Confused re batteries

[The Medway Handyman]

Hi All

Using my 14.4 volt drill driver today, big screws into MDF, lots of hammer drilling with blunt bits, flattened both batteries. The driver (Performance Pro - yes I know) has a battery check button with red/yellow/green LEDs and both batteries showed red and were really flat.

I measured the voltage when I got home and both showed 14v. Having said that, my meter will only show smaller DC voltages as a decimal, so both came up as 0.14 volts on the display. When charged - charger light changes to green, button on driver shows green and they obviously have plenty of charge - the meter shows 0.16 volts

What don't I understand here?

[meow2222]

The difference is in current delivery. A flat battery gives sod all current. Put a load on it and V drops to about zero pronto.

NT

[The3rd Earl Of Derby]

Batteries need to be measured under load.

-- Sir Benjamin Middlethwaite

[Dave Plowman (News)]

Ni-Cads have a very small range of output voltage when not under load. Not at all like a disposable. They also tend to 'recover' when not used so the off load voltage goes up after a while.

[Ian Stirling]

Dischjareging batteries to 0V really, really damages them, they might last 10 recharges or so. Cutting them off at ~14V means you get 90% of the power out, but they last hundreds of charges.

[meow2222]

true for lead acid, but I thought the other way round for nicads, hence chargers that discharge them fully first to avoid memory effect.

NT

[usenet]

The 'memory' effect is *not* what makes 99.9% of NiCd batteries fail.

There is such a thing as memory effect in NiCd batteries but it only occurs after several hundred *identical* charge and discharge cycles. It was first observed in NiCd batteries used on a satellite where the regular discharge and recharge by solar cells caused the effect.

NiCds in domestic use are incredibly unlikely to get the sort of regular charge/discharge cycling that causes this failure. You're much more likely to damage cells by trying to discharge a battery (i.e. cells connected in series) completely and thus causing reverse charging of the weakest cells.

The best way to treat NiCd batteries used in cordless tools is to recharge them *immediately* any significant reduction in power is noticed as this will prevent reverse charging of the weaker cells.

Only if you can discharge individual cells is discharging a safe thing to do. It might be useful then if you are going to fast charge the whole battery as you are less likely to overcharge some of the cells.