Re: Good Girl Gets a Motor (was: Correcting my EARLIER Instrument Notes on EV Power)


That's a good setup. You'd have to spend a lot more money to get a trolling motor with electronic speed control, and 30 lb. thrust should move Good Girl just fine. The runtime graphic is necessarily just a rough guide, but it does take into account Mr. Peukert. That motor should draw about 30 amps at full "throttle". It'd be nice to do some tests with an ammeter to see what it draws under typical conditions pushing Good Girl at the different speed settings. I did such a test with a 30 lb. Minn Kota on Pickle years ago but, alas, the results got lost in a computer upgrade since then. One of these days I'm gonna rig up a portable ammeter for doing trials like that, so maybe I can go out in Good Girl sometime for some test runs.

Get a digital voltmeter (cheap from harbor Freight will do) and keep an eye on the battery voltage now and then as you go along. I charged Tuffy's flooded cell batteries if they got down to 12 volts under light load -- my usual pace getting back to the dock after a sail -- which I figured was close enough to 50% discharge. The only really accurate way to to tell the state of charge by voltage is to let the battery rest for several hours before testing. Not very practical for us! <g> Here's some basic info about the Peukert effect and what I think is a practical voltage/state of charge chart, because it agrees with what I've been doing. <g>:

Don't sweat it if you have to go a little below 12 volts to get back to the dock, but recharge the battery right away, and promise to try to treat the battery better in the future. <g>

"Smart" battery chargers are getting cheaper and more common. I wouldn't be surprised to discover that they're the most common kind above the cheapest ones nowadays.

On 4/8/2021 10:32 AM, Josh wrote:
It's a Minn Kota 30lb thrust and the battery is a Duracell deep cycle 80a/h. I saw what I think you were describing with the Peukert effect when I was looking at runtime and how drastically it reduced as the draw increased. Reminds me of the energy to go from displacement to planning.
Think the charger I have is a smart charger but will be checking.
John <>
If you can't be a good example, then you'll just have to be a horrible warning. (Catherine the Great)
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