Thread: DC 14.4v Down To 12.8v?

Hey fellas i donno where else to ask this online, i picked up a new boat the other day and it came with a old trolling motor designed to be used with lead acid batteries it's 12v DC, instead of running heavy AGM 100amp hour battery i want to run a 12v lithium battery but the AGM/motor input voltage is rated to 12.8volts a lithium battery is 14.4volts, now the trolling motor manufactor states i can use a lithium battery so long as i don't go over 85% speed/power

Now the trolling motor will be GPS controlled so i won't have any input on what % of speed it uses to "spot lock" as it's automatically self controlled, is there a way of stepping down the voltage from 14.4volt to 12.8volt, i know i can step down 24volt to 12volt but it's unclear if the step down buck's on ebay actually step down to 12volt or 13.8volt which is too high

Amperage side of thing's the trolling motor can pull 50amp's at 12.8volts

What i am trying to achieve is half the battery weight and double the duration or run time, guy's are reporting run times of 6 hours on a AGM battery to 4 days on a lithium the same size

Any help of advice is appreciated i am just technically challenged with electronics and new technology

If you run slightly undersized wires with a long run you will be below the 12.8 volts you are chasing .

For your application I would be surprised if the motor ever tried to run at above 85% for more than a few seconds.

Without knowing the equipment it’s hard for me to give you a good answer.

50amp is fairly high to be messing about with and so you need to be sure that the step down can handle that.

I am surprised that the step down has no provision for adjusting/tuning the voltage? This should be standard if a product is designed for the open market unless it’s a very closed source / vendor locked Apple iPhone type product model.

With regard to voltage, I am again slightly surprised with the strict voltage requirements.

A lithium battery will have fluctuating voltages under load. Further more there is always a voltage drop going from full charge down to a safe discharged battery state.

Most of the DC systems I have seen that are battery powered have provisions and protection for such voltage fluctuations rather than “a strict 12.8v or it will explode”.

For the stuff I do, 12.8v (+/- 1v) has never mattered except for very special circumstances such as parallel charging.

Just plug it in and run it.

Its nominal 12V, not critical to decimals of a volt
A fully charged lead acid battery be it flooded, glass mat or whatever is 13.5V

I had a light bulb moment when it was explained to me that a rechargeable battery (like a lead-acid battery in a car) needs the charging system to have a higher voltage than the battery. Otherwise there will be no charging going on.
The often seen 13.8 is a common figure for alternator output voltage for example. Voltmeters commonly show that at one end of their scale.
With the engine stopped, the voltage reading will be more like the true level in the battery.

It's a Minn Kota brushed trolling motor guy's it can only run on lead acid, if it's ran off a lithium the voltage is to high if the trolling motor is used over 85% of power it will burn out the circuit board as it's not designed to run on 14.4 volts that is why older trolling motors must be ran off a battery on it's own cannot be hooked up to a outboard engine for charging

I'm just thinking of running a 24v lithium battery with 2x 30amp 24v to 12v step down buck's in parallel

Originally Posted by gazza2009au

It's a Minn Kota brushed trolling motor guy's it can only run on lead acid, if it's ran off a lithium the voltage is to high if the trolling motor is used over 85% of power it will burn out the circuit board as it's not designed to run on 14.4 volts that is why older trolling motors must be ran off a battery on it's own cannot be hooked up to a outboard engine for charging

I'm just thinking of running a 24v lithium battery with 2x 30amp 24v to 12v step down buck's in parallel

I dont know the particular model and you probably have far more experience in trolling motors, but why does the manufacturer have a section on lithium batteries if they are not suitable for use?
https://minnkota.johnsonoutdoors.com...-compatibility

I do know that a lot of drag racers use 16v batteries so they do not have to run an alternator, none of the electronics used in the systems designed for 12v have any issues that I am aware of.

Here's the info https://minnkota-help.johnsonoutdoor...ecommendations

Guy's can i build a 3 cell prismatic lifepo4 battery each cell is 4.2v charged so 12.6v as a 3cell battery i looked into this last night but the cost's were pretty up there, looking at EVE 280ah cell's, just not sure if a voltage sag will make the battery useless

Originally Posted by gazza2009au

Hey fellas i donno where else to ask this online, i picked up a new boat the other day and it came with a old trolling motor designed to be used with lead acid batteries it's 12v DC, instead of running heavy AGM 100amp hour battery i want to run a 12v lithium battery but the AGM/motor input voltage is rated to 12.8volts a lithium battery is 14.4volts, now the trolling motor manufactor states i can use a lithium battery so long as i don't go over 85% speed/power

Now the trolling motor will be GPS controlled so i won't have any input on what % of speed it uses to "spot lock" as it's automatically self controlled, is there a way of stepping down the voltage from 14.4volt to 12.8volt, i know i can step down 24volt to 12volt but it's unclear if the step down buck's on ebay actually step down to 12volt or 13.8volt which is too high

Amperage side of thing's the trolling motor can pull 50amp's at 12.8volts

What i am trying to achieve is half the battery weight and double the duration or run time, guy's are reporting run times of 6 hours on a AGM battery to 4 days on a lithium the same size

Any help of advice is appreciated i am just technically challenged with electronics and new technology

Hi Gazza.

A pair of 50 amp silicon diode rectifiers in series with the power supply will reduce your voltage by approximately 1.4 volts, three will drop 2.1 volts. Each diode will drop 0.7 volts.

An easy and cheap way of loosing a few volts ! Check the voltage under load to make sure its in the range that you want !

Originally Posted by gazza2009au

It's a Minn Kota brushed trolling motor guy's it can only run on lead acid, if it's ran off a lithium the voltage is to high if the trolling motor is used over 85% of power it will burn out the circuit board as it's not designed to run on 14.4 volts that is why older trolling motors must be ran off a battery on it's own cannot be hooked up to a outboard engine for charging

I'm just thinking of running a 24v lithium battery with 2x 30amp 24v to 12v step down buck's in parallel

Hi Gazza,

Its more likely that the reason that recommendation is made is because charging generators output a lot of voltage spikes that would kill electronic circuits !

Had a quick look on line and see heaps of 12.8V LiFePO batteries for sale.

I think you are getting confused between the Charging V versus the Output V.

Screenshot 2024-08-24 at 3.19.11 PM.png

Again no first hand experience but from the way I read it may be possible to limit motor speed using the One Boat Network app.
https://johnsonoutdoors.widen.net/co...ions_Guide.pdf

Appears you can limit in 1/2 increments and set a button for max speed (10)

Could also be wrong but that’s the way I read it.

Originally Posted by BaronJ

Hi Gazza.

A pair of 50 amp silicon diode rectifiers in series with the power supply will reduce your voltage by approximately 1.4 volts, three will drop 2.1 volts. Each diode will drop 0.7 volts.

An easy and cheap way of loosing a few volts ! Check the voltage under load to make sure its in the range that you want !

How do i wire them in as in positive terminal than rectifier than the power cable to the trolling motor?

Or do i cross the terminal's? I'm electronically challenged..

Hey bob i was getting lithium lifepo4 mixed with lithium polymer

Lithium polymer charges to 4.2v per cell 3x cells in a 12v battery

Lifepo4 charges 3.6v per cell 4x cells in a 12v battery so my idea of a 3 cell pack won't work