Ive been running my two 36v MK Ulterra trolling motors on two different boats with a 36v Lithium for about ten years. Just heard of this a couple months ago. My trolling motors have spent a ton of time on full power. Didn't know it was an issue. Im aware of it now but I don't really change how I operate.....
Overbuild your connections and wiring for sure.

 

I’m on year # 5 with a 36 volt minn kota and a Lithium Pro battery. Not the first issue yet. I would guess your wire overheated due to a bad connection. Got another boat with a 24 volt and no issues

 

Could’ve been a lose connection but not likely am well aware of what lose electrical connections can do. I tightened the screws down with an impact when I installed the plug. Minn Kota said the power wire from the trolling motor to the plug isn’t rated for 25.6 volts and that was the reason it melted. I

 

Bull puckey.... they use the same wire on the 36v....

 

I’ve been running a 36 for about 2 years and it often starts out above 36. I’m not a EE, but I find it odd that the tolerance specs would not tolerate a 1v variance. I don’t believe them…. Same thing for the circuit board. Look at the specs on almost any device you own and you will see stamped on the side voltage something like 10v-15v operating range.

I fish down south where the wind is strong and trolling distance is long. I run mine on 10 a lot. I posted a story here about losing my steering a while back. I ran mine on 10 for way more than an hour.

All my connections are coated with di-electric grease.

 

I think the difference might be I don’t have a true 24 volt lithium battery. My battery is actually rated for 25.6 volts. Maybe that’s not accurate this is my first experience with lithium on my boat. I agree I was shocked the wiring melted that fast. I ran it on 10 only for a few minutes.

 

I think my 36v was really about 39-40v and never had an issue. The connection needs to be rated for the current, which is what causes the heat.

 

As Themudflap mentioned, 12v batteries in series will have higher voltage than your lithium. After a full charge on your 12v, check it. If the battery is good it will be somewhere between 12.6 and 13.5. Add them together and it will exceed your lithium. If your 12v starting battery is being charged by an alternator it will maintain 12.6-14v. Pretty sure our engine wiring is far less robust than a trolling motor. Maybe Minn Kota is right but plenty of evidence they aren’t. If there tolerance is +- 1v, then when mine dies eventually I’ll look for another brand.

 

Where are you going in such a hurry On #10.
I have the same motor and a 24VDC Lithium batt.
I have read the same thing about running on 10 speed at 26plus volts.
Broke down 3 miles from the launch earlier this year. I could see on the batt management sys that I had 80% batt life. Took off on 10 speed and was burning like 56amps and stated I could maintain for 8hrs. So back off to 9 speed and on to the next fishing spot. Did not want to chance 10 speed for a long period of time. Just me…..
Fished all the way home with the trolling motor. The BMS systems are great!
I asked my Minn Kota mech over in Tomball and he stated run it on full with no problems.
If you do the math on 24V vs 26VDC amp draw. Not much difference to really count.
ThinkIng you had something loose.

 

Per Minn Kota on that motor is 56amps full load
Use 60a breaker
Use no 6 wire for 20ft

 

Remember, wire length is double actual length...20 ft boat.. front to back.. is considered 40 ft.

https://minnkota.johnsonoutdoors.com/us/support/trolling-motor-installation-guides/trolling-motor-wiring-and-battery-guide

"When calculating wire length for a trolling motor, if the battery is ten feet from the trolling motor, what is the correct wire length 10 ft or 20 ft.?"


When calculating wire length for a trolling motor, you need to account for both the positive and negative conductors, as the circuit requires a complete loop from the battery to the motor and back. If the battery is 10 feet from the trolling motor, you measure the distance for both the positive wire (from battery to motor) and the negative wire (from motor back to battery). For a 10-foot distance:

• Positive wire: 10 feet
• Negative wire: 10 feet
• Total wire length: 10 + 10 = 20 feet

So, the correct wire length is 20 feet, assuming a straightforward run with no additional slack or routing considerations. Always check the trolling motor’s manual for specific recommendations, and consider adding a small buffer (e.g., 10-20%) for installation ease or future adjustments."

 

As the 2 previous post mentioned, might be wires from battery. Depending on where your trolling battery is located probably need #6 wire minimum or up grade to size #4 if it's a 20'+ run. Plugs and their connections can get corrosion, replace plug with a new one (cheaper than swapping out batteries), new connections (no splices) from plug to battery.

 

we use AI, google or Youtube often for detailed questions. This reminds me of using a higher voltage plug or a cord which is not correct to charge a cell phone. If the plug is higher voltage than the phone is suppose to have your phone will often overheat or blow up. There is no solution to the phone issue except to use a correct cord and correct voltage of charge box. We bought solar chargers for one of our boat batteries.

 

Looking at Minn Kota’s recommendations, the Quest motors can handle the high power on lithium batteries. The brushed motors could have issues at high speeds. Lead acid batteries will drop voltage running high speeds quickly. Lithium batteries hold the voltage longer thus can create overheating conditions on brushed motors.
I want to upgrade my optima batteries to lithium for my Ulterra. I fish fresh water so I should be good with lithium. No currents to deal with.

 

Got picts of the wiring? Like others, I'd bet the wiring was too small or has some corrosion, bad crimp, etc.

 

Sorry I don’t have any pics. It’s a 3 prong twist lock plug and the wiring melted where it connects to the male end of the plug. I used an impact to tighten down the screws but I now believe it was due to loose wiring. The wire size is 6 gauge.

 

" Kinda disappointing because I’m guessing this means I can’t use spot lock. "

Can you explain why spot-lock would be affected?
I'm about to install the EXACT combo you have and I cant understand how it would affect
spot lock, unless you're in raging wind and/or current

 

My point was if what Minn Kota told me was true about not using full power (10) then spot lock can and probably does us full power when necessary I just wouldn’t know when that happens. I’m thinking that was a CYA statement from Minn Kota given all the reports of people not having any issues.

 

Gotcha. makes sense.

 

I run mine on 10 all the time with a 24 volt system with no problems, but my wires are the biggest i could get by with and close to the trolling motor