question for electricians

It does depend on what type of charger and what type of re-chargeable batteries you have. Assuming you have the basic (cheapest) NiCD batteries and a simple charger then the answer is Yes, because these simple chargers use the same amount of energy regardless of how many cells you insert.

The more expensive NiMH/Li-ion batteries/chargers use a charger that is usally called a 'smart' charger, since they work out how much power is needed and gradually reduce as the cell charges up, empty cells will not consume any energy. So the answer here is No!

PS: I'm not an electrician but think I'm correct in this

 

If we are talking about AA/AAA batteries it only costs fractions of a penny to charge anyway so I'm not sure you would save a great deal. Also you need to use a charger that matches your batteries - you can't use a Ni-CD charger for NiMH (although some do have switches on them).