NiCd and NiMH cell failure-modes?

[G6Tanuki]

I've got an old "Titan" brand electric drill here - must be about six years old. It uses 18V NiCd battery-packs.
Recently I've noticed that if I fully-charge a pack (using the specified 18V snap-in fast-charger) then remove the pack and leave it for a week or so, it's all-but dead when I come to use it.

Bung it back in the charger, go and have a cup of coffee or two, and when I come back an hour later the charger-light's gone to green and the battery's got enough oomph to let me hammer-drill eight 150mm deep 10mm diameter holes in hard concrete before showing any signs of tiredness.

Recharge the battery afterwards, take it out of the charger, and a week later when I come to use it, it's all-but dead again!

Clearly the pack has adequate capacity to take a full charge and then do a decent amount of work but is self-discharging to uselessness after a week.

Any ideas why?

I've also seen a similar sort-of effect with some AA NiMH cells: 2450mAH "Energizers" bought about 3 years ago, recharged at 0.1c for 15 hours appear OK if used immediately but if charged and left standing, out of a batch of 20, 5 will have leaked-away their charge to the point where their no-load terminal voltage drops to under 1V after less than 20% of their capacity has been drawn.

Most odd! And annoying to have these relatively-recently-bought 2450mAH cells perform a lot worse than the Uniross-branded 1300mAH NiMH cells I've had for a decade and which have seen many hundreds of charge/discharge cycles but are still going strong with no noticeable capacity-depletion.

[Refugee]

NiCad cells are well known for self discharge but NiMh are better.
Lithium cells fare much better if you can cope with them having a higher volts per cell.
I converted one power tool to run from a 17Ah Dryfit that I dragged out of a thicket of brambles last year.

[paulsherwin]

You are describing a common failure mode. The cells appear to charge normally and deliver power, but they have a dramatically accelerated self discharge rate. It often used to happen with early mobile phones with NiMH batteries after they'd seen a few hundred charge cycles.

[julie_m]

There probably will be just one rogue cell in the series string that has aged prematurely and is preventing the battery from working properly. If you have a spare pack, you might try to "make one good one out of two". Re-celling battery packs is usually only worth it if you have a really special bond with the tool .....

[G6Tanuki]

I don't think it's one rogue cell: when the pack has done its week-of-rest-after-charging, the terminal voltage goes from 18V down to about 6V [insufficient to light any of the 'traffic-lights' on the drill's battery-health indicator].

A charge gets everything back to the sort of performance the packs had when new, but they won't hold a charge.

I *suppose* I could waste an afternoon splitting the packs, testing each cell individually, selecting the best, and maybe build one good pack out of two?

But life's too short and a new Li-ION-powered drill, two 2AH packs and a charger is less than £150. It has to be said, LiION technology totally outclasses antique NiCd - it's like comparing a LED flashlight to a candle!

[Refugee]

You can get two drills with batteries and chargers from the discount supermarkets for the price of an extra battery for an older model.
They get better and lighter every year with lithium batteries.
The advantage of buying two budget units is that you can fit different points in them to save time changing over during work.

[The Philpott]

I've not tried making a good NiCd battery from selected used cells, but think it would turn out not worth the labour.

This reluctance to hold charge for more than a few hours is very familiar! Although NiCd will tolerate a fast-charge a better solution for longevity (i would say) is two battery packs and a slow charger.

Lithium is trampling everything else it's true, which puts me firmly in the cordless tool dark ages.

Dave

[G6Tanuki]

Originally Posted by The Philpott

Although NiCd will tolerate a fast-charge a better solution for longevity (i would say) is two battery packs and a slow charger.

Lithium is trampling everything else it's true, which puts me firmly in the cordless tool dark ages.

Dave

These days everything I come across is fast-charge (tradespeople on building-sites don't want to lose money waiting for batteries to recharge: so 'one in-the-tool, one in the charger' and half-an-hour to fully recharge is normal, even if it does somewhat shorten the life of the battery. I'm happy with this - my NiCd ones have lasted at least four years.

LiION is definitely the way to go, I agree: a friend has a Bosch LiION chainsaw (with 36V 4AH battery-packs) that cuts through firewood as well as my two-stroke petrol one, and is a lot easier/safer to wield when you're using it up a tree!

[Bazz4CQJ]

I think this is an effect where you are getting a thin layer of ions adsorbed on the plates (due to polarisation effects) which impede current flow, even though the charge is still in the cell. I'm sure I've witnessed it too. The brief connection to the charger disperses the layer. Sorry this is vague, but the keyword I think for any further research is 'polarisation'.

I suspect that a quicker way of recovering the the power would be to put a very heavy load (aka, a short) across it for an instant, but this may not be in the best interest of battery life.

[mark_in_manc]

That's funny - I repaired two 18v NiCd drill-driver battery packs today. In the past I have zapped cells which would not take a charge with a car battery, which brought them back to life for a bit. But today I snipped out the dead cells and soldered in some used ones saved from another dead pack. Initial results are great - a nice long charge (not showing 'full' after just a few minutes) and loads of torque.

This would all be a faff if I were needing to do it at work, but for home use I don't mind. If anyone wants to send me any duff sub-C cell battery packs (or even a Makita Ni-Cd drill-driver body they don't use anymore - the Li-Ion ones have a different body - and who wants multiple chargers on the bench for different marques) then feel free to PM me