Reviving 14v Ni-MH batteries

I don't know about you, but I have a Nicad and NiMH rechargeable battery charger. I just switch from one to the other accordingly.

no. Nothing really works. Bin em

Nevertheless, I'd try a quick nicad-style zap first.

I've heard that a 'reconditioning' charging pattern after zapping makes it last - never tried it meself tho

NT

I doubt it. Is this the one involving the freezer or the high current one? Brian

Really the only way to fix dodgy battery packs is to go inside, weed out the cells which are duff and replace them with the same types. However it might just be that when you look inside you decide to change them all. From the standpoint of replacing them with anything other than what was there before, I'd be very wary as chargers will be set up for the type originally supplied. Normally I find battery packs go due to one r more cells being lower capacity than the others and thus tend to get reverse charged by the rest when they go down. You can detect this when the speed or power of the appliance gets weaker after a while running.

Even fixing the duff cells will not match them up as others will fail one by one. depends on how lucky you feel. Brian

I'd not say never, but it is less likely it will work. I have found some triple A cells can be reclaimed for a while by keeping in a freezer for a week and then charging them, but the success rate is less than one in 3, and even then they do self discharge faster than the rest do.

Brian

Since no one else has offered you any worthwhile advice and (especially!) the 1st 4 video hits on googling "revive NiCad batteries" fail to show the optimum/correct method to 'unstick' NiCad cells that have been left unused for protracted enough periods sufficient to allow the dendritic crystal growth which shorts out the cell, making the unchargable on the standard charger, I've decided to offer you my advice.

First off, NiCads can, after seeing several charge/recharge cycles, develop dendritic shorts when left unused for long enough to go flat. This then shorts out the cell sufficiently to stop most 'fast' chargers being able to raise the cell voltage more than a few millivolts above zero.

NiCads, if left on a C/30 slow charge normally mitigate the effect of dentritic shorts by the act of the newly forming short being 'zapped' by the existing charge on the cell.

The problem arises when Nicads, which have been subjected to this C/30 charging regime for a year or so (cordless telephone handsets being the classic case in point), are then removed from the care of the charger and allowed to self discharge (usually a matter of a month or three with part worn cells). The lack of any charge in the cell allows the dendrite to short the cell without being zapped.

Apart from the fact that this renders them unchargable by conventional means doesn't actually harm the chemistry of the cell like it would in the case of a lead acid cell (and lithium is even _more_ fussy about its care than a lead acid cell!).

The best way to remedy this situation (dendritic shorting) is to use a large value electrolytic capacitor (10,000 to 100,000 microFarads) charged up to 3 or 4 times the cell voltage via a current limiting resistor and literally 'zapping' the cell or battery by connecting it across the cell/battery terminals either directly or via a heavy duty contactor switch using short thick leads.

Monitoring the cell/battery voltage will allow you to see when these dendrites have been blasted away allowing the rapid charge pulse to instill the initial charge required to allow the cell/battery to be connect to its normal charger to complete the charging cycle.

The theory is that the dendrites are disrupted by being melted into cadmium globules dispersed within the electrolyte, removing the short circuit and allowing the normal standard C/14 or fast rate C/6 to C/2 to electrolytically dissolve the material out of the electrolyte and plate it back onto the electrode from whence it came (high duty pulse charging offers the best method of achieveing this condition, as well as reducing the propensity for dendritic crystal growth in the first instance).

Unfortunately, NiMH suffers failure from other modes that don't include dendritic failure as the wiki link below shows:

This is the relavent paragraph:

"NiMH batteries do not fail in the same way as NiCd or lead?acid batteries. NiMH batteries fail in two predominant modes that are somewhat interrelated. The metal hydride material used for the negative electrode undergoes gradual corrosion in a strong alkaline environment. This corrosion results in less negative active material for hydrogen storage and also consumes water from the electrolyte."

So 'zapping' a NiMH cell/battery pack won't, unlike in the NiCad case, revive it. The only reason for the treatise on 'zapping' NiCads is to dispel the mis information shown in all those youtube videos on reviving dead nicad batteries.

Sadly, it turns out that not all of the advice you were given, as I initially implied, was worthless. TNP's advice to bin them "Is the _right_ answer!", to emulate the eponymous Gordon Angus Deayton of HIGN4U fame. :-(