How Long Will That Battery Last

[keninaz]

I know quite a bit about deep cycle batteries as I have had many in the past on RVs and such. I have not however ever had a trolling motor.
I have a 12' aluminum boat, deep V style.
I am well aware that current draw depends on such factors as drag, load and wind in a lake.
Therefore there is nothing standard about current draws for a given speed by the manufacturer I suspect.
However, how do you plan for battery life?
Let's say that I have almost no wind and I want to troll quite some time in my boat with just me in it on say speed 2 of a 40# thrust motor.
Has anyone actually measure the amperage draw on their motor under certain conditions or is is just a matter of guessing on how long a typical croup 27 battery would last?
I am currently working on the boat and trailer fixer upper I bought and I will purchase a motor and battery within the next month or so and it would be nice to hear some real life experiences.
I am older, have a bad back and I would really hate to have to row back in.
I would also not like to spend the money for a spare battery for "just in case."
I am well aware of battery voltages and cutoffs and I can monitor that easily during the use of the battery.

 

[fishrdan]

Re: How Long Will That Battery Last

The easiest way to calculate run time for a trolling motor and battery is to divide the battery reserve capacity by the TM's current draw. With a group 29 battery (something like 210 min reserve capacity) I can troll constantly at speed 2-3 for around 4 hours in my 14' jon boat. Speed 2 in the beginning and as the battery wears down and the motor slows a bit I bump up to speed 3. This motor draws around 40+ amps, so the calculation is pretty close. If I were to push the battery I could probable troll for a full 5 hours, but I switch over when I notice the battery getting low, you can hear it in the motor.

I don't have any equipment to measure the current draw, if I did, I would have measured it... I'm like that Some of the TM's have a max current draw stated on their schematic/exploded diagram, a Minn Kota Endura 46# speed coil motor is 42 amps max. [EDIT] From what I've experienced, the current draw is pretty close for speed setting 1-2-3-4-5.

One thing about speed coil trolling motors, their current draw is almost the same at all speed settings. So running at speed 1-2-3 is not saving that much power compared to running at speed 4-5. Digital trolling motors on the other hand, use far less power running at a slower speed since they use PWM technology. Minn Kota and MotorGuide say you can run up to 5 times longer on a "digital" motor. That's probably a stretch since no one runs their TM at the slowest speed all the time, but you can certainly double or triple the run time in real world conditions.

 

[keninaz]

Re: How Long Will That Battery Last

Interesting.
I may just have to pony up with the extra dollars and get the better motor.
Thanks.

 

[bruceb58]

Re: How Long Will That Battery Last

The easiest way to calculate run time for a trolling motor and battery is to divide the battery reserve capacity by the TM's current draw.

The battery reserve capacity spec is not the capacity of the battery. You have to do further calcs to determine it. The reserve capacity is how many minutes a battery can supply 25Amps before needing to be charged. A battery reserve capacity is specified in units of minutes, not amp/hours. If you want to know the approximate amp hours, take the reserve capacity divided by 60 and multiplied by 25. That will give you amp hours which you can then divide by the trolling motor load.

 

[fishrdan]

Re: How Long Will That Battery Last

The battery reserve capacity spec is not the capacity of the battery. You have to do further calcs to determine it. The reserve capacity is how many minutes a battery can supply 25Amps before needing to be charged. A battery reserve capacity is specified in units of minutes, not amp/hours. If you want to know the approximate amp hours, take the reserve capacity divided by 60 and multiplied by 25. That will give you amp hours which you can then divide by the trolling motor load.

Right,,, but my calculation is still close. My TM draws around 45amps or so max, divide that by the RC (210) and you get what about what I'm seeing from my batteries, around 4+ hours each.

If the TM manufacturer's provided current draw specs, at each speed, you could use AH and calculate from there. But, they only provide the max current draw, and that's at stall speed, so figuring out the actual motor draw while running in normal conditions would require test equipment. Or,,, run the TM a couple times and see how long the battery lasts each time.

 

[BoatNoobie]

Re: How Long Will That Battery Last

Right,,, but my calculation is still close. My TM draws around 45amps or so max, divide that by the RC (210) and you get what about what I'm seeing from my batteries, around 4+ hours each.

If the TM manufacturer's provided current draw specs, at each speed, you could use AH and calculate from there. But, they only provide the max current draw, and that's at stall speed, so figuring out the actual motor draw while running in normal conditions would require test equipment. Or,,, run the TM a couple times and see how long the battery lasts each time.

To my knowledge, the RC is created by a draw of 25 amps. So according to the specs, should be a little over 2 hrs.

 

[bruceb58]

Re: How Long Will That Battery Last

Right,,, but my calculation is still close. My TM draws around 45amps or so max, divide that by the RC (210) and you get what about what I'm seeing from my batteries, around 4+ hours each.

You are taking a spec that has a unit of minutes only and dividing by your current and come up with a result in hours?

Here is the correct way to calculate it. If the RC = 210, the time @ 45A would be 210 x 25/45 which would equal 117 minutes. More likely it will be less as the curve for current draw vs capacity is not linear due to internal battery resistance and other small factors.

Best thing to do is never to exceed the 50% discharge point of the battery which is around 12.0V to 11.8V measured with the battery at rest with no load. Exceeding the 50% point reduces the overall capacity of the battery.

 

[fishrdan]

Re: How Long Will That Battery Last

That's the problem when trying to figure out the run time of a trolling motor, too many variables and some of them unknown, load, motor draw, battery capacity. Exactly how much power does the TM draw during use,,, don't know as those numbers are not published. Exactly how much will the battery deliver, depends on how good the battery is, temp, state of charge.

The 45(?) amps I mentioned is MAX draw, but if you divide RC by that number, it's close to what I've experienced by real world run times. I'm running 2 group 29 batteries and know that I'm hosed if I want to fish longer than 8-9 hours. It's possible to run a bit longer, but at that point I'm paralleling the batteries through the battery switch. Even at that performance of the TM is diminished, and yeah, probably hurting the batteries....

 

[bruceb58]

Re: How Long Will That Battery Last

The 45(?) amps I mentioned is MAX draw, but if you divide RC by that number, it's close to what I've experienced by real world run times.

You might as well divide it by the speed of light because the two results still don't mean anything.

 

[fishrdan]

Re: How Long Will That Battery Last

You might as well divide it by the speed of light because the two results still don't mean anything.

OK, I'll bite, so what's the correct way to figure out how long a battery will last with a trolling motor??? Remember, we don't know what the current draw is during normal operation, we only know what the max current draw is at stall speed.

The calculation of 117 minutes is way off, that's less than 2 hours. I can easily run for 4 hours on 1 battery rated at 210RC.

Like I said, what I posted is what I've experienced in real world conditions...

 

[bruceb58]

Re: How Long Will That Battery Last

OK, I'll bite, so what's the correct way to figure out how long a battery will last with a trolling motor??? Remember, we don't know what the current draw is during normal operation, we only know what the max current draw is at stall speed.

You can't calculate the time unless you know both variables which are average current draw and battery capacity. If you don't know both you won't have an answer and dividing the reserve capacity by a stall speed results in a bogus number.

Best thing to do is either measure the current, which will be touch because a clamp style current meter that measures DC is very expensive, or charge the battery and run the trolling motor until you reach 12V and you will get the time.

 

[fishrdan]

Re: How Long Will That Battery Last

Best thing to do is either measure the current, which will be touch because a clamp style current meter that measures DC is very expensive, or charge the battery and run the trolling motor until you reach 12V and you will get the time.

Clamp on battery meter go for it. Test it out and let us know what your results are.

As far as running the trolling motor until it reaches 12V... How is the OP going to do that when he doesn't even have the trolling motor or battery yet? I'll let you test that one out too, when I'm out trolling around I'm more worried about fishing than testing out trolling motor current draw.

You know, this is just to find out how long a battery will last with a trolling motor, were not calculating battery usage for a trip to the moon...

I'm sticking with my calculation. Is it 100% accurate, no. Will it get the OP in the ball park, for his use of a 40# TM on a small aluminum boat, yes...

 

[bruceb58]

Re: How Long Will That Battery Last

Your calculation means zip.

 

[keninaz]

Re: How Long Will That Battery Last

When the speak of reserve minutes there appears to be no standard.
I have already seen 20 amp and 25 amp loads quoted.
Without a standard it means nothing, nada, zip
I prefer the old style of deep cells where they just listed the amp hours.
If you are interested the group a battery in means little either.
For instance I have researched and found the following variances within the same groups depending on the maker~

Group 24 70-85 AH
Group 27 85-105 AH
Group 31 95-125 AH
4 D 180-215 AH
8D 225-255 AH
Gold Carts and T 105s 180-255 AH (6 volt batteries)

Now I see the variances by group getting larger with bigger batteries too.

I know I had quite a nice setup in on 5th wheel we had running the t 105s.

So within manufacturers specifications there is quite a variance and many like noted just wanted to quote reserve minutes without telling you the specifications or load testing that was involved at all.
Further, as originally stated, Minn Kota anyway as near as I can tell does not even discuss current draws.
Does your Minn Kota manual tell you what the typical draw is? You can't download a manual at their site so I don't know.
So I guess the only way to guess is to look at the line fuse and figure the maximum draw will be less than that.
I also find it funny that lots of folks keep pointing towards the newer digital motors. What I am finding so far is that they are not coming close to up the 5times the range as the ads claim on the same battery, thrust for thrust.
I can tell you this. I will have more information before I buy from a selling of motors.
I spent my life working on electronics and electrical stuff before retiring and I will get details one way or the other if I have to find someone with a boat and motor and I will take current readings while he operates it.
And yes, I do have a DC Amp Meter that will go 100 amps and know how to use it needless to say.
And if the manufacturers want to play the game with only quoting reserve minutes I will know there standard for testing as otherwise it means nothing.
It's not all rocket science.

 

[fishrdan]

Re: How Long Will That Battery Last

Means zip,,, but it works I'm all ears to hear of other calculations you might have for figuring out run time for a trolling motor and battery, with the available printed/online specs.







The only current ratings I've seen for TM are on the schematic/exploded parts diagram, where they also list the maximum current draw of the TM at stall speed.

http://www.fish307.com/mk_parts/2003/transom/endura/Endura%2046.pdf

 

[bruceb58]

Re: How Long Will That Battery Last

The same motor on two different boats is going to have different current draws. The only way is going to be to measure the current.

Capacity of batteries with the same case size will vary greatly as its not the case size that determines the capacity but the area and volume of the plates inside.

There is nothing magical to math but apparently its very challenging to some people even when explained over and over.

 

[fishrdan]

Re: How Long Will That Battery Last

The difference between the size of boats the OP and I have (and I have had in the past) is marginal. I've used this size of TM on 5-6 different small boats (mine and friends) and the run time is similar, it's a marginal difference at best. A bigger affect on run time is load in the boat and fighting wind/waves/current.

I do agree about the mathematics being quite a challenge as I've only seen 1 calculation offered that uses the known parameters, that's even close to the expected run time...

Let's say that I have almost no wind and I want to troll quite some time in my boat with just me in it on say speed 2 of a 40# thrust motor.

That's exactly what I do ALL THE TIME, a couple times a month for at least 15 years, had 3-4 small boats and all of them had had similar run times. Does anyone else have similar experiences they would like to share?

 

[GLENN M]

Re: How Long Will That Battery Last

an automotive amp gauge is cheap,one that measures 60 amps is common as dirt did i say cheap,the guy wants to know approximatly how long he can go and not row back.nevermind text book crap,just anyone with real time info

 

[Socal Pat]

Re: How Long Will That Battery Last

an automotive amp gauge is cheap,one that measures 60 amps is common as dirt did i say cheap,the guy wants to know approximatly how long he can go and not row back.nevermind text book crap,just anyone with real time info

I agre with this and Fisherdan. While Bruce is paralized onshore with the calculator the rest of us are trolling and creating real-world data. The number crunchers will tell us how wrong we are, but in the end we experience what we experience. I myself have experienced a 4-6 hour window on a group 27 Wallmart deep cycle pushing a 14' tinny at lower speed. This was on a 2001 Minkota transom mount 38lb IIRC.

A piece of advice for the OP from another old guy that hates to paddle... just get a smaller spare battery for emergency use. It'll get you back if need be. Hey the extra expense is just part of boating right?

 

[sasto]

Re: How Long Will That Battery Last

I agre with this and Fisherdan. While Bruce is paralized onshore with the calculator the rest of us are trolling and creating real-world data.

An optimist says the glass is half full.
A pessimst says the glass is half empty.
An engineer says you obviously have the wrong size glass.

Good Day!