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Author Topic: NIMH stored discharged, can it it be recovered?  (Read 275 times)

N2KD

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NIMH stored discharged, can it it be recovered?
« on: December 13, 2022, 01:34:47 PM »

Have a couple of 9.6 volt NIMH batteries that were stored discharged for a couple of years.

I know this is the worst thing for them, but can they be restored and charged to even half capacity?

Had watched a couple of YouTube videos that
zapped batteries with higher voltage and got
back some or all of the original capacity.

Have you had luck with this?
What momentary voltage would you hit it with?

Just need to get some voltage in it so that the smart charger can charge it, then do a couple of deep discharge cycles?

Appreciate any suggestions.

Thanks!
 
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WB6BYU

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Re: NIMH stored discharged, can it it be recovered?
« Reply #1 on: December 13, 2022, 03:32:48 PM »

I’ve treated a lot of NiCad batteries this way,
but haven’t needed to do any NiMH cells yet.

Such treatment works if the cause of failure
is crystalline growth in a cell that shorts it out.
The zap melts the crystal hairs, at least to the
point where the battery will work, but doesn’t
keep them from growing back eventually.
I used it to get radios back on the air while we
waited for new batteries to arrive, but it
isn’t a long term solution.

Other failure modes, however, won’t recover.

I used a current-limited (0.5 A) adjustable
power supply.  A standard 12V supply will work,
but will want a resistor in series to limit the
current.  Probably 100ma might be a reasonable
max current, which could be a small light bulb. 
And higher voltage may work better for a
nominal 9v battery, since all the cells are in
series.

The first step is to put the battery on the power
supply with the current limiting resistor and see
if the voltage recovers.  I generally use clip leads,
and connect my multimeter across the battery to
monitor the voltage.  For a single cell, if I see
more than about 0.1 V then it isn’t a dead short,
and probably can’t be recovered this way.  For
a multiple-cell battery, you might have some
shorted cells and others good.

In fact, if you just want enough voltage to get
your charger to work, this may do the job all
by itself.  Just don’t leave the battery on too
long - at 50-100mA you should see any voltage
within a minute or two.  (You might try this
first with about a 1 K resistor, which will
allow the battery to charge slowly without
overcharging.)

To try to recover a shorted AA cell, I used a
large capacitor, like 25,000 uF, with a voltage
rating above that of my supply.  For a 9V
battery, 1000uF might be enough.  You just
want to make a good spark when you short
it out.

I put the current limiting resistor between
the power supply and capacitor.  A lamp
actually works well here, as it gives a visual
indication of when the capacitor is charged.
Then a separate set of leads go to the
battery:  I had some brass bolts that I used
as contacts at the ends of my clip leads,
otherwise the clips can get pitted.

Let the capacitor charge up through the
resistor, then, observing polarity, touch the
leads to the battery terminals.  There
should be a spark as you do so.  The burst
of current from the capacitor hopefully will
eliminate the short circuit, then the
residual current (through the resistor) will
provide charge for any recovered cells.


So the power supply voltage, and capacitor
size, determine the amount of zap, and the
current limiting resistor sets the charging
current once the capacitor is discharged.

This may take a few tries, and some
experimentation.  It’s easier with
individual cells, as you don’t have to worry
about overheating the good cells while
trying to recover the last couple, but may
be worth a try if the cells are shorted
internally.  If they are open instead, or just
won’t take a charge, then it won’t help.

N4ATS

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Re: NIMH stored discharged, can it it be recovered?
« Reply #2 on: December 14, 2022, 08:05:29 AM »

The ZAP needs to be the SAME voltage as the cell and it MUST be hooked up backwards for about 3 to 5 seconds , this will blow the Dendrites from the plates allowing normal charge again. Dendrites are typical in both N and LI batteries. I do it ALL the time for old batteries and it works EVERY time and have been doing it for 30 years
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WA3SKN

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Re: NIMH stored discharged, can it it be recovered?
« Reply #3 on: December 14, 2022, 10:03:05 AM »

What have you tried so far?

-Mike.
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WB6BYU

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Re: NIMH stored discharged, can it it be recovered?
« Reply #4 on: December 14, 2022, 11:34:38 AM »

Quote from: N4ATS

The ZAP needs to be the SAME voltage as the cell and it MUST be hooked up backwards...




Clearly different methods work, as I do it the other way.

The important thing is for the capacitor to deliver enough
current through the cell to zap the crystalline whiskers
without damaging the cell itself.

I happen to prefer the matching polarity method, as the
residual current will recharge the cell and give an indication
of whether the treatment worked.  It does, however,
require a higher voltage (and a current-limiting resistor) to
overcome the forward voltage of any other cells in series.

I've heard of folks doing it direct from a car battery with
no current limiting, too, but the potential current is quite
high, making it more risky.

N2KD

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  • Posts: 69
Re: NIMH stored discharged, can it it be recovered?
« Reply #5 on: December 15, 2022, 07:36:01 AM »

Interesting, obviously this is part art and part science with different thoughts and methods.

Hadn't heard of reversing polarity, but given sufficient zap I'm sure that works too.

Appreciate everyone's input, I'm getting a
variable power supply in a few days and I'll
see what works. Want to be able to limit current during the experiment.

First snowstorm, I'll grab a beer and some safety glasses and let you know how I make out. Start conservative and work my way up
in voltage and current.

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KB1GMX

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Re: NIMH stored discharged, can it it be recovered?
« Reply #6 on: January 03, 2023, 08:58:47 AM »

First order of business..

NIMH is NOT NiCd treating them as such is destructive.
They are nickel based but not the same.

NiMH best best is start with the correct charger and see
if they "wake up".  Also inspect to see if corrosion due to leakage
is occuring.  Leaking cells have poor life as they dry out from the
failed seals.

Over charging or use of excessive voltage can potentially cause
failures and they may be "energetic" when doing so. 

See BatteryUniversity.  https://batteryuniversity.com/articles
especially https://batteryuniversity.com/article/bu-807-how-to-restore-nickel-based-batteries

Best bet if the batteries power something important, replacement.
Cells of the correct size with tabs or complete batteries are obtainable
if the original vendor has expired or as a way to replace them cheaper.



Allison
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