There is a thread in the sci.electronics.design usenet newsgroup that reveals extreme reduction of capacitance due to DC bias, even well below rated voltage. I recently bought some 0805 capacitors in various values from 2.2 uF to 47 uF with 16V and 25V ratings. Here is one example:
The part number is GMR21BR61E226ME4L.
I used the Murata web tool
# and found:
1 V 20 uF
5 V 9 uF
12 V 4 uF
15 V 3 uF
25 V 2 uF
At rated voltage, actual capacitance is only 1/10 of the nominal value!
A 1 uF 25 V capacitor in 0805 (part number GRM21BR71E105KA99) is as follows:
5 V 0.8 uF
12 V 0.7 uF
15 V 0.7 uF
25 V 0.5 uF
Here is an article that explains this in a bit more depth:
Here is a detailed data sheet for a similar TDK capacitor:
Note that the voltage rating is given as 1V (35V), so the specifications only apply with 1 volt of bias. The voltage coefficient is actually -95% at rated 35 VDC, so this 2.2 uF capacitor would act like a 100 nF (0.1 uF) device.
This effect gets worse as the C*V^2/volume ratio increases. I was wondering why I found such high capacitance and voltage ratings in such small packages and low cost (like $0.25 each).