Discharging a 14.4V NiCad power pack?

[Lobster]

Hi

I have a cheap'n nasty cordless drill with a correspondingly cheap'n nasty charger (ie it's not intelligent enough to discharge my NiCad batteries before recharging them). There's aften quite a bit of juice left in the powerpack after the useful power has gone, and I'd like to be able to empty the remaining charge before plonking them in the charger, in order to prolong battery life.

I thought of knocking up a little gizmo with a 12V car bulb attached, into which I could insert the batteries on the way to the charger. Does that make sense? Or could that damage the batteries in some way? Or would there be enough charge to blow a 12V bulb? How long would it take for the charge to reduce effectively to minimum - presumably some time after the filament stops glowing? Is there a more effective resistive load I could use instead?

The power packs are 14.4V, and are the usual sort; ie a roughly cubic block with a sticky-out bit with contacts on which slots into the base of the drill handle.

Thanks David

[Ian Stirling]

Yes, never do this. If you don't terminate at around 1v/cell, you will certainly reverse charge one or more cells. This is very bad for them, and will rapidly kill them. If you own a voltmeter, and can supervise, then this method can be used.

[The Natural Philosopher]

Wel it won't make an ounce of difference, but don't let me stop you ....

by 14.4v I take it you mean 12 cells. 12 cells an any state of discharge will not be above the 13.8v that is the rated voltage of a car battery witha decent alternator goimng, so no probs with headlamp bulb.

Just recharge em. They go on and on until a cell goes, and then you just re-cell em.

[Dave Plowman (News)]

There is no benefit discharging a Ni-Cad before recharging - except in that it might prevent overcharging on a timed charge with a nasty charger. Indeed *over* discharging Ni-Cads is likely to do far more damage.

[RichardS]

Have a read through Peter Parry's website detailing NiCd battery technology/behaviour/charging (esp the "memory effect" page).

charger may well be the culprit for reduced battery life....

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk

[John Rumm]

The problem you will have (especially with the cheaper battery packs) is that the cells will not be that well matched. As a result there is a danger that one cell will be totally discharged long before the others. If you load a pack as you describe and drain the whole pack you will end up reverse biasing one or more of the cells and damaging it.

I have often seen the serious (read anal!) fast electric model guys using a small value shunt resistor and volt meter to discharge each individual cell in a pack to about 1V before recharging. This is however not easy to do with a sealed battery pack where you can't get at the individual cells.

Generally you should only run the tool until the performance starts to drop off (indicating the battery pack is almost discharged). Then charge it. You may get best results from getting a better charger (with delta peek detection) for the pack. That way you can more reliably charge from an unknown state of discharge.

[John Flax]

Is there any reason why you shouldn't wrap an elastic band around the switch so that the battery is run down with no load?

I leave my electric razor 'on' until the battery is run down and put the rechargeable batteries from my digital camera in a torch.

[Dave Plowman (News)]

It's extremely bad practice to run a Ni-Cad battery pack absolutely flat, and can do it harm. Nor is their any need to. Ignore the so called memory effect - it's a myth in practical applications.

[Lobster]

OK, so looks like I've saved myself a job then! Thanks for all the replies.

However - on another tack, I use a Goodmans GBC1100 charger to charge up my AAs for the digital camera etc; it calls itself "intelligent", and has different settings for NiCad and NiMh cells. On the NiCad setting, it enters an indicated 'discharge' phase before it starts charging the cells. Is this a load of old baloney; should I dump this device?

Thanks David

[Ian Stirling]

It probably doesn't do anything especially beneficial. But it probably does little if any harm.

Memory can be a problem for a very small number of devices that cut off at a certain depth of discharge every time. (NiMH and NiCd)

A very similar phenomenon happens when you charge NiMH especially for a long time slowly. This can result in a battery that performs poorly until it's discharged and recharged.

[Dave Plowman (News)]

It doesn't do any harm to discharge cells first - but to a controlled state. What does the harm is to run them *totally* flat as in leaving on a torch etc. With a power tool, simply recharging when the performance starts dropping off is fine.

[John Rumm]

Note also the charger described discharges individual cells as opposed to a battery pack (i.e. a packaged collection of cells). Hence you don't have to worry about the possibility of reverse biasing a cell etc.

[usenet]

... and discharging them is pretty well pointless anyway.