Confused re batteries

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

Batteries need to be measured under load.

-- Sir Benjamin Middlethwaite

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.

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.

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

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.