Newbie needing battery info

[fathairypirate]

Hello all,

I'm wanting to get a good battery for a minn kota endura 34 on a <12ft inflatable raft for fishing hard to reach places. It will be my primary power source and only used for trolling, not starting a small outboard or running any electronics gear.

I have never bought a battery other than an automobile battery for my cars. I have no experience with marine batteries, but was recommended a maxx marine deep cycle from walmart (12v @ 125ahr $65.00). I've searched around but can't find a good FAQ for marine batteries as to what I need to do to ensure longevity both during use and during storage. Info seems varied and highly opinionated. I have done enough research to know that the maxx battery is rated for starting, trolling and RV use, but is this really a good way to go because I will exclusively use this for trolling and will probably run the battery down low during use (which is 1-3 times a month).

Can anyone point me in the right direction or offer their advice please? Remember I am complete newbie to this scene so acronyms and obscure terms will need to be spelled out for a beginner. This might include information such as recommendations on a good battery recharger and proper use of said item.

Thanks!

 

[Silvertip]

Re: Newbie needing battery info

For a trolling motor you need a Marine "Deep Cycle". The next consideration is "what size". That consideration deals with the physical size of the battery. Sizes are typically specified as "Group 24, 27, 31" etc. So whatever battery size (and therefore weight) you have room for is a big consideration. Next is the amount of run time you need. When selecting a battery the rule of thumb is buy the largest "capacity" you have room for and your wallet can tolerate. Run time is determined in part by the label on the battery. The number to look for on a Deep Cycle is "Reserve Minutes: xxx @23A. The larger (physically) the battery is the more power can be crammed into it so what these numbers mean is at a 23 amp draw, the battery will be able to deliver that amount of current for xxx minutes. Your motor has a maximum current draw of about 30 amps which would be at full speed, perhaps into the wind or fighting river current. But for simplicity lets use 23 amps siince you likely won't be running full speed all the time. So at a 23 amp draw, a battery with say 180 reserve minutes, would run the motor for about 3 hours. I just purchased a group 27 deep cycle at my local farm store for $73.00 and some change. It was rated 200 minutes @23 amp draw so it would run for approximately 3-1/2 hours before it would be unable to deliver 23 amps. Another way to look at this would be if you had a larger motor -- say a 50# thrust which draws about 42 amps, then that same battery would run the motor for only 1/2 that time. 42 amps is nearly twice the current draw of 23 amps so the run time is therefore 1/2 as well. Be aware that a group 27 deep cycle is a very heavy battery so if you have to tote it very far it would be wise to have a small cart of some sort for that purpose (unless your name is Bubba and your physique is worthy of the name).

 

[fathairypirate]

Re: Newbie needing battery info

Thanks for attempting, but you might have missed the fact that I'm a beginner and have no knowledge of jargon. What are the differences between the 'groups' of batteries (you mentioned 24, 27, 31, etc.) Does the size/weight go up as the 'group #' goes up? Which group is the battery i mentioned before fall into?

For the runtime, as stated I'm using a minn kota endura 34. I think it's a 12 volt motor, so what are the math calculations needed to figure out how long it's going to last? You asked how long i'd need it to run. I'd imagine 3 hours at the shortest outing. Of course, I'd like to run it 5 hours if possible. Recommendations?

 

[Silvertip]

Re: Newbie needing battery info

Did you not see the "physical size" (group 24, 27, 31 etc) in my response. Physical size is the dimensions of the battery. A group 24 is smaller than a 27 or 31. A 31 is bigger than a 27 or 24. The numbers you gave regarding the battery you referenced are meaningless because they do not relate to "physical size". Again, read the label on the battery - it tells you what size it is. Engine starting batteries have CCA (cold cranking amps listed) for example. Deep Cycles do not because they are not used for that purpose. Likewise starting batteries don't list "Reserve Capacity" because they are not deep cycles and not used for that purpose. Capacity is the AHr rating or Reserve Minutes, or Cold Cranking amps. It is all on the label. Bigger numbers are better in all cases. As for run time, I used your motor as an example. Re-read my post. I can't tell you which battery you need precisely because I have no idea how you plan to use the motor. All we can do for you is run the numbers for maximum current draw for your motor. Running slower is just like your car -- better gas mileage the slower you go (to a point at least). Trolling motor manufacturers don't publish current draw numbers for every possible speed but they do publish a maximum because it is easy to test for. If it is necessary for you to run 5 hours I suggest you look at something no smaller than a group 27 with the highest reserve capacity you can find. I see no reason for buying the high-tech AGM and other high-end batteries. A good old lead-acid battery from your local farm or discount retailer will work just fine.

 

[fathairypirate]

Re: Newbie needing battery info

Did you not see the "physical size" (group 24, 27, 31 etc) in my response.

I did, but you didn't explain until last post that as the numbers go up, so does size/power. Thanks for the clarification.

The numbers you gave regarding the battery you referenced are meaningless because they do not relate to "physical size".

I was using those numbers because I have heard that whatever # thrust a trolling motor is, that's how many amp hours it pulls at full power. So by that logic, my 34# minn kota supposedly pulls 34 amps per hour. If I use the amp rating of the battery (125) the math would indicate that it would run for 3.6 hours. From your statements, it would appear you use a different means to figure out runtime, which you did not state.

So at a 23 amp draw, a battery with say 180 reserve minutes, would run the motor for about 3 hours.

What equation did you use to come up with this so I can do some mental math when shopping in a store? It would appear that it's basically the same outcome as my above mentioned 'amp math'.

Thanks for your patience.

 

[Silvertip]

Re: Newbie needing battery info

OK -- one more time.
1) Whoever told you whatever the thrust is is what the amp draw is was wrong. A 55# PowerDrive for example draws 42 amps. Your 34 Endura is not listed in my data but it will draw somewhere in the 30 amp range. So for purposes of this discussion we will use 30 amps. That is the MAXIMUM current your motor will draw. Before we go farther, the only way to determine what the actual current draw is at slower speeds is to measure it. You do this with an ammeter connected in series with the positive post on the battery and the positive wire in the troller cable. Put the boat in the water and measure the current at the various speeds. Then and only then can you figure run time at slower speeds. You will need an ammeter that can read a MINIMUM of 40 amps. Most hand held meters cannot do that so a simple panel meter from Radio Shack can be used.

2) Batteries are like buckets of electricity. Bigger buckets can provide longer run times.

3) Forget the CCA (cold cranking amps and AHr (amp/hour) ratings as these are specs for CRANKING batteries. Deep cycle batteries don't contain this info. For this discussion we will use RESERVE MINUTES (xxx) @23 amps.

4) Based on 30 amp draw we can calculate run time as follows. The test current spec is 23 amps so you need to divide 23 by 30 which comes to 0.77%. Why do this? Because your motor draws more than 23 amps. The delta is 0.77%. Now whatever battery you are looking at you simply multiply the Reserve Minutes by 0.77. Why? Because you are drawing more than 23 amps to the reserve minutes must be reduced by 0.77. In the case of a 200 minute battery, you would be able to run about 154 minutes or abour 2-1/2 hours. Now FOR EXAMPLE: lets say you motor draws only 23 amps at speed #3. 23 divided by 23 = 1 where I went to school. So 200 x 1 is 200. At that speed you could run for 200 minutes.

5) So the only number you need to remember is 0.77. Multiply whatever the reserve minutes is on whatever battery, whatever group size, and whatever price point you find to determine the absolute MINIMUM amount of time you can run on those minutes. Note I said MINIMUM because that is at maximum current draw which drains the battery quickest. Run time would be longer if you slowed down. Just like your car -- better mileage at slower speeds (to a point).

6) The relationship between reserve minutes, test spec (23A) and group size is simple. 23A is the test spec for all sizes. Bigger batteries (group size) can provide 12 volts over a longer period.

Does this help? It boils down to a big tank of gas or a small tank of gas.

 

[fathairypirate]

Re: Newbie needing battery info

Just so you know, I got the idea straight from the Minn Kota FAQ on their website:

# What is Amp Draw?

* Amp draw is the measurement of electrical current drawn from a storage battery (or battery), while the motor is being operated.
* Minn Kota's published amp draw figures represent actual on-the-water conditions at full power (with all of the motor's wiring, switches and circuitry in the electrical system as the test measurement is taken).
* Approximate length of running time when operating a motor at full power can be determined with a motor's amp draw and the battery amp hour rating.
* As the speed setting is reduced the motor amp draw is also reduced.
* A motor drawing 30 amps at high speed may only draw 5 amps at the slowest speed. The same motor that will theoretically run about 3.4 hours at high speed will run for over 20 hours at the slowest speed.

*EDIT* I also just found this, their calculations:

* Example: A single 120 amp hour, 12-volt battery and a 12-volt trolling motor drawing 30 amps at the highest speed setting will run for about 3.4 hours.

#The formula for the calculation is as follows:

* .85 A.H.R. = Hours of running time (A.H.R.= Battery Amp Hour Rating)
M.A.D. (M.A.D. = Motor Amp Draw)

Which is to say (.85 * 120ah) / 30 = 3.4

 

[Silvertip]

Re: Newbie needing battery info

Yup -- except we are talking about DEEP CYCLE batteries. So what do you do when the A/hr rating does not appear on the label? Deep Cycle battery labels generally don't carry an amp hour rating which is why I explained how to determine run time using reserve minutes. Minnkota is also not being fully truthful either. Older motors draw nearly as much power at slow speeds as they do at full speed because they used "speed coils" (wire wound resistors) which drop the voltage to the motor. The voltage that is dropped by the resistors turns to heat so there is no savings in these motors. PowerDrives use a pulsed system to power the motor at slower speeds so current is greatly reduced. Nevertheless, you asked about batteries, I answered your questions using very real examples. I happen to have owned many trollers over the years so I do speak from experience. You have the info you need so shop, buy what you need and go boating. Don't overthink this. If you need five hours run time you need a big battery. If you run wide open for extended periods you need a bigger one. If you don't have room for a bigger one you need to compromise. I'm done here.

 

[fathairypirate]

Re: Newbie needing battery info

Thank you for gracing me with your wisdom.