Trolling motor + LiFePO 4 battery questions

Greetings,

Last year, I powered my kayak with a 34lb thrust, 5-speed trolling motor running on a 12v 55ah LiFePO 4 battery. I could be out for ~6 hours trolling at various speeds throughout the day, over distances of 8+ miles on flat water. I never once ran out of battery power, but on numerous occasions, I believe I was right on the edge based on the (inaccurate) reading from my trolling motor.

Question one: what would be the smallest, lightest “backup” battery I could pack along in the event my primary gives out? For example, there is a generic 20ah deep cycle LiFePO 4 on Amazon for $70 that weighs only 4lbs. Would that little guy run (for an hour or so) to get me back to the dock in the event my main battery runs dry late in a trip?

Question two: I’m looking at updating my motor to a 70lb thrust with variable speed (compared to the 5 speed I have now.) The new motor draws anywhere from 5-65 amps, while the existing 34lb thrust motor draws 7-34 amps. Given that the new motor would draw almost double the amps at top speed, is it reasonable to conclude that it would also eat up my battery at a corresponding rate? (In other words, perhaps the 55ah battery will no longer last my entire day.)

Thanks for any advice — the setup I have now was my first and it has been awesome but want some backup and a bit more power in the winds and waves!

Yes the little battery should work, I just don’t know how much actual amp draw they can safely handle.

And the second question, it wouldn’t last the whole day if you were running on high the entire time. However you will likely be using the same amount of power as the small motor, just have some backup power incase you need it. May even be a bit more efficient depending on the motor.

Also if the batteries don’t have them internally you’ll definitely want to install a low voltage disconnect made for lithium batteries so when the battery does get low you can’t overdraw it.

LiFePo batteries are rated in Ah (amp hours) so a 55 Ah battery will provide you with 55 amps for 1 hour or 1 amp for 55 hours. It is not a perfect formula, and they don’t want you to discharge it completely dead (usually down to 30%).

So your 55Ah battery has approximately 38.5 available Amp hours. Your original trolling motor should run about 5.5 hours at low speed. Your experience say that it is not perfect math but it is close.

Your theoretical back up battery should last about 2.0 hours at low with your original motor. (20Ah * .70 available current / 7 low speed current draw) = hours of operation.)

Hope this helps.

Rather than a backup battery, have you considered a paddle?

tswoboda wrote:

Rather than a backup battery, have you considered a paddle?

Haha, yes, I also have a paddle but paddling a wide and heavily equipped fishing kayak is akin to paddling a river barge. You can do it, but if my battery dies 3 miles out and it is getting dark or bad weather is coming, in I definitely don’t want to paddle my way through it given the choice.

broeker wrote:

…but paddling a wide and heavily equipped fishing kayak is akin to paddling a river barge.

Much akin to the canoe paddle I keep in the ski locker of my Nautique.

djshannon wrote:

LiFePo batteries are rated in Ah (amp hours) so a 55 Ah battery will provide you with 55 amps for 1 hour or 1 amp for 55 hours. It is not a perfect formula, and they don’t want you to discharge it completely dead (usually down to 30%)…..

Thanks for your detailed response, that tracks roughly with how I’ve been doing my math but I neglected to subtract ~30% from the total.

Do you know if there is any reliable method to tell when a battery hits that level?

My understanding is that LiFePOs discharge at a flat rate and therefore can’t provide an accurate reading of what is left in the tank. When I start out on a trip, the meter on my trolling motor reads 13.2 at top speed, and by the end of a trip I’ve seen as low as 11.5 but I don’t have any way of translating that into something meaningful (other than when it hits 11.5 I start to worry.)

Search for a voltage chart for you particular battery. Will look something like this, or perhaps a line graph. On LiFePo’s 20% is okay to hit.

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Using a voltmeter to keep track of remaining battery life in a LiFePo4 is probably not the best. Using a shunt device that measures amps out/amps in will tell you exactly how much life is left. They are relatively inexpensive and easy to install, especially with a single battery system.

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