o1marc,
do you have a multimeter so you can test what's going on with the charger? you can use a large spool of small wire as a power resistor if you don't have something better suited & test voltage coming out of the thing...
a lot to unpack in this thread to point you in the right direction. it really depends what your power system setup & usage requirements will be.
I highly recommend getting a battery tester before buying any new batteries. And a multimeter. you can find each for $10-20 on amazon.
when I buy a new battery (starter or deep-cycle) I always bring the battery tester (Solar BA5 and another I don't remember the name of (check-a-volt?) to the store - it typically requires testing 4-5 batteries to find 1 acceptable battery. for example, it will say on the battery it's supposed to be 1200CCA but test at 600, the next one will test at 768, the next at 900, and finally you'll find one that tests at 1265 (above spec). this has happened to me even when the voltage matches (indicating they're charged similarly).
depending on your use buying a $15 tiny window solar-panel vehicle battery charger/trickle charger could be enough - but house batteries are a totally different story. also on the subject of vehicle/starter batteries - you do not need to spend $200+ to salvage a sulfated (really dead) battery. I've fixed half a dozen just using an old DC power supply & epsom salts (I know there's controversy on the subject but I don't argue with results). I made a youtube video of how to do this search my username 'Triskite', vid from 2014
In general I think most battery chargers/maintainers similar to the one you linked are overpriced and kind of a waste of money (except for the NOCO brand but even then...). They generally only do a few voltage steps and have very little room for customization to match your specific battery profile. You'd be better off buying a solar battery charger & a DC power supply on ebay/amazon. It would be cheaper, too.
I'd post links but it won't let me due to my low total post count.
For reference, we started out with 8x of the Deka/Duracell Sam's Club 200AH deep-cycle golf car batteries - but lead acid is an absolutely tragic battery chemistry to live with long-term if you have steep power requirements (we are off-grid with no generator and lots of smarthome stuff // electronics, and go in all-seasons including negative degree winter with our water lines active & anti-freeze protection requiring power).
Now we run 16x 200AH 3.2v LiFePO4 (8s2p, CA180Fi model) with the Duracell GC2s in parallel (they act as an emergency backup // enable me to change wiring on the lithium circuit without losing power). For non-solar battery charging the best I've been able to find with variable/adjustable voltage is the Turnigy 1200W 50A Power Supply. (the mosfet on our solar wifi lithium charge controller blew so right now I have IFTTT turn off the smart outlet it's plugged into when any cell voltage gets above a specific level, voltage level is decrypted and sent to cloud by a raspberry pi running openhabian)
Sorry for rambling a bit, it's late and there's a lot to think about with skoolie power