I recently got a backup car I will not be using often. I have read reviews of battery maintainers and read that they frequently fail, or improperly keep charging continuously, destroying the battery. Does anyone have experience with a brand that has worked well for them? Thanks!
Battery Tender. Many models to fit many needs. As low as $35 or so
My Battery Tender (brand) is 13 years old, works great.
I’ve never had a problem with the battery chargers/ maintainers I’ve used over the years from Schumacher.
Tester
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The only brand I have ever bought was Schumacher, and they continue to work just fine, many years later.
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My Shumacher battery charger must be over 30 years old. Still works great.
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I have one that cycles on and off between 14 Volts and 13.2 Volts. Works well for storage. Problem is they don’t make it anymore. Second problem is it broke, and the board is potted.
A 3 stage charger that does 14.4 Volt absorption and then drops down to 13.2. Needs to do 4 hours at 14.4 to ensure a full charge before storage, or a car or marine battery will sulfate. Deep cycle batteries don’t require this.
You could get an old unregulated battery charger and put it on a timer so it comes on for 15 minutes every day. If it takes 3 months of continuous charging to ruin the battery, then 1 out of 96 would be 24 years until the battery is ruined.
I’ve had good luck with the Noco Genius. Comes in various levels of power.
I bought my Schumacher when I had my 66 Mustang, sold the Mustang in 84 when my son was born, didn’t make a good family car, still have the working Schumacher and the son moved to Texas a couple years ago.
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Some control modules are also potted, yet companies successfully rebuild them
So there’s a chance you could repair it
The only difference between lead acid starting and deep cycle batteries is the design of the lead plates. The chemistries are the same. So they are both subject to the same issues with sulfation if left in a continuous state of discharge.
Mine still works perfectly. Purchased many years ago, probably late 70’s. Good quality product. I used it again just the other day. Not exactly light weight though. Pinhead diy’er, that’s me!.
There is something about the 12 Volt starting & marine batteries that is different. The 12 Volt seal lead acid batteries used in electric sooters and UPS units will fully charge at 13.5 Volts, given enough time. Whatever they did with the flooded starting and marine, they will not fully charge at 13.5V. Not sure if it’s the lead calcium or lead antimony for vibration resistance, or they modified them somehow to make them more tolerant of vehicle 14.4V charging systems, but they do not charge at 13.5 or even 13.8V. I learned this the hard way.
Came across a discussion forum where someone described the exact same thing. “I’ve had a lot of experience with the 12 Volt marine batteries. Mostly bad.” Said they have to charge at 14.4 Volts for 4 hours to fully charge. The others like the 6 Volt aren’t like this.
I put a new marine battery on a solar charger set at 13.8. One of the FVP from menards. It got drained once a week or so and it took 2 to 4 days to charge back up. It is now badly sulfated and at half capacity after only 10 months. Going to warranty it soon.
There are various was to charge flooded, lead acid batteries. I was looking for the profile I followed when I was paying particular attention to this and found an example here:
A few years back, I bought this pretty fancy “smart” charger that had all the bells and whistles. I put it on my boat batteries- deep cycle marine- and found it was stopping well short of full charge. I went around to various battery types and manufacturers I had and found it was highly variable but almost always stopped short of what I considered full charge.
So when it was done, I put it on my old “not smart” Schumacher I’ve owned for 40+ years and it was charging them. When it was done, they were indeed charged to what I felt was a full charge. So I took the “smart” charger back and am still using my tried and true “dumb” charger.
Marine batteries last me 4-5 years using this charger and they get used very often so lots of cycles. I gave up looking so closely at the process 
It should do a 14.4 Volt absorption charge after the bulk charge cycle ends.
The graph you shared is from Power Sonic, a maker of sealed lead acid or AGM batteries. Those will fully charge at 2.25 Volts per cell at room temperature, given enough time. The 12 Volt automotive and marine batteries won’t.
My last outboard battery lasted 13 years, did not need a lot of oomph to start the 90 horse outboard. It is a 4 week a year boat as property is split among family. Summer cabins so no electric over the winter. I guess it is called a marine/rv battery, need the screw on terminals. So my maintainer would take a day maybe to charge the battery to full. Info is 535 miles a way on maintainer but thinking it was a sears purchase a few decades ago.
So I hook up the maintainer when I get up there, 1 time a year.
We had a Windstar with faulty brake light switch that took me a bit to figure out, in the meantime maintainer would charge the battery up in 20 minutes for a start.
One hour before I have to use the car I ask Alexa to turn on the battery charger for one hour. Thought about buying a maintainer but I already have a regular battery charger. Of course this is only for sub zero days when the Panther sits inside the car shelter for extended periods.
I should have been more specific perhaps. I’m talking about my trolling and electronics batteries. Those are deep cycle marine. Most time is spent on the electric trollers and various electronics like depth finder, cameras etc. They get a ton of use. Starting batteries in the boat last 2 to 3x longer.