Calculating Battery Life for Trolling Motor

[KSanford33]

I have a 30lb thrust trolling motor, and a marine battery that has 685 cold cranking amps and 140 reserve. How do I calculate the running time for the trolling motor? And please, just like in school, show all work (or at least explain how you got there). ?

[A-Jay]

Far too many variables to even come close.

Go fishing and see how it goes.

If you get home with juice to spare, you're good.

If you find yourself cursing your battery.

Get bigger one or perhaps add a second one.

Good Luck.

A-Jay

[MN Fisher]

5 minutes ago, A-Jay said:

Far too many variables to even come close.

Not exactly

 

140 reserve translates to 58ah

RT/60*25 - https://battlebornbatteries.com/battery-reserve-capacity/

 

A MinnKota C2 30# will draw 30 amps at speed 5, 5 amps at speed 1

https://www.trollingmotors.net/blogs/selection/86961351-calculating-motor-run-time

 

Now it's just simple math

At top speed, you got just under 2 hours before the battery is completely drained...which is a No-No for Lead-Acid batteries (wet cell, gel-cell, AGM) - so call it an hour.

If you go slow the whole time - you can effectively get 6 hours.

 

Your actual time will be between those two based on what speed you go for how long.

[A-Jay]

13 minutes ago, MN Fisher said:

Not exactly

 

140 reserve translates to 58ah

RT/60*25 - https://battlebornbatteries.com/battery-reserve-capacity/

 

A MinnKota C2 30# will draw 30 amps at speed 5, 5 amps at speed 1

https://www.trollingmotors.net/blogs/selection/86961351-calculating-motor-run-time

 

Now it's just simple math

At top speed, you got just under 2 hours before the battery is completely drained...which is a No-No for Lead-Acid batteries (wet cell, gel-cell, AGM) - so call it an hour.

If you go slow the whole time - you can effectively get 6 hours.

 

Your actual time will be between those two based on what speed you go for how long.

 

I stand by my OP.

But this looks really impressive.

A-Jay

[Woody B]

@A-Jay is right.  Unless trolling no one is going to be running the trolling motor constantly at the same speed.    

 

The math is simple but doesn't always relate to real world conditions.   

[Jig Man]

That simple math might be accurate if the battery were new and fully charged, the trolling motor was in perfect running order with no worn brushes and there was no wind or current.  Otherwise I see it as hypothetical.  Which places me in A-Jays camp.

[KSanford33]

32 minutes ago, MN Fisher said:

Not exactly

 

140 reserve translates to 58ah

RT/60*25 - https://battlebornbatteries.com/battery-reserve-capacity/

 

A MinnKota C2 30# will draw 30 amps at speed 5, 5 amps at speed 1

https://www.trollingmotors.net/blogs/selection/86961351-calculating-motor-run-time

 

Now it's just simple math

At top speed, you got just under 2 hours before the battery is completely drained...which is a No-No for Lead-Acid batteries (wet cell, gel-cell, AGM) - so call it an hour.

If you go slow the whole time - you can effectively get 6 hours.

 

Your actual time will be between those two based on what speed you go for how long.

Thank you sir. 
 

@A-Jay I’m sure you’re 100% correct as far as real results, I just wanted to get the mathematical side of it. Especially knowing that for a full day out I’ll need another battery. 

[Way2slow]

A-Jay is a lot closer with his reply than that chart.  If you depend on that chart, make sure you take someone that can help paddle.  Mathematically that looks correct, but there are some variables that are not accounted for when doing the math.

First off, it's not a true deep cycle battery if it has a CCA rating so you can bet it rated using the 20 hour method.  Meaning a 100 amp hour (Ah) battery is only that at about 5 amps per hour.  You are not gonna go very fast at five amps per hour.

As the load is increased. internal resistance increases (creating loss due to internal heat) and causes the Ah to drop.  At 30 amps, you can figure at least a 20% drop in the Ah capacity of a battery

The next variable they fail to account for is, that 100Ah is based on when the battery has reached full discharge, that's going to be somewhere around 11 volts or slightly less.  Even at 11.5 volts the TM is going to be slowed down to a ridiculously slow pace, to the point it's almost useless, so that's another 20% of battery capacity that's going to be useless to you.  

With all that hidden loss, your run time is only going to be about 60% of what the math says it's going to be, and if it's not a good, almost new battery, it's going to be even less.  So, good luck on thinking you're going to get what that simple math says.

I should also mention, this is all based on analog TM, no digital style. Digital motors use pulse width modulation to run the motor so the more off of full speed you run, the greater the run time you can get over the conventional TM.  If you are using a digital TM, when just easing around casting, you can easily triple your run time.

In general, the Reserve Minutes (RM) are a much more accurate way to calculate one mathematically.  RM is based on a 25 amp load (with most batteries) which is a little more realistic to what you could be using over a 5 amp load in the 20 hour rating.  You are still going to lose about 20% because the battery voltage being to low to run the TM at speed when it's discharged. and you are still going to lose some capacity as you go over that 25 amps used to rate the battery but it won't be as great of a loss as with the 20 hour rating.

No matter the rating method, no battery is going to last what the math says it will because of those variables. 

[WRB]

#1 the OP is using a cranking 12V battery from his description #2 we don’t know if it’s a marine deep cycle.

#3  we don’t know the battery to TM wire size or length.

A-Jay 1st reply is spot on!

Tom

[KSanford33]

So considering the fact that I’ll need another battery for what I want to do regardless, can I wire a traditional car battery in series (or parallel, I can never remember which) to extend the usable life?

[GaryH]

1 hour ago, KSanford33 said:

So considering the fact that I’ll need another battery for what I want to do regardless, can I wire a traditional car battery in series (or parallel, I can never remember which) to extend the usable life?

You run them in Parallel. It’s best to run the same type and same age for your best results. + to + and - to -  I would not run a automotive starting battery in parallel with a deep cycle marine battery. 

[WRB]

Auto batteries are not designed to be discharged, not deep cycle. Car batteries have lighter weight cases and thinner plates that make up 2V cells. A 12 volt battery has 6 cells 2V each connected internally in series to = 12V. 

Deep cycle marine batteries have stronger battery cases and thicker plates in each cell design to be fully discharged and recharged hundreds of times.

You never want to put a weak battery in series with a strong new battery, it’s the weakest link no better then the old battery.

2 ea 12V batteries wired parallel; + to + (positive)  and - to - (negative) = 12Volts.

Think of amps as water in a hose, volts the pressure to push the amps though the hose. If the wire (hose) is too small the pressure (voltage) drops. Wire size is important, larger = less voltage drop with DC current, the reason battery cables are large diameter to reduce over heating.

At the end of the day It’s simpler to use larger group size battery 29 or 31 deep cycle marine battery then trying to wire 2 batteries parallel.

Next item to consider is your battery charger!

Tom

[Bankc]

In order to safely run two batteries in parallel (or series), you'll need to buy two new batteries.  Never mix two old batteries in parallel or series, even if they're the same brand, because you can't guarantee they have the same amount of wear. 

 

You could run two batteries and just switch between them (either with a switch or manually removing one and hooking up the other).  That would get you same runtime as two batteries in parallel (or more) and avoid any dangerous situations like batteries exploding or catching fire because only one would ever be connected at a time.  

 

The best bet if you're looking to maximize trolling motor range or speed is to go with a lithium (LiFePo4) battery.  They can be drained down much further than traditional batteries (they should shut themselves off if you drain them too deeply to prevent damage), have a slightly higher voltage, much flatter discharge curve, and are much lighter.  They also have a much longer lifespan, so over the course of the battery's lifetime, they could actually be cheaper than traditional batteries.  And while they also can be hooked up in series or parallel (sometimes depending on the battery in question), you can find 24v or 200Ah lithium batteries fairly easily, which tend to be cheaper than buying two 100Ah 12v batteries.  

[Deleted account]

On 11/5/2022 at 3:38 PM, KSanford33 said:

I have a 30lb thrust trolling motor, and a marine battery that has 685 cold cranking amps and 140 reserve. How do I calculate the running time for the trolling motor? And please, just like in school, show all work (or at least explain how you got there). ?

You would need the amp draw at the different speeds, and then calculate the watts used per unit time at each speed, and then an approximation as to the actual usage at those speeds. You could hook up a meter that actually reads those #s real time if you were so inclined, OR, you could do the experiment of running the battery down on continuous use per speed, and voila. 

[WRB]

Just curious....what boat are you using this 30# thrust TM on?

Tom

[KSanford33]

4 hours ago, WRB said:

Just curious....what boat are you using this 30# thrust TM on?

Tom

It's a 10ft Pelican Predator. Small enough to still fit in the bed of my truck.

[WRB]

Makes sense now.

You need 1 group 27 AGM deep cycle battery.

Tom

[KSanford33]

1 minute ago, WRB said:

Makes sense now.

You need 1 group 27 AGM deep cycle battery.

Tom

Thanks Tom. This should go without saying, but I'll listen to your advice on anything regarding fishing.

[Woody B]

I may be disappointed some day but from my experience lithium batteries are magic.   The 88 amp hour group 27 conventional deep cycle battery that came with my boat was really weak after 4 to 6 hours in the wind.  (2022 Tracker Classic XL)   I bought a 125 amp hour lithium (overkill, buy once cry once).  I've fished multiple days without charging in the wind and never had it below 50%, or noticed any drop in performance.  

 

My battery is an Ionic, and cost a BUNCH.   If I were you I'd get a 100 amp hour lithium from Amazon.  It wouldn't be any more expensive than a group 27 deep cycle.   You would need a charger that's lithium compatible though.