Hey guys, l have a new to mid 90s 15' sea nymph (aluminum flat bottom) with a 30 hp merc jet. It's also mid 90s. I was told that it's a 50 HP powerhead that becomes a 30 out the pump so they consider it a 30 and that's what it's marked as. I'm only specifying this because it's my understanding that they don't do it that way anymore. I believe now this motor would be rated as a 50 HP. I could be wrong. But I want you to understand the size we are talking about. 

 

This boat is set up for dual batteries but has none. My buddy has a similar boat and runs one big deep cycle marine type 31 for the trolling motor but then cranks the engine and runs the fish finder off a very small car batt. 

 

I've never set one of these up but does that sound about right? A big type 31 seems like an awful lot just to crank over this merc. Do any of you guys know how small of a battery I could get for just cranking over my motor? Seems like a good way to save some weight.  

Normal boat battery setups have the cranking battery for starting the motor, running pumps, electronics, lights etc. and 1 -2  or 3 batteries dedicated to the trolling motor.  That depends on the required volts of the troller (12-24 or36).  For your cranking battery, normally your motor will have a minimum Marine Cranking Amps (MCA) requirement.  Depending on how many accessories you wire up to it will determine how much above that minimum rating you will go based on the battery’s Reserve Capacity which is usually stated in amp hours. 

3 hours ago, TOXIC said:

Normal boat battery setups have the cranking battery for starting the motor, running pumps, electronics, lights etc. and 1 -2  or 3 batteries dedicated to the trolling motor.

Where-as I went one more

Cranking - that's all it does

House - runs lights, pumps, sonar, etc

TM - that's all IT does.

 

That way I can run the sonar/livewell until the battery conks out and still be able to start my motor.

 

Course the 25 DOES have recoil backup

My fishing partner has 5 batteries in his 20ft Ranger with a 250 4 stroke merc.  3 lithiums for the PowerPole Move trolling motor, 1 lithium “house” battery for all of his graphs and accessories and 1 AGM for solely cranking the big motor.  He has an onboard 4 bank charger for the 3 lithiums and the cranking battery and he carries a small portable lithium charger for the house battery. It won’t be long before he swaps out the AGM for a lithium now that they have refined them to be used as cranking batteries as well.  

Thanks guys. I just want to figure out the minimum cranking amps? Is it labeled somewhere?

Manufacturer Recommendations for a Mercury 2 stroke 50hp and below is 245 CCA, 323 MCA, 52 RC@ 25amps, 40AH@ 20AH Rate.  That is for the motor alone.  You start adding graphs and other things to it, you are going to want more Reserve Capicity.  Also, never hook a trolling motor up to your cranking battery.  Wire them to their own batteries.  

Motor should list what it wants for a minimum - I'd get something rated 20% more ( CCA or MCA) above that. To play it safe most outboards want between 200 and 300 - so a 500 CCA/MCA would cover you just fine.

 

my 25hp Envirude wants a minimum of 375 - so I dropped a 500MCA into the boat.

I just went from 1 battery to 2 batteries for the first time. 5 batteries might last me all season without needing a charge 

Any group 24 automotive battery will have plenty to start your engine.

If you want a full days TM and have electronics, then opt for the 31 AGMs for both.  With one your stuck with a 12V TM.   Wet cell are passe for these apps.

If there's no space for 31s, put the biggest one in the fits.   The AGMs can do both, last years longer so there's not need to have two types.   

I'm leery of lithium still.  Why?   If they catch fire?   That little boat fire Ex won't put them out.   Toxic fumes when burning.   No where to run to when you're on the water, and most fishermen I know don't stack up to water walkers!!!   But weight savings is a plus and can get one 36V vs 3x12.   

My first boat was a 17 ft Tracker with a 50 HP motor and a 12V trolling motor.  I ran 2 dual purpose group 27 batteries in parallel for the trolling motor.  When the TM batteries got old I would take the best one and make it my cranking battery.  I can’t ever remember the motor not cranking.  I carried jumper cables just in case so I could jump off the TM batteries in a pinch.  I ran the electronics off the TM batteries.  This can sometimes cause interference on the electronics but I never had a problem.

25 minutes ago, Tennessee Boy said:

12V trolling motor.  I ran 2 dual purpose group 27 batteries in series for the trolling motor.

I hope you meant 'in parallel' - series connection would give you 24v, contra-indicated for a 12v TM.

3 hours ago, MN Fisher said:

I hope you meant 'in parallel' - series connection would give you 24v, contra-indicated for a 12v TM.

I did,   corrected.

On 5/7/2025 at 12:51 PM, fish'n Jim said:

I'm leery of lithium still.  Why?   If they catch fire? 

 

The first generation of lithium batteries were more prone to that.  The newest ones out there aren't any more likely to catch on fire than anything else in your boat.

 

 

On 5/7/2025 at 12:51 PM, fish'n Jim said:

I'm leery of lithium still.  Why?   If they catch fire?

 

2 minutes ago, gim said:

The first generation of lithium batteries were more prone to that.

First generation lithiums for boats were Lithium-Ion, and yes, they were very prone to fires if physically damaged. Latest gen is LiFePo4, which is almost impossible to set aflame, and doesn't burn near as hot or long if they do catch fire.