Jason;
Since I have been using the same controller for over a year now, I thought I would weigh in here.
First, the LEDs on the side of the controller is the voltage levels available coming to your loads. The 15V light being on all the time basically means that your panels are in enough sun light that it's getting that voltage level to the battery)ies) and the screaming speaker is about to kick in.
This little unit is OK for what is does and it's cost as a "starter kit". I still use mine, but with a slightly different idea. I use mine as a PDU, power distrubtion unit. AND for that it works perfect. HOWEVER, some of the units have been found to have a major soldering problem.
Open the top and look straight down at where the 3,6,9 VDC plugs are at. Look amd make perfectly sure the Diodes are NOT grounding to the metal box. This problem short the battery charging system so the only amount of charge you'll see going is less than 3 V.
Second do what Ghurd says, he talking real life knowledge here, take a look at the SGx units, go with the larger unit if you do intend on growing the system. I have the SG4 and it works without fail.
The outpout of the SGx can then be connected to the PDU and you'll have a nice system that is very functional a stable.
Also one other item that I have found to be VERY usefull because I have ordered and installed 7 already, it Ghurd's dump controller, if you put this gem in place just after the SGx unit it will dump any excess power to whatever you want it to, and will keep that nasty horrible screaming speaker thingy from sounding off, OR stuff an Q-tip into the speaker
As for the current battery:
I see no reason to "chuck" a good battery. If you are getting this system to be usefull and stable, this battery is perfect for doing just that, no need to WASTE anything.
If you need it, I can take a picture of my SG4 so you can see how I put it together for easy ADD-On of more panels, once I can afford them:-)
Hope this helps;
Bruce S