Posts: 2752
Location: Mauston, Wisconsin | mseybert - 1/16/2009 6:17 PM
I found some calculations online that give an estimate of runtime for trolling motors at a given setting. This is probably not a hard fast calculation, but a reasonable estimate.
For Example: A 55 pound 12V trolling motor is likely to draw 1.2 amps per hour at 100%. Say you have a battery with 100 amp hour rating you might expect to get:
7.5 hours at 20% power
3.8 hours at 40% power
2.5 hours at 60% power
1.9 hours at 80% power
1.5 hours at 100% power
I found it interesting and probably just a good guesstimate.
Mike - You are right it is a guesstimate, although a poor one. As Shep said above if you cut the load in half the runtime will be 3X. You get the same effect by adding a second battery. That is definitely a provable engineering fact for lead-acid batteries. The unknown author's chart doesn't fit that fact. f. ex. from 80% to 40% is a 50% cut, yet the author has simply doubled the hours. He really misses the mark at 100% power.
Ampere hours (Ah)
Tested at 77 degrees Fahrenheit. It is the amount of current (in amps) a battery can deliver, multiplied by the amount of hours, without falling below 1.75 volts per cell (10.5 volts on a 12-volt). Most marine/RV-deep cycle batteries are rated on a 20-hour discharge rate. Example: a 100 Ah battery can deliver 5 amps for 20 hours (amps x hours = Ah).
There's also something wrong with his ampere figure of 1.2A
Basic Electricity for Muskie Fisherman 101:
Electrical engineers (like Shep & myself) measure electrical power (real power) in watts.
1 Volt (V) x 1 Ampere (A) = 1 Watt (W)
so given 12V x 1.2A = 14.4W Your body radiates more energy just resting. Does anyone honestly think a 55# TM only draws 1.2A at 100% load? I don't. Actually a quick google search turned up this ->
http://www.buy.com/prod/minn-kota-edge-55-lb-thrust-trolling-motor-...
50 Amps Max draw. That's quite a difference. How long would a 100AH battery last on this motor at different power levels? Well if I look at a Trojan 27-AGM 12V
Deep Cycle Battery it's rated 100AH. It has a marine reserve capacity of 175 minutes at 25A@77F.
100% power (50A) = ~58 minutes
66% power (35A) = ~ 116 minutes (1.93 hrs)
50% power (25A) = ~175 minutes (2.91 hrs) - marine reserve capacity
33% power (15A) = ~ 348 minutes (5.8 hrs)
25% power (12.5A) = ~ 525 minutes (8.75 hrs)
15% power (7.5A) = ~ 1044 minutes (17.4 hrs)
10% power (5A) = ~1200 minutes (20 hrs)
For those whom may be interested - 1 hp = ~746 watts
So the 55lb trolling motor uses ~ 600W at 100% power or ~ .8hp of electrical energy.
Are we having fun yet? I agree with Lambeau- go for the most power you can afford - both in battery and TM capacity. If you don't have it, you can't use it!
Al
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