Memory effect is well known for NiCd rechargeable batteries. If not completely discharged on a regular basis, the batteries may appear to be fully recharged, but deliver very little power before there's nothing left. Functionally discharged at this point they may still appear to be fully charged if only the voltage is measured.

The same effect appears for NiMH (Nickel Metal Hydride) batteries, although less rapidly. The "Battery Chemistry" link at 17 Nov 05 - 05:16 PM recommends fully discharging NiMH batteries approximately once every 30 recharge cycles.

Some more sophisticated chargers will actually include a partial (or nearly complete) discharge step before recharging begins. The cycle required for best performance of a given set of batteries depends on the specific kind of batteries. If your "expensive piece of equipment" recommends a specific battery type, and if it came with a charger, it's safest to use batteries of only the recommended type and to use the charger that came with it, or at least another charger specific to the battery type.

With cheaper devices, the "throwaway" rechargers are unlikely to include any sophisticated charge profile control, so they can be considered interchangeable with others of like kind.

Most of the common rechargeable battery types have a cell voltage of about 1.2 volts, so a charger designed for 1.5 volt "Rechargeable Alkaline" (or Carbon-Zinc) batteries may overcharge them. Any charger designed for common rechargeables (except Alkalines) should only supply 1.2 volts, so can't "fully charge" an alkaline (or a Carbon Zinc) battery, but may put "some life" back into them.

I have noted, for NiMH batteries, that a first few charge cycles for a new set of batteries doesn't give particularly good performance; but the performance seems to improve after a few charge cycles. I haven't seen any "professional opinion" to support that this does happen; but at least in my Digital Camera it seems to be the case. The opposite is more likely with Ni-Cads, with each successive recharge giving back a bit less, even with regular discharging to avoid "memory effect."

John