Deep cycle batteries have fewer, heavier plates than starting batteries, as they serve a different purpose: a constant, low amperage draw, rather than a several-hundred-amp burst of power for starting an engine. Don't connect starting and deep cycle batteries. They charge and discharge at such different rates that one will destroy the other, or else the isolator will pack it in in short order. Deep cycle batteries charge and discharge best at 10 amp hours or lower. An alternator has trouble with them because it wants to jam 50 to 60 amps in. On my powerboat I use the alternator for the starting battery, but charge the cabin batteries, two surplus CPS units (computer constant power supply) when I get to shore power. I've discovered a new wrinkle with the trolling motor on my fishing boat: a new 12 amp CTC charger overheats if I use it outdoors on the new deep cycle battery. If I charge it out of the sun in my garage, it is fine at 12 amps. I guess the charger needs a fan. Bad design, and at a high price. I wish I had my trusty Walmart 10 amp back again.
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