Trying out the battery test now with my modded C5 and S3. Will update once I have results.
I have found a problem with my mod. When I'm trying to use the C5 as an amp only and I feed it line level input(2Volts) I get noticeable distortion when playing music.Did not notice it cause previously just tried it out with my S3 jack out and my DAP and it sounds clean at my usual listening level (low gain,around 8~10 volume).Did a test with another external DAC with volume control and with my voltmeter measuring and sure enough feeding it more than 0.9 volts causes it to distort.Doh! I was actually feeding the C5 2 inputs, one that's soldered on(although not powered) and another with the C5 jack input causing the distortion.
Wanted to use a switch as a first solution but couldn't find one small enough to fit the case and that can be easily mounted on one of the faceplate. So I remove all the soldered wiring from the C5 input and UD-100 output.Soldered a 1/8 inch jack wires to the UD-100 output and feed the C5 input using the 1/8 jack(below photos should make it clear what I'm babbling about).So I'm a happier man now with this improved mod and bye bye distortion!!!