Trolling Motor Battery Life?

[meerkat]

Can anyone help with a realistic expectation for how long I can run my trolling motor on one charge??? Running between 4 & 6 power I am only getting about 6 hours before the batteries are dead (and I mean completely dead - registering about 8 volts).

This is a newly installed Minn Kota 70 PowerDrive with 2 brand new 27 series batteries from Walmart. I have a 3 bank on-board charger which shows fully charged & I have tested the charge with a volt meter. I have had the batteries load tested and they tell me they are fine.

Any ideas about what could be the problem? Am I expecting too much?

 

[Silvertip]

Re: Trolling Motor Battery Life?

Well sir -- lets run some numbers and see what it shows. Your motor draws 42 amps MAXIMUM meaning running at the fastest speed. Now electric steer takes power too so the amount of turning impacts run time as well. Running between #4 and #6 is about 1/2 speed which does not mean 1/2 of 42 amps. We really don't know what that is because those numbers are not published anywhere and the only way to determine it exactly would be by installing an ammeter in the POS lead to the motor. But -- lets assume current draw at those settings is in the 23 amp range. Your group 27 batteries are probably rated at about 180 minutes reserve @23 amp draw (as specified on the battery label). So at the "assumed" 23 amp draw you batteries should drop out of spec at about 3 hours. Because you have two batteries in series does not double run time -- parallel batteries accomplish that. So if you are able to run 6 hours you are indeed punishing the batteries severely and the lesson learned here is that your motor probably draws less than 23 amps and more like perhaps 15 amps at those settings. You can't expect more than that from those batteries. They are doing their job but won't last long if you are running them down to 8 volts.

 

[meerkat]

Re: Trolling Motor Battery Life?

Silvertip - thanks for the comeback - that helps me understand what is going on.

As a follow-up, please help me understand the battery label & ratings. The label on the battery has 3 specs: Marine Cranking amps @32F = 720, Cold Cranking amps = 600, and then "Amp Hrs = 115". I know the first 2 don't really have any relevance here but is "Amp Hrs" the rating you mean when you say "180 minutes reserve @23 amp draw"? If so, can I reasonably go to a bigger (more reserve) battery? I have room in the boat for bigger batteries (so long as I can figure out how to slide the cost by the accountant at home).

 

[Silvertip]

Re: Trolling Motor Battery Life?

Are your batteries Deep Cycle, or Dual Purpose (Starting/Deep Cycle). The only reason I ask is that the CCA numbers don't normally appear on straight Deep Cycle batteries. They normally carry the Reserve Minutes: xxx @23A. I went through this discussion with another poster but he didn't have a battery and was wondering the same thing you are. The Amp Hr rating is sort of the same thing as the reserve minutes but I will try to explain what the numbers mean. Reserve Minutes XXX at 23A means the battery will deliver 23 amps for xxx minutes before it can no longer meet that load. So if your motor drew 23 amps at some speed setting, the battery would maintain that load and motor speed for whatever period the XXX minutes was. On a battery labeled with Amp Hours the number means the battery would deliver 115 amps for 1 hour or 1 amp for 115 hours. So using the same 23 amp trolling motor draw we used above, you would stuff 23 into 115 and where I went to school that would result in exactly 5 hours of run time. So you see -- things work out the same either way but the reserve minutes is a more concrete specification since they guarantee 23 amps for the specified period. The motor will obviously run longer than the reserve minutes but it would be losing power because the battery has begun the sharply dropping portion of the discharge curve. You might want to check group 31 deep cycles (not dual purpose) or if you have the room, buy another pair of group 27s and parallel them or include a switch to go from one set to the other. That would get you double the time you have now.

 

[meerkat]

Re: Trolling Motor Battery Life?

The label says "deep cycle" but in looking around on the internet at different "marine batteries" is seems that the manufacturers have blurred the lines between starting and deep cycle because many of the batteries specifically labeled "deep cycle" do carry the CCA numbers.

One thing I did notice on the specs for the different batteries is that there is a wide discrepancy on two batteries with otherwise very similar specs. One 27 series battery is listed at 50 pounds and another is listed at 80 pounds. Do you have any idea of the performance implications of the weight difference? Other than weight all the other specs are very similar.

But in any event thank you for the explanation of what the specs mean. I am now much more informed and that will help a lot when I purchase new batteries.

 

[meerkat]

Re: Trolling Motor Battery Life?

Sorry - meant to say wide discrepancy on weight between two batteries with otherwise very similar specs...

 

[Silvertip]

Re: Trolling Motor Battery Life?

Weight is probably a good indicator of construction - meaning thicker, heavier, and a bigger number of plates go into the battery. This allows it to have more "reserve minutes" or higher CCA numbers. While I don't have performance specs for every battery ever produced nor do I consider myself a "battery expert" one can look at the CCA, Amp/Hr, Reserve Minutes and come to some conclusions. While a relatively big battery and a relatively small size battery may have the same CCA rating, what is missing is the "time factor". In other words how long will the battery deliver xxx CCA which is important if you have a "cranking/starting" battery. The big battery will likely spin the engine far longer than the small battery. If the engine should turn balky that is important. Small piston powered airplanes for example have pretty small batteries but since one does not routinely start and stop the engine in flight that's not an issue. However, on the ground, those engines do not spin very long on those batteries. Reserve minutes and Amp/Hr are far better measures of "run time" since they include both a load and time factor. Yes -- you will find variations in specs within a given battery physical size. Reserve minutes uses an industry test load of either 23 or 25 amps however (mostly 23 amps from my observations).
For starting your boats engine or the engine in your car, look for CCA AND Amp/HR ratings. For a trolling motor look for Reserve Minutes XXX @23A or Amp/Hr. Bigger numbers are better in all cases.

 

[bruceb58]

Re: Trolling Motor Battery Life?

If you want your batteries to last longer, don't run them down below 12V. You NEVER want to fully discharge a battery...even a deep discharge one. The more you discharge them, the more their capacity is diminished.