Nope you nailed it Korishan. You can use a separate power source to make sure the meter turns on, then use the VIN to measure the voltage of whatever your testing, you can tie them together if your powering it from the same thing you are measuring. Just make sure you run an extra black wire from the ground terminal if you want to prove stuff like a multimeter. Also these little 4 digit meters were less than $1 on eBay..the extra resolution is nice.
I'll do a quick video and upload it....gimme ten minutes and it'll be uploaded for ya.
Well done. Cells or packs in parallel should be st the same voltage or something is very wrong. Once you connect them in parallel, they balance themselves out to equal voltage potentials. Under load they may deviate a little due to internal resistance which is why you should match the cells in each pack by their IR prior to the build... in series, just wire up the meter to each packs +\- and make sure they are all at the same ground potential.
Video is up YouTube search "the combat engineer 18650" or click the link for my channel