Trolling motor battery questions

[Kent]

I have a 74 pounds of thrust 24 volt Minn Kota trolling motor. I purchased two of the best rated batteries that I could find locally (Trojan, deep cycle, 130 amp, 7.1 hours run time at 15 amp and 820 marine cranking amps). On my first trip (which was early in 2005) after fully charging the batteries I tried using it for trolling just to see how long it would last. I was able to troll almost steady with it (varying the speed from slow to fast) for about 5 hours before the batteries run down. Last Saturday, after fully charging the batteries (onboard with my Dual Pro charging system), after a maximum of 30 minutes of slowly moving around looking for fish to vertically jig to, the batteries were too low to operate the trolling motor. I removed the batteries and charged them (using my automatic charger set to deep cycle setting) and after letting them sit overnignt took them to Batteries Plus to have them checked out. The guy at Batteries Plus said the one battery had lower cranking amps than he would like to see, but that it was a deep cycle battery so no problem and everything else checked out fine. Is it possible that I have internal resistance in one or more of the batteries that is causing the problem (if so how do I check for that) or could something else be causing the problem? I should also explain that I purchased my first Dual Pro charging system used off of eBay and when I started having similar problems to the above and Batteries Plus said the batteries weren't charged I purchased a new Dual Pro charger in 2005. I should also note that with my single battery charger at times the charging meter will slowly move down until the green light turns on, indicating it is fully charged, and other times the arrow stays up high. I have decided that the charger is actually working because the batteries are charged and the water doesn't boil out of the battery (but my logic could be faulty). Can a deep cycle battery develope memory? If one is supposed to drain the batteries every time before recharging how does one accomplish that? Any help would be appreciated.
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