We used the Schumacher 6 amp chargers for HS robotics for years. Very reliable.
I have a dual Noco genus on the boat, to maintain both batteries. Also reliable.
I also have a cheap computerized charger(s) from Amazon...... Had one that worked fine for about 4 years, lost it in a lightning storm along with a pool robot, can't blame the charger. Replacement still going strong.
I also recommend an analyzer. I have this one:
Car Battery Tester