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Deep Cycle battery and trolling motor question

Discussion in 'Power Electronics' started by Madsalts, Feb 22, 2019.

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  1. Madsalts


    Sep 25, 2014
    Part 1
    When running a deep cycle battery, I know that current draw can be used to calculate run time as a factor of Amp Hours of a battery. I also know that batteries should only be drained down to about 50%. So, if I'm running a trolling motor, should I calculate run time assuming that I can "use" all of the amp hours listed on the battery, or just half of these?

    According to this site ( ... r-run-time), if I have a 100 AH battery and a trolling motor that draws 20 amps, then:

    100 amp hour rated battery / 20 amp draw = 5 hour run time

    Is this realistic, though? Does this mean that the battery will be dead, and drained below 50% after 5 hrs, or does this mean that after 5 hrs the battery will be half drained?

    Part 2
    Furthermore, deep cycle batteries usually, if not always, have their amp hour rating determined by continuous load of 5 A/hr over 20 hrs. Whatever the answer to the discharge question above, in order to determine real world working conditions in which I'd draw from 7 to 20 amps, then would I want to use a fudge factor to approximate real world conditions? I know that there are formulas to determine battery discharge at different loads, but these formulas could only get me so far since there are so many variables. If I were to use a fudge factor, what's a good approximation of what value I should use, 25%? 50%?

    If I have an motor drawing 15 amps from a 100 AH deep cycle battery at 60 degrees F, am I really looking at about 1.6 hrs run time?
  2. Bluejets


    Oct 5, 2014
    Many of the battery Ah ratings will deteriorate as the load current increases so it is not a simple matter to openly say 100Ah/20A = 5hours like you have done.
    There will be discharge curves one can apply and one of the best places to get detailed info is probably battery university.

    You can find them here.......
  3. Madsalts


    Sep 25, 2014
    thanks, I'll check it out.
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