Two batteries for small fising boat

[robert291]

My 15 hp motor came with a Group 24 starting battery. I happened to have a dual purpose battery (also Group 24), so I hooked that up instead. It also powers a small trolling motor and a fish finder. I'm only getting about 30 minutes of trolling time before the battery drops below the level needed to power the fish finder. Then I have to use the manual starter for the outboard, of course.

I'm looking for a low-cost way to combine the two batteries. Here are some possibilities I've identified:

1) Voltage Sensitive Relay (MaySpare lists one on Amazon for $40). Sounds like it might do the job, but I wonder if it requires other parts, such as fuses.

2) Add-a-battery kit from Blue Sea Systems ($125). Includes a switch and an Automatic Charging Relay. I don't understand why the switch is needed if the relay is really automatic. Looks like the ACR alone is only $85.

If could invent my own, it would be a two battery switch with a meter for whatever battery is selected. My outboard starts very quickly, so I think the starter battery would be topped off within a few minutes. Then I would flip a switch and direct the alternator current to the deep cycle battery.

Thoughts?

Robert

 

[TyeeMan]

Sounds like you might have an old battery, or a battery low on electrolyte.
I don't know what pound thrust your trolling motor is, but it should be able to run for hours and your battery should still have enough juice to start your outboard. As for your outboard charging your battery to full in a few minutes, , highly unlikely. A small outboard only puts out a few amps of charging power at idle/trolling speed, and not much more at full throttle.

How about upgrading to a group 31 dual purpose battery? Then you would have enough power to run your trolling motor and fish finder for hours and still have enough power to start your outboard.

 

[airshot]

Yep, poor battery.....I had a group 27 deep cycle battery powering my accessories in my 16' boat, could run my trolling motor for 4 hours with power left over for other accessories.

 

[BWR1953]

I bought a Nocqua battery with charger for my electronics. Lasts forever.

 

[airshot]

Dual purpose batteries will not maintain as long as a true deep cycle!! If your serious about using your trolling motor for hours get a true deep cycle group 27 or 31. If you have an issue with weight, get a small utility battery such as a wheel chair battery as a starting battery for your 15 hp motor. Small in size but pack a big punch. I bought one as an extra battery to operate various 12v tools and toys I have. About the size of a motorcycle/UTV battery, mine powers my 30 gallon sprayer, my trolling motor for an hour in my pond, have used it to start my snoblower and lawnmower. Keep it on a maintainer so it is ready when needed, mine is over 5 years old and still works perfect. More than enough power as a start only battery for your motor......

 

[robert291]

Yep, poor battery.....I had a group 27 deep cycle battery powering my accessories in my 16' boat, could run my trolling motor for 4 hours with power left over for other accessories.

Does this battery get recharged by your engine? Do you have a separate battery for the starter?

 

[robert291]

Dual purpose batteries will not maintain as long as a true deep cycle!! If your serious about using your trolling motor for hours get a true deep cycle group 27 or 31. If you have an issue with weight, get a small utility battery such as a wheel chair battery as a starting battery for your 15 hp motor. Small in size but pack a big punch. I bought one as an extra battery to operate various 12v tools and toys I have. About the size of a motorcycle/UTV battery, mine powers my 30 gallon sprayer, my trolling motor for an hour in my pond, have used it to start my snoblower and lawnmower. Keep it on a maintainer so it is ready when needed, mine is over 5 years old and still works perfect. More than enough power as a start only battery for your motor......

I already have a separate battery that I could use for the starter. Came with the boat. What I’m trying to figure out is how to combine the two.

 

[robert291]

I bought a Nocqua battery with charger for my electronics. Lasts forever.

Does it also power the starter? Or do you have a separate batter for that?

Sounds like you might have an old battery, or a battery low on electrolyte.
I don't know what pound thrust your trolling motor is, but it should be able to run for hours and your battery should still have enough juice to start your outboard. As for your outboard charging your battery to full in a few minutes, , highly unlikely. A small outboard only puts out a few amps of charging power at idle/trolling speed, and not much more at full throttle.

How about upgrading to a group 31 dual purpose battery? Then you would have enough power to run your trolling motor and fish finder for hours and still have enough power to start your outboard.

The electrolyte is good. I just put it on the charger overnight and I’m getting 13.4 volts right now. Not saying that a new deep cycle wouldn’t help, but I already own a new battery for the starter, so I’d like to combine the two with some kind of switched system.

Hoping some one here has some experience with that.

 

[robert291]

Dual purpose batteries will not maintain as long as a true deep cycle!! If your serious about using your trolling motor for hours get a true deep cycle group 27 or 31. If you have an issue with weight, get a small utility battery such as a wheel chair battery as a starting battery for your 15 hp motor. Small in size but pack a big punch. I bought one as an extra battery to operate various 12v tools and toys I have. About the size of a motorcycle/UTV battery, mine powers my 30 gallon sprayer, my trolling motor for an hour in my pond, have used it to start my snoblower and lawnmower. Keep it on a maintainer so it is ready when needed, mine is over 5 years old and still works perfect. More than enough power as a start only battery for your motor......

 

[robert291]

I already have a separate battery for starting the outboard. I’m just looking for a way to combine the two so that the deep cycle battery gets recharged (a little) when the engine is running.

Are you talking about a solar maintainer? I think that might be helpful, too, but I thought a switch or relay would be a good first step.

 

[dingbat]

I already have a separate battery for starting the outboard. I’m just looking for a way to combine the two so that the deep cycle battery gets recharged (a little) when the engine is running.

Are you talking about a solar maintainer? I think that might be helpful, too, but I thought a switch or relay would be a good first step.

To get an intelligent answer you need to know the current draw of the trolling motor, duration of use between charges and the charging rate of the outboard.

Couple of things to understand going forward.
1. Outboards put out very little current, if any while idling.

2. Solar “Maintainers” are not chargers. A deep cycle will not be happy

3.Leaving a battery discharged for any length of time is detrimental to life expectancy.

4. A switch or ACR is only effective if your engine output and run time is sufficient to replace the energy used by the trolling motor.

 

[Thirsty Endgrain]

Does it also power the starter? Or do you have a separate batter for that?

The electrolyte is good. I just put it on the charger overnight and I’m getting 13.4 volts right now. Not saying that a new deep cycle wouldn’t help, but I already own a new battery for the starter, so I’d like to combine the two with some kind of switched system.

Hoping some one here has some experience with that.

How do you know the electrolyte is good? Did you test the specific gravity? And did you check the level? How old is this battery? Does your charger have de-sulfation capabilities? Proper maintenance of FLA batteries include de-sulfation appx. 1-2x/mo.

A lead acid battery (assuming this is what you have?) doesn't hold 13.4V for very long, and measuring that voltage after charging not an indicator at all of your battery health. You need resting voltage. If fully healthy they rest at 12.6V. If they rest at 12.5V they have appx. 70% of their capacity. They accept charge at between 12.9-13.4+ or so.

What you need to know is the resting voltage of the battery, i.e. fully charged and then taken off the charger for ideally 12-24 hrs. or so. But even 2-3 hrs. after being removed from charging will help tell you.

Batteries can also hold a "surface charge" meaning that, they could tell you a good resting voltage (12.6V) but as soon as you apply a load the voltage drops below what is required to successfully run stuff. This happens when the lead plates grow crystals.



Maintaining a lead-acid battery is almost like owning a pet. You gotta feed 'em and brush 'em and take them for walks, and they'll live a long time.

Agree with others, your little outboard is not going to charge that battery in the way that you hope it will. Outboard charging is basically trickle maintenance, it's not "charging."

Battery systems are simple math. Your appliance uses x amps for x hrs. That is your amp hour requirement, or Ah.

I don't have a trolling motor but let's say it uses 20A at 12V, and you use it for 2 hours.

20A x 2H = 40Ah

Now let's say your G24 battery contains something like 80Ah of capacity when new.

After 2 hrs. of trolling you've used 40Ah of your available 80Ah. That is 50%, and your battery is effectively discharged, and going further than that will begin prematurely degrading your battery, voltages will begin to suffer and electronics start to malfunction etc. Since your battery is degraded/sulfated you are not getting nearly that.

I could go on and on. The reality is that it sounds like your battery is going bye bye.

If you want something that is mess maintenance get a lithium battery. You need a LiFePO4 specific charger, but you can run them down to 20% and still have usable voltage, and it doesn't really phase them, they just charge back up and keep going. I would start by figuring out your demand, increasing it by 30%, buying the appropriate sized battery, hook it all up, and go fishing.

 

[airshot]

I already have a separate battery for starting the outboard. I’m just looking for a way to combine the two so that the deep cycle battery gets recharged (a little) when the engine is running.

Are you talking about a solar maintainer? I think that might be helpful, too, but I thought a switch or relay would be a good first step.

Cant run or charge two batteries together unless they are the same type of battery. Install a switch then run/ charge each one seperately. Your best bet for overall health of your batteries is to charge them at home and keepbthem on a battery maintainer. That is how I keep my boat and motorhome batteries and they hold up well for 5 plus years. Starting and deep cycle batteries each need their own type of maintenence

 

[BWR1953]

Does it also power the starter? Or do you have a separate batter for that?

Nope, it's for the fish finder only.