When plugged in those transformers use electricity. Generally it's not a lot of electricity but enough of them over time does at up.

But the question isn't the electricity used, but rather the constant charging harming the machine/battery.

I've heard divided opinions. Some say for maximum life it's best to charge a battery, disconnect it, and run it down and repeat the cycle. Others say keep it plugged in, it doesn't mater. Take your pick. Fortunately, most of the things with rechargeable batteries are either relatively inexpensive items, or are of a type that replacement batteries can be bought.

mike is right, all the plugged in items in a house draw power and it does add up. This is called the "phantom load". If you're going away for more than a few days, it's a good practice to unplug all those things (including TVs) that draw power in a standby mode.

PS - I think power supplies/wall warts that power electronic devices are theoretically rectifiers, not transformers. No matter, the "transformer" term is widely understood and used.

They are both rectifiers and transformers. The transformer converts 120VAC down to a lower AC voltage, and the rectifier converts it to DC for battery charging.

Older NiCd batteries DO develop memory, and lifetime is extended by not leaving them on the charger all the time, but charge, unplug, use, recharge. Newer NiMH batteries do not have much of a memory problem, so it doesn't matter so much.

One thing to keep in mind is the both batteries and chargers can fail and cause fires. Rare, but it happens. It's better to leave things unplugged.

"When plugged in those transformers use electricity. Generally it's not a lot of electricity but enough of them over time does at up."

Not enough to matter.

An unloaded transformer has a VERY small power dissipation.

Even the with a rectifier circuit on the output of the transformer, the capacitors charge up and the leakage is VERY tiny.

This whole thing is caused by some fool that measured the current without looking at the phase angle to the voltage.

Just like the 'power factor corrector' charlatans.