I think you’re right…I did the math, but forgot to double the amps being drawn.

If your trolling motor was drawing 40 amps @ 24v, that’s 80 amps at 12v, so you’d barely get 2 hours of near-max run time, at that rate.

At the same time, my current 2 AGM’s are rated for 100AH, but they can only be drawn down about 40% without worrying about damaging them. That means I’m currently working with about 40AH of power, at 24v. Not taking into account efficiency losses, I would need 80AH at 12v to get the same amount of time/power out of my trolling motor. Since you can draw LiFePO4 batteries down 90%, I have about 162AH at 12v to work with…that’s twice as much as I’m actually able to use from the 2 100AH AGM’s. The AGM’s weigh 68lb/ea and the 180AH LiFePO4 weighs about 50lbs, plus 5lbs for the converter.

I’m not sure I’m going to go this route, but it’s something I’m considering. Two smaller lithium batteries, in the 50AH range, wired in series, would give me about 45AH @ 24v, but only weigh around 30lbs, total. The specific energy of LiFePO4 is that much higher than lead acid.

The bottom line is lithium batteries weigh a lot less, and can be drawn down a lot more, so you’re getting about 4 times as much usable power out of the same amp-hour rating. Alternatively, you can cut the weight by 1/2 to 2/3’s and still have the same available power. One final advantage to lithium is you don’t get the voltage sag that you see from lead acid, which means you don’t have to keep turning up your trolling motor all day, to get the same amount of thrust from a given setting.

Lots of things to consider, but I’m not sure the step-up converter is the way to go…

I am looking into this same question. I am building LiFeP04 (lithium iron phosphate) batteries that will serve multiple functions. I would like to build them as 12v batteries, for the convenience of powering a lot of different 12v devices. While I could put two of them in my boat, wired in series for 24v, that increases the weight I have to carry. That results in hauling around a lot more power than I might need for a day’s worth of running the trolling motor.

With a step-up converter, 12v-24v, you draw twice as many amps, with an efficiency loss of 5-10%. However, if you are using a single 180AH LiFeP04 battery, capable of a full C discharge rate (180amps), and your trolling motor only draws 50 to 60 amps, peak, then you would be drawing 100 to 120 amps (at max speed for your motor). This means you could only run for 3 hours or so, under perfect conditions…probably 3 hours for a 50amp draw and 2.5 hours for a 60amp draw. However, keep in mind that is running your trolling motor on its highest setting, continuously. If you run it at 50% of max, you more than double the amount of time you can run it. Is 5 to 6 hours of run time, at 50%, sufficient to your daily trolling motor use needs? For many of us, that’s plenty, so a single 180AH LiFeP04 battery (~50lbs) and a 4lb converter would save you about 80lbs of weight, versus a comparable SLA/AGM (lead acid) solution.

Thoughts, observations, rude comnments?