my treating it as the same is probably wrong.

--- End quote ---

Right. Quote from Energiser appnote explaining polarity reversal (both electrodes reversed): At the point both electrodes are reversed, substantial hydrogen gas evolution occurs, which may
result in battery venting as well as irreversible damage.
james_s:
Either way very deep discharge is generally best avoided due to the risk of cell reversal, however sometimes stuff happens and it would be convenient if all chargers had an override function to jump start the charging process on cells that are too low to detect. Draining down below 0.5V is not ideal but it is something that I have had happen often enough to be annoying. Unlike Li-ion, there is no risk of "exciting" failure modes so even if a cell is damaged there is rarely any harm in attempting to charge it. Even if one is no longer in tip-top condition I see no reason to dispose of it if it is still reasonably usable.
13hm13:

--- Quote from: IanB on July 30, 2019, 01:45:22 pm ---
--- Quote from: ogden on July 30, 2019, 07:03:40 am ---
--- Quote from: 13hm13 on July 30, 2019, 06:05:28 am ---After a test with the paper-clip trick on the C204, the charge did conclude (green light) ... but both batts. were too hot to touch. This is not normal.

--- End quote ---

Hot NiMh's in the end of fast charge is completely normal.
--- End quote ---

Batteries too hot to touch at the end or charging is not normal. It is a sign of bad batteries or bad charger. If you have a "fast" charger that regularly does this it should not be used.

At the end of charging batteries should be mildly warm at best, certainly not hot.


--- End quote ---
Yes.  The Maha 204 (or the 9000 in fast mode) does leave the batts. quite warm. I've never been comfortable with this in any case. But the paperclip trick was prob. a good 20 degrees hotter.
Rick Law:

--- Quote from: 13hm13 on August 01, 2019, 02:38:57 am ---...
...
Yes.  The Maha 204 (or the 9000 in fast mode) does leave the batts. quite warm. I've never been comfortable with this in any case. But the paperclip trick was prob. a good 20 degrees hotter.

--- End quote ---
If I understand you right, you are saying when you start it with the paperclip (verses other way of starting charge with the very same battery), that made a difference in the battery temperature at termination?

Did you remove the paperclip after charging starts?

Once it starts charging and the paperclip is gone, it should not make a difference how you start it -- unless with your charger under the mode you selected, it does something such as testing how much current it can take and use that as the charge current.  If it does auto-detect charging parameter, it might have started with parameter the battery doesn't agree with.  In this case, the paper-clip trick is not for you.  Your charger doesn't agree with it.  Please don't do it.

Regarding fast charge, many articles I've seen suggest that lower charge current is better for long-term health of the battery.  If you are already uncomfortable with fast charge, that is one ready reason to switch over to lower current charging.
David Hess:
As far as I know, the only danger from low or zero cell voltage for NiCd and NiMH batteries is cell reversal.  Lithium and lead-acid should never be deeply discharged.
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