Use lithium house battery for small trolling motor?

[Lorelei072]

If I install a bank of lithium batteries in my van, is it possible to remove one to run the trolling motor when going out in a boat, and be able to put it back at the end of the day? Or do I need a dedicated battery?

I’m in a conundrum with the small boat & trolling motor I recently bought. I received a 70-lb lead acid battery with it that’s just too much for me to handle. I’d like to get a lithium battery for it, and I need to set up power in my van as well.

 

[jonyjoe303]

You can do it, but if the bank is in parallel when connected back to the other batteries it has to be the same voltage. If your batterybank is at 13.4 volts and the the battery you removed is at 12.9 volts, you need to get it back up to 13.4 volts, thats the recommended method to connect batteries in parallel.

 

[Lorelei072]

Are you saying it has to be replaced by another 12.9 volts to maintain the 13.4 volts?

Is there a way to disable the system on my boating days?

 

[jonyjoe303]

The battery you use for trolling will discharge as you use it, while the house batterybank remains the same voltage. The trolling battery has to be charged back up to the same voltage as the battery bank, before you connect it back... If the voltage differential isnt too much you can just connect it back, at most 3 or 4 amps will flow from the full battery to the low battery until they equalize.
You dont want to connect a completely discharged lithium battery back to a full batterybank, I have seen 25 amps go from a full battery to a low battery.

 

[bullfrog]

You could just leave the battery you have in the boat and charge it with a small generator and a suitcase solar set up. Most lakes have some sort of grid power available for charging trolling batteries if they have a lodge or fish cleaning station so maybe all you need is a good charger. Outboard motors can have a charging system installed specially designed to charge trolling batteries while running the outboard. The custom lithium battery packs built for trolling motors are usually pretty large. This does raise a good question though, could a DC to DC charger be used to isolate and charge the trolling motor battery up to up to the charge level of the battery bank?

 

[DLTooley]

A trolling motor is likely to be a heavier discharge. I’d consider how my house system is set up. A second solar charge controller would help on that aspect of the disparate batteries, but not with the load issues.

If I was doing this regularly I’d also rotate which battery I used so they age roughly the same.

Another option would be to purchase a lithium battery dedicated to the boat and maybe other portable uses.

 

[Lorelei072]

What is I just buy one lithium battery and 100 watt solar now, use it for boat & house, then say next year add 2 more lithium batteries with a 200 watt solar system for house use? I understand they would be 2 separate systems from the solar panels to the batteries. Then do the 2 systems join in an inverter?

 

[ldsreliance]

No, you don't want to use two different battery banks on the same inverter.  You could pull it off without problems if you really knew what you are doing but for where you are at right now that would not be a good idea.

What size trolling motor do you have?  Is it 12V?  I can help you size a battery if you want.

But your idea of having a lithium battery for the boat and recharging it with a 100W solar panel is a good one.  Most of my boat customers don't use the whole capacity of their battery every time they go out and a 100W panel will provide enough Amps to recharge the battery within a day or few days.  Obviously, if you go out on the boat every day then you will need more solar.

 

[Lorelei072]

It is a 12v. I’d love to hear your choice for battery size. I was having a hard time finding any reference for amp hour run time, but I managed to find a guy on YouTube demonstrating 2 hours 40 minutes continuous run time out of 50AH on a Minn Kota like mine. Another guy had a different motor, 100AH, and got 3:35 continuous run time.

 

[ldsreliance]

What is the model of the motor?  They have a bunch of different models of 12V motors.

Please keep in mind that it will also depend on how heavy you are on the throttle.  But if you can tell me the model I can give you a fairly accurate estimate.

 

[Lorelei072]

Minn Kota Endura Max… In the river I have to go full throttle to get upstream about 75% of the time and 25% going downstream to maintain control of my direction.

 

[XERTYX]

Its gonna depend on the thrust of the model you have. I found a pdf file from minn kota that gives values for max amp draw and minimum size circuit breakers as well as wire gauge.

I copied this from that file.

30 lb 30 
40 lb., 45 lb. 42 
50 lb., 55 lb 50 

So that's what they claim is the max rated amps on their various models.

 

[ldsreliance]

Lets say it is the 45 lb thrust version. The max Amp draw on that motor is 42A. So lets plan for worst case scenario... 100% throttle all the time and then you can guesstimate from there based on your usage habits.

Lets say you use the motor an average of 2 hours (actively running) when you go out on your boat.

42A x 2h = 84Ah

Add in about 10-20% excess capacity to compensate for voltage drop, hot temperatures, or whatever other imperfections you may have.

84Ah x 1.1 = 92.4Ah

So in that scenario I would recommend you select a 100Ah battery. That way you have a little extra capacity in case you need to use the motor more than 2 hours.

In the real world, you won't be at 100% throttle for all 2 hours so the battery will last longer. How much gets complicated fast but 2.5-3 hours is a safe bet.

Hopefully that helps you out a little bit. If you can tell us the exact thrust rating of your particular model and a better idea of how many hours you are on the motor each trip we can dial that in a little bit.

 

[XERTYX]

From what I read quickly when I was googling the amps for different thrust motors I found something I hadn't thought about. The 55lb thrust motors do obviously use more amps to achieve that thrust. But at lower throttle they provide as much thrust as smaller models but use less amps because there is more winding in the motor making it more efficient.

The OP would actually be better off if they have the higher thrust model and just didnt quite put the spurs to it as hard.

 

[Lorelei072]

@XERTYX Interesting point. I will have to go down to my storage unit and verify but I don’t think I’ll have time today.

@LDSReliance I appreciate the formula! What is the biggest lithium battery available? I have to choose run time vs. budget vs. battery weight but would like to know what the outliers are.

 

[ldsreliance]

@XERTYX Interesting point. I will have to go down to my storage unit and verify but I don’t think I’ll have time today.

@LDSReliance I appreciate the formula! What is the biggest lithium battery available? I have to choose run time vs. budget vs. battery weight but would like to know what the outliers are.

There are tons of sizes.  The most common size is 12V 100Ah, which is roughly Group 27 size in dimensions.

If you want to browse around on my site, it has a pretty good selection of sizes and spec levels.  Kind of a "good, better, best" type deal with the MillerTech Economy ones being cheaper, MillerTech Premium being better, and the Jericho ones being best.

https://store.ldsreliance.com/

 

[Lorelei072]

Thank you. I appreciate the options for cheaper American-made batteries. I won’t buy anything made in China in protest of live organ harvesting of Falon Gong practitioners