I will use a 2000 mah battery as an example:  When new, charge at 100-200 mah for at least 14-16 hours.  This will form the cells.  Go fly the model or cycle the battery and continue to charge at the low overnight rate for the first 3-4 times.  This low rate should not overcharge or harm a battery.

Once you have cycled the battery 3-4 times ( I just fly), then it is OK to fast charge.  Most Nimh batteries can be fast charged safely, but don't crank the charge rate up too high.  I personally don't charge over a 1/2 C rate.  So for a 2000mah battery, I will fast charge at 1000mah or 1 amp.

I almost always slow charge my batteries for at least 6-8 hours before flying.  That will usually top off a battery that has been sitting for a month or two.  If I want to be sure your battery is fully charged, after a long slow charge plug it in to a fast charger and let it "top off" the charge at the 1/2 C rate.

A good 4 cell battery should read about 5.5V fresh off charge.  A 5 cell will be about 6.6V.  A load test at 500mah will drop the voltage around 0.2-0.4 V but it should rebound quickly when you remove the load.

If you fast charge a new Nimh battery right off the shelf, don't be surprised when it fails.  This is the same advice I give to all my customers.  Those that choose not to follow these guidelines often return in a few days with a "bad" battery.

Now you know.

Gene T.