I use an old wall wart, seriously
A 50 or 100mA unregulated 12VDC wall wart will work fine with a car battery.
I wouldn't go bigger, but it will just use more water.
You don't want the peak voltage to go over 16-18V, or it can cause hard sulfation, an even worse kind.
You do need to add water regularly to any unsealed wet cell battery, anyway.
The new tenders have some fancy desulfation modes, but if you take care of it, it won't ever sulfate to begin with.
If you keep one long enough, it will fall apart internally.
I've only seen that happen once; no it wasn't my battery, but an older neighbor.
Red '96 Cougar XR-7 240k mi. '02 4R70W, PST DS : '03 PI engine, 04 maf, 24lb injectors, 2.5" exhaust, '02 4r70w + Jmod, DirtyD0g TC + cooler + 3/8" lines, 255 walbro fp. Alpine system.
Black '96 Cougar XR-7 (Lazarus) 210k mi PI Intake, '02 4R70W, Jmod, PST DS, GrogTune, Konis, Mark LCA+Poly, racecougar Custom Engine Chain, and JL and racecougar Bracing.
Black '97 Tbird Limited Edition, '02 4R70W, 255 walbro, PST DS, PBR Brakes&SS lines, Toicko Blues & Springs, GrogTune.
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