I have mixed experience with Solar Battery maintainers. I used them for several years and they appear to work if you get a panel of sufficient size.

But, the circuitry in one apparently went bad and killed one of my Batteries. Since that time I have not used them again. But that was back in the 1980s; perhaps they are better made at this time.

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84 300D, 82 Volvo 244Gl Diesel