I have been upgrading my blazer for a couple of years now. I just finished with a new stereo system and placing 6 KC off road lights on the vehicle. Prior to that I installed new HID headlights and signal blubs to the vehicle. Now if I let vehicle sit for a week the battery drains, or when I turn on the 6 lights while running the Voltmeter goes way down and the lights are not bright. The vehicle now is starting to lose electrical power idling.


Do I need a new Battery or install a second one, or should I upgrade my alternator to one that can handle the additional load.

 

I would upgrade your alternator and your battery.

 

Battery draining in a week may be normal, but probably isn't. This may be related to your new stereo. It may be requiring more power to run something while the truck is off. Easy test is to pull the stereo fuse and see if sitting for a week drains the battery. If you have a voltmeter handy you can measure the current coming out of the battery with everything connected and then again with the stereo fuse out. Figure out what is draining your battery one fuse at a time.
As for the low voltage while everything is running (lights/stereo/HIDs/etc) that would be the fault of your alternator. What's happening is that your alternator can't supply enough current to keep up with the load imposed by your equipment. If it's a stock alternator, it may be around 100Amp output (less when idling). So when your 6 lights are on (if each is 100W@12V) they're drawing 50Amps alone. Then add your headlights (55W@12V) they're drawing 9Amps combined. Don't forget your truck itself needs some amount of current just to run ~30-40Amps. You can easily see that your little stock alternator is too small for such a large load. You can test by revving the engine (with everything on) while not moving and and see if the light output or voltmeter increases. I guarantee that it does.
Remedies for your issue:
1) larger alternator (150+ Amps) (pricey)
2) Do the "Big 3" wiring upgrade (search it on here, you'll find it)
3) Downgrade some of your equipment or don't run it all at once (but where's the fun in that right?)

You might be thinking of upgrading your battery... This won't help you much.
Things it will do:
1) allow your truck more time to sit before the battery dies
Things it won't do:
1) improve your voltage/current output

If you put in a second battery, it will only allow you to run things longer while the truck is not running. Think of it this way, when the truck is running, your power is coming from your alternator. when it's off, it comes from the battery.

Don't forget when looking into alternators to keep in mind that they output different currents at different RPMs. You want as much current at as low an RPM as possible (aim for your idle RPM to output your required current).

Good luck, hope this helps