Your batteries will be happy with higher voltage but the solar panel controller should cut the voltage off before the battery overcharges. What puts the juice back in your batteries is the amp output of the panel. A light bulb can take 5 amps per hour out of your batteries. Start adding more lights, radio, fans, etc. and the amp hours taken from the battery bank in a period of 8 to 12 hours can be significant. Your 120 watt panel will not provide the rated amperage for very long. Shading and sun angle among other things will reduce what is going into your battery. To determine how effective it's charging you need to do a power analysis. Figure what is running and for how long to come up with amp hours used. Then look at the panel to see how much it is replacing to your battery banks during daylight hours. You may or may not be in the plus column. But just guessing will not get you the answers. On cloudy days, the solar panel output will be very little. Of course having the solar panel will be much better than nothing at all, but it may or may not provide all of your charging needs. At night it will not do anything for you and in the morning it will have to start making up for the deficit. Here is an excellent article on solar panels. It for installation on a boat but the info is almost identical, http://www.boatus.com/boattech/articles/solar-panels.asp . Hope this helps. Chuck
