Charging 10 AA nimh batteries in series
Your best bet is to locate a NiMH charge controller IC and build your charger using the recommended circuit for that IC.

Charging batteries is a tricky process if you intend to do it without destroying the cells.  It is even trickier if you have a large number of cells in series.

Figuring out how to do it right is a job for a professional engineer, and finding all of the needed conditions has been the subject of many a research project.

The ICs will have all of this knowledge and know how "baked in."  If you just need to build a charger, this is the easiest way. 

If you are building the charger to learn how to make such a controller, you will need to do a lot of reading.  You can probably also learn a lot just by looking at the datasheets for charge controllers.



As an example, there is the MaxDS2715.  

This chip is built to control the charging of up to 10 NiMH cells.  It is configurable for the number of cells, includes connections for the charge current sense resistor and for a thermistor.  Proper charging requires regulated current and voltage, both of which this chip provides.  Among the criteria for "full charge" detection is the temperature of the cells - this chip has the needed connections for a thermistor to monitor the battery pack temperature.

You will, of course, have to provide a powersupply that can deliver enough voltage and current to the chip.

There are numerous other chips out there.  Given the keywords you can find in the description of this IC, I expect you can find others easily enough.  Any large supply house (Mouser, RS, etc.) should have a good selection.