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Depends on use of boat, sitting time, bilges auto on, down rigger use, charging system of motor, etc.... Not a simple answer. However, the Blue Sea system is the most efficient "bang for the buck" out there. There may be other manufacturers of the same engineering but I never tried to research because I am pleased with the Blue Sea product. When my boat was in its slip, once a month I would take my charger down there with auto shut off. I thought the charger was broken the first time because my secondary battery shut off the charger in the first 30 minutes. In fact, one of my mentors actually designed and was making this system in his shop in Petaluma years ago and gave the technology to Blue Sea (Blue Sea refined), a Mr. Fred Fritz.

Garris