DIY Solar Panels

I’m trying to build a DIY Uninterruptable Power Supply to run some electronics in a remote location with intermittent grid power. I plan to use two 115Ah deep cycle batteries attached to a two stage battery charger. When grid power is on I want to charge the battery bank with grid power, but I also want to supplement with solar panels. I purchased a 30A solar charger (Sunforce), a 16v 20A regulated power supply (Samplex), and a battery isolator (to use when I get solar panels to keep them isolated from the power supply). I thought that the regulated power supply would power the charger when the grid was up, and solar panels (added later) would charger in the absence of power. PROBLEM: When the regulated power supply gets over about 5 amps, the charger basically stops working. The output voltage starts fluctuating and it eventually shuts off. Is there some way to get this working???
Oh and I know I could just get a higher end solar charger that will do all this, but the costs are very high. . .I’d like to spend under $300, thus the DIY aspect.
Thanks for the response, but not that I already purchased a batter isolator to prevent backfeeding either the panels or the power supply. Also I don’t have any solar panels right now, just the power supply, so solar panels aren’t the issue. I think there must be something fundamentally different about the DC that is coming out of my regulated power supply and a solar panel! Another interesting note, the solar charger says right on the side, "Only use with solar panels, do not use with other power sources". . .but WHY???? Seems like it should work to me. . .?

Sounds like you have the power supply in parallel with the solar panels, feeding the charger, right?

If so, you’re putting power into the solar cells and the charger at the same time. If those are 12V panels, you’re putting 16 volts on them and probably back-biasing them. The solar panels are designed for direct connection only when they are a higher voltage than what they’re feeding.

The simplest way to fix this is to use a power contactor (large relay) — DPDT. One side to the solar cells, the other to the power supply. Have it pass the power supply to the charger when the grid is on and the solar cells to the charger when the grid is off. That’s what you’re trying to do any way, right? Since your electronics are running off of the batteries either way (right?), the momentary switch shouldn’t cause any problems.

Here’s one for 20A at 28VDC for $16 from digikey.com. The coil is energized with 100VAC, so it just hooks to the main (if you use 120). I’m sure there are similar relays with 240 VAC coils, if needed. You could put two in parallel if you need a higher rating than 20A.