NiMH problem, solved

[BrightLight]

I just bought a bunch of NiMH AA cells from Battery Station, and a Maha 401 (slow/fast) charger. Two of the batteries wouldn't take a charge (on slow). (Battery Station offered to replace them.)

I then realized I hadn't tried them on fast-charge; doing that, the charge took. After charging them on fast, slow charge worked also.

My conjecture is that they self-discharged so low that the slow charge couldn't force itself through, but once fast charge did, they were OK.

 

[kitelights]

My guess is that you're right. I understand that if they are too low (reverse charged) that you can sometimes shock them back. To my knowledge the Maha 401 is the one of the highest output (1000mA) consumer chargers on the market. BTW the box doesn't say it, but the website says that the 401 has a lifetime warranty.

 

[Lurker]

New NiMH cells are stiff until broken in. This can make them resistant to taking a charge until they have been cycled at least once or preferably several times. It also makes them appear to have lower capacity at first. However, I am surprised the Maha didn't have enough oomph to overcome this even in slow mode. I have only heard of this being a problem with solar chargers.

 

[snakebite]

the cheap chinese cells often come with a few duds in a large lot of like 50.
they are at 0 volts right out of the box.
fast chargers will wake them up but they will rapidly self discharge.
worst are the generic china noname that look like they were printed with a 9 pin dotmatrix printer.

 

[mattheww50]

Often times batteries that won't charge have small internal short circuits. At low current, they are shorts, so no energy goes into the battery.

Fast chargers often provide enough current to actually burn up the small internal short circuits. Once that is done, they charge just fine. It is well known that fast chargers have the ability to 'fix' some nominally dead cells.

 

[BrightLight]

Sometimes dead 6v gell cells can be revived by "flashing" them from a (12v) car battery, too. That technique isn't recommended for the explosion-averse.

 

[star882]

"Sometimes dead 6v gell cells can be revived by "flashing" them from a (12v) car battery, too. That technique isn't recommended for the explosion-averse."
I have heard some stuff about zapping shorted NiCds with a high voltage capacitor(like those in switching PSUs).

 

[Tomas]

One of the home-brew chargers I used to have when I used a lot of GE "D" cell NiCd's for my photoflash packs had a huge 10 volt 2F electrolytic capacitor (a little bigger than a soup can) that I could charge up to full charger voltage (about 6V).

If a cell refused to charge, or hold a charge, it was often from the growth of conductive crystals that shorted them out. To try to shock them back into service (by vaporizing the short without harming the battery, I'd drop the cell into the extra "special" slot and push the button that discharged that capacitor through the cell. If the cell didn't revive with one to three zaps, it probably wasn't going to, but MOST of them would pop open their little eyes and pay attention. /ubbthreads/images/graemlins/huh.gif

(I have NO idea if this would apply to the newer generation NiCd's or to NiMH's.)

/ubbthreads/images/graemlins/thinking.gif

 

[Ray_of_Light]

The NiCD cell recovery is done by discharging a 100,000 uF capacitor charged at about 100 V trough the cell. This sends a few milliseconds high current surge (of thousands of amps), strong enough to vaporize the crystals that shorts the cell without producing other side effects.
Since the current surge would also melt any switch in the circuit, an high power SCR diode is used for switching.
This technique is not good for NiMH, since the type of fault in this type of battery is different. Direct battery shorts are caused by cadmium crystals, which is not contained in NiMH.
Dead gel cells can sometimes be recovered by reverse charging, until some sulfuric acid is formed, and than they can be charged normally. Lead sulphate sometimes acts like a "semiconductor" layer, preventing any charging current to flow.

Hope it helps

Anthony

 

[MrAl]

Hello all,

I've brought back several NiCd's this way myself using
a simple 9v dc wall wart that had an internal cap of
about 2000uf.

Using two aligator clips, touching the defective batterys
contacts for about one second did the trick in most cases,
but sometimes it took three or so more times to burn out
the internal short.

I used to do one particular battery quite often as it
kept shorting out.

I've also noticed that the charge retention isnt as good,
probably because when the battery gets to that point it
isnt as good at holding the charge anymore (not because it
was zapped). This means the battery is on it's way out
anyway :-) so it's time to think about purchasing a new
one.


Take care for now,
Al

 

[star882]

"The NiCD cell recovery is done by discharging a 100,000 uF capacitor charged at about 100 V trough the cell. This sends a few milliseconds high current surge (of thousands of amps), strong enough to vaporize the crystals that shorts the cell without producing other side effects."
100,000uF @ 100v?
Where did you get a capacitor that big(I'm guessing some type of industrial equipment)?

 

[Tomas]

Even the car audio guys have some monster capacitors. Here's a site that has some 1,000,000 uF, 20V capacitors to help stiffen your power source for high power car amps ... Certainly don't want any power droop when pumping out sounds that can be heard two counties over ... /ubbthreads/images/graemlins/ooo.gif

You REALLY want to get into industrial strength capacitors, though, try the railgun folks - some of them are using absolutely massive capacitance for their power source for the rails. CDE makes some really nice ones. (Capacitors, not railguns!)

/ubbthreads/images/graemlins/huh.gif