50ah lithium max discharge current

[1GThompson]

It’s time to replace my group 27 Optimas trolling motor batteries. They have served me well for 7 years. I would like to go with 50ah lithium batteries. I think they would give me plenty of run time with how I fish.

My concern is the maximum discharge current. Most 50ah batteries have a maximum discharge current of 50 amps. Minn Kota specs say my 80lb Fortrex pulls a max of 56 amps, but I know a lot of guys are using 50ah lithiums as trolling motor batteries.

Have any of you had any problems exceeding the maximum discharge current with 50ah lithium batteries?

[Bill Reynolds]

Over current is one of the protections set in the BMS (battery management system) of the battery. It may vary by battery mfg, but I would be surprised if it is much over 50 amps. I have 50 AH batteries on my 80# Ultrex and the max Continuous current is 50 amps.

it is important to note that the max current draw on the Ultrex is “Locked Rotor”. The way I see this happening is that you run into something solid that holds the TM prop against rotation while holding power on the motor. It certainly can happen and if the current continues long enough, the BMS will disconnect the battery to protect it. If the BMS takes the battery off line, you will have to disconnect the positive cable at the battery and wait up to 30 minutes for it to reset.

That being said, it is unknown how long the the 50 amp current has to sustain before the BMS kicks in and whether the TM circuit breaker catches it first. In that regard, Minn Kota recommends a 60 amp breaker in the Ultrex 80# circuit but I use a 50 amp. To my knowledge, Ranger has always used 50 amp breakers in all their 24 volt TM circuits. You can see the advantage in this instance.

one of the characteristics of a permeant magnet motor Is that it will only draw as much current as needed to overcome the motor load. While conditions vary, this chart was produced on the TM on my RT188 with no current (river flow) and 15-20 mph wind.

I ran a current gauge on my front deck for about a year just to satisfy my curiosity. The highest amps I saw was 42 and that was in river current so strong that I was slowly moving downstream.
To be clear, it all depends on load and other variables so these numbers are just rule of thumb.

Sorry for the long answer.

[Fishysam]

So max discharge you stated only happens for a very short time when the boat is not moving, as the hull starts moving the propeller load decreases some my 112 ultrex from a stop or in a 180* momentum change will peak at 44amp discharge on power 10. But when hull speed in real it drops down to 33amps. So i have about 1.5 hours of use on power ten on my 50ah x3 12v system

the hardest use i have done was prefishing docks, i used 30% in a hour. But i also fish in 30+mph winds in the wind and they make it 7+ hours without anchoring or tucking away, i only seem to make it 4-5 hours in 5' waves before im beat soooooo unless your really hard on batteries 50's work and I personally would compare useable power in a 27 lead batter to a 50ah lithium.

ps as lead dies you have to turn the speed from 3 to 3.5 to 4/4.5..... lithium will run speed 3 till the end since voltage stays more consistent. Then because your not creaping up the throttle your getting better economy per hour... if that makes sense

[1GThompson]

Very good information Bill….
Yes I meant maximum continuous discharge current...
I was thinking 56 amps was locked rotor and/or initial starting current when the trolling motor is set to 100. However, I wasn’t sure because I’ve never put an amprobe on to check it. I figured if the current did get that high, it probably didn’t stay there too long.
Thanks guys

[lpugh]

50 AH lithium's will have comparable run time as most grp 27 agm batteries

[billnorman1]

I bought two 50AH Lithiums from an outside source. The specs said they were compatable with marine use and trolling motor application. They have a B M S and are rated at 50 amps. I use them on an 80# Minnkota, have never had an issue with them. They are rated at 50 Amp discharge rate.
I do not use my trolling motor on maximum speed for EXTENDED periods of time. I do not believe the T/M draws 50+ amps when it is used on full speed and turning in water.
The T/M is wired with 10 gauge wiring from the plug on. 10 gauge is only rated for 30 amps it will melt down at 50 especially at any weak junction.

[WranglerSS]

Checked my amp draw yesterday on the Ionic app. 24v Ultrex on rabbit pulls 42 amps on a loaded 21' 10" Bullet.

[Impulse Lithium]

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[1GThompson]

WranglerSS…Do you have 50ah Ionics? And if so, have you ever had a problem exceeding the maximum continuous discharge current?

[Woody b]

Bill (and others) correct me if I'm wrong. In the real world you're going to see a lower amp draw when using a quality lithium battery than you'll see with conventional lead acid battery. There's quite a bit more voltage drop with a lead acid battery when compared to a quality lithium battery.

IE in heavy current with my (12 volt)Terrova on 10 the voltage drops less than 2 tenths of a volt with my board sponsor, quality lithium battery. With the (undersized) fully charged lead acid battery that came with my boat the voltage dropped 1 volt with the TM on 10.

So, on 10 with my lithium battery voltage is 13, amp draw is 39. 13X39 = 507 watts.
On 10 with my fully charged lead acid battery voltage is 12, amp draw is 42.25 12X42.25 = 507 watts.
The wattage required is going to remain constant. The number of amps required goes up as voltage goes down.

This doesn't seem significant but this is with a fully charged battery. As the state of charge drops voltage falls off a bunch with a lead acid battery. The voltage curve is basically flat with lithium.

So the lead acid battery gets down to 60% charge the voltage will be dropping to 10 volts or so. Amp draw is up to 50. A lithium battery at 60% charge is still going to have well over 12 volts, with the load applied.

However, I do believe the steady voltage a quality lithium battery provides increases the importance of correct sized quality wiring and good connections.

[WranglerSS]

WranglerSS…Do you have 50ah Ionics? And if so, have you ever had a problem exceeding the maximum continuous discharge current?

I have the Ionic 24v 50ah lithium and never had a problem exceeding the max. The Ultrex on high pulls 42 amps so there is a bit of a buffer to the 50 amp max draw.

[Bill Reynolds]

Woody b,
I agree with your synopsis. It is thought provoking and makes me wish I had recorded the voltage at each of the speed settings when I created the chart. I still have the shunt resistor and amp gauge and if I still had the lead acid batteries I would be tempted to re-install it and repeat the test. I would expect to see a low voltage drop even with a fresh battery because of the high internal resistance of Lead Acid.

What I observe is that as the battery voltage drops, the boat speed slows indicating that there is less current flow.
I remember when fishing long days, I may start out with the TM on 30% and be at 60% or 70% by the end of the day. This phenomenon becomes even more marked as the lead acid batteries age, but not so with Lithium. For sure I am pleased to have a battery with a flat discharge curve, I don’t even think about the TM throughout the day.

[yam911]

Bill (and others) correct me if I'm wrong. In the real world you're going to see a lower amp draw when using a quality lithium battery than you'll see with conventional lead acid battery. There's quite a bit more voltage drop with a lead acid battery when compared to a quality lithium battery.

IE in heavy current with my (12 volt)Terrova on 10 the voltage drops less than 2 tenths of a volt with my board sponsor, quality lithium battery. With the (undersized) fully charged lead acid battery that came with my boat the voltage dropped 1 volt with the TM on 10.

So, on 10 with my lithium battery voltage is 13, amp draw is 39. 13X39 = 507 watts.
On 10 with my fully charged lead acid battery voltage is 12, amp draw is 42.25 12X42.25 = 507 watts.
The wattage required is going to remain constant. The number of amps required goes up as voltage goes down.

This doesn't seem significant but this is with a fully charged battery. As the state of charge drops voltage falls off a bunch with a lead acid battery. The voltage curve is basically flat with lithium.

So the lead acid battery gets down to 60% charge the voltage will be dropping to 10 volts or so. Amp draw is up to 50. A lithium battery at 60% charge is still going to have well over 12 volts, with the load applied.

However, I do believe the steady voltage a quality lithium battery provides increases the importance of correct sized quality wiring and good connections.

I think you have that backwards.

For a given load, the higher the voltage of the battery the higher the current flow will be. The lithium battery, maintaining a higher voltage, will result in higher current flow.

[lpugh]

I think you have that backwards.

For a given load, the higher the voltage of the battery the higher the current flow will be. The lithium battery, maintaining a higher voltage, will result in higher current flow.

No sir, the higher voltage creates the stronger magnetic field thus requiring less current flow, this is a fact with all electric motors
The higher voltage is capable of pushing more current, but the motor does not need it to operate at its designed rpm
This is why most motors are pulse modulated at peak voltage, way more efficient than variable voltage as variable voltage means greater current draw and more heat

[bobcoy]

Yesseree, Just follow the Law. OHM’s that is. Due credit to Jimmy’s Law

[Woody b]

With a resistive load like a heater or light bulb amperage and wattage will increase when voltage is increased. With an inductive load like a trolling motor the wattage remains constant so watts law applies. As voltage goes up amperage will go down. It's very common to confuse resistive loads with inductive (and capacitive) loads.

[billnorman1]

A motor and a transformer draw maximum current at start. That's why you get an arc when you plug the charger in. Once a motor gets up to speed a factor called counter E M F takes over and makes an effective resistance out of a direct short like an armature. If you ohm an armature of a trolling motor you would probably get a factor like one ohm with an ohmmeter. If that was the case without counter E M F it would quickly melt down with 24 or 36 volts across it, but with counter E M F it just keeps humming along, unless it gets loicked up or frozen and if that happens, something's going to fail soon.