I bought one of these
http://www.amazon.com/DROK-Regulator-Adjustable-Converter-Indicator/dp/B00HHQ0VP4

couldn't find a datasheet, but this was on the description on the website i bought it from.
Indicator : dual color indicator , charging indicator light is red , the green light means fully charged ( No load is green )

its a DC-DC buck converter that I am using to step 36 V battery to 5 V powering a servo motor and a Raspberry Pi.
As soon as the buck converter converts power the LED colour turns blue (meaning green i guess if referring to website description)
when i connect 1st load(servo) it still operates and its blue, but as soon as i connect my raspberry Pi it turns red.
i got scared and unplugged the pi.
if anyone has any knowledge of why this is the case (what does it mean by charging? inductor charging? it should charge thats how it drops the voltage), let me know. plz and ty

by the way, servo and raspberry pi require 5V, so i connected them in parallel, as for the current the Pi needs minimum of 750 mA and the motor lets just say 3A

 

I'm just guessing, but it might be changing mode, from CV to CC or vice versa. You didn't mention how you set it up and set the voltage (or current). Until you know how it all works, I suggest you use an automotive bulb as your load instead of your Pi. This will allow you see exactly what's going on, and you can use a multimeter to measure voltages and so on to gain confidence on how it is operating.

The pessimist would say the supplier exaggerated the current capability, and your load is simply too much for it. But you don't have enough data to conclude that yet.