Charging 10 1.2V nicd batteries
The fully charged voltage on 10 NiCd batteries is about 1.4V * 10 = 14V. So the first thing you will need is a voltage source higher than that. To keep it safe you really need 1.5 * 10 = 15V minimum.

NiCd batteries are charged with a constant current. You don't say what the rating on your batteries are, but let us assume they are 500mAhr. Trickle charging them is C/10 or 50mA. That's nice and safe and will fully charge your batteries in 15hrs (you add 50% to the time because charging is not 100% efficient)







simulate this circuit &ndash; Schematic created using CircuitLab

To keep things cheap you can use a resistor to supply this current and calculate the resistor value as:

$$R = \frac{V_{source} - V_{batpack}}{I}$$

Let's say you find an 18V source. Then 18 - 14 = 4V. And you want to charge at 50mA so your resistor value is 80 ohms. 100 is the closest you will get which will charge at: 4/100 = 40mA.

Edit:

I should add that the current will vary as the batteries charge. So a discharged battery at 1.1V will charge at (18 - 11) / 100 = 70mA. This current will fall to 40mA as the battery comes up to full charge.

With a C/10 charge rate, 14 hours is safe even if they are partially charged to begin with. You are "trickle charging" at a "charging rate" and the batteries are designed to full cope with this rate. Any faster than this you will have to verify they are charged - the battery pack voltage will be above the nominal 1.25V * 10 = 12.5V when you disconnect the charger and check with a meter. At 1.45V/cell (or anything above 14V for your pack), they are fully charged. Expensive chargers do all this for you.

You can trickle charge at a "top up" rate by adjusting to C/20. So for 500mA batteries, trickle charge at 500/20 = 25mA. You can do this indefinately without damage. Although to charge at this rate from discharged will take, well, days!