Battery chargers are not noted for having clean nor stable power. I would never use one for ground power, even if it was connected to the battery in parallel You would be better off purchasing a good 20 amp regulated power supply to use to run your avionics when on the ground.
If you get a good Battery Minder that is calibrated for the battery you have (Odyssey, Concorde, etc.), that works great as a power source to keep the battery up while you are using the panel.
I started out with an old 10 amp battery charger during the panel build and eventually changed out to a dedicated 30A 12 vdc power supply that can be adjusted with a voltage trim screw. Initial voltage setting was 12.8 so it would not charge the EarthX batteries, I later changed it to 13.4 when I added the isolated external power connector circuit.
The 30A DC power supply I used got good reviews for clean power on a couple of the HAM radio sites, picked it up from ebay for $25. Also picked up a DC volt and current meter for $10 and an enclosure for $25. I recycled an old PC power supply for the 115 power plug, on/off switch and fan. 10 AWG Zip cord from local big box finished off the parts. I guess I have about $80 total invested and can run the entire electrical system all day with the batteries completely isolated.