Battery charger

1. Simple circuits

http://www.simple-electronics.com/2011/09/simple-universal-battery-charger-low.html

This charger can charge batteries 1.2V,1.5V,3V,4.5V,6V,9V and 12V with different charging currents ranging from 50mA up to 700mA.

charger schematic

Part List:
C- 220uF electrolytic capacitor 25V
D1- 2.7V zener 0.5W rated
D2- 1N4001 or any diode 1A rated
R2- 560 ohms 1W
Transistor- TIP41C,TIP31C or any similar NPN transistor. see  pinouts below
R1- value is dependent on the charging current. See table below.

R1 vs charging current

TIP41C/TIP31C pinout

To protect the battery from damage, dc input voltage is specified. This tabulated input voltage also protects the charger from overheating (may result to damage) and minimize the charger loss. See table below.

Input voltage vs battery voltage

If you are planning build a more "universal charger", you can use the schematic below. It is just the derived version of the charger above. You add only the selector switches where you can turn one R1 at a time to select the magnitude of current you need.

Universal charger with current selector

Note: This charger is not automatic and you need to track the time when will the battery fully charge to avoid damage.
How long will it be fully charged?
Time = battery capacity / charging current; battery capacity is usually in mAh or Ah. (Ah=1000mAh)