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Powering a 12v Trolling Motor

Aphers

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Nov 17, 2020
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I'm trying to get to grips with LFP batteries and thought I could dip my toe in the water by installing a small pack to power my trolling motor.
I'm going to use the motor on a small tender, and would be quite happy with a full throttle duration of 20-30min- it''s just for getting to and from a mooring. The motor draws 64A at full power, so I reckon any battery over 20Ah will be big enough for me. I'll be taking it ashore, or aboard the yacht, to charge. Small and light is good- as is cheap, obviously!

There are plenty of 'drop in' type batteries on eBay at low prices, but it is the BMS which lets them down, with maximum discharge currents of 1C. So I would need a 64Ah battery, which is overkill.

Daft newbie question no 1: what if I took power directly from the cells, bypassing the inbuilt BMS? Could I still charge via the BMS, or would that not work? (Yes I know in this scenario there is no automatic low voltage cut off).
Question 2: I could buy a bare pack of balanced cells, no BMS, and draw power directly as above. Does the lack of BMS make charging it more difficult?

Thanks in advance
 
You don't just size batteries for capacity. You also size them for the load. Since your going to pull 64A from the battery THAT is the driving design factor, NOT a 20-30 minute need.

You need a 64Ah+ battery.
 
Thanks, some of the packs I've seen (e.g. from GWL) claim a max current draw of 600A even on a little 20Ah pack.
So I was thought it was the BMS rather than the cells themselves that limited you to 1C.
 
Those are usually instantaneous or pulse currents. Most cells only allow for a 1C sustained draw. Many BMS allow for a significant "surge" above their max continuous rating.
 
For example:

"Maximum continuous discharge current for 30min from full charge= 120A or 3C"

Run this via a VSR or battery protector, or even just keep track of the time/voltage and manually disconnect.
Then recharge... carefully... would I need a cleverer charger with no BMS to protect the cells?
Or is there is a BMS out there that will do all that and not cost a fortune? I can't find anything rated for this kind of current that doesn't cost more than the battery itself. Hence trying to reinvent the wheel and work around the problem.
 
That's a different type of LFP battery with Yttrium. More commonly referred to as LYP.

A BMS would require you to cut open the battery and attach it to the cell groups. The way it reads is that the cells are perfectly balanced, and they don't need a BMS - just operate them within the voltage range.
 
How big is your trolling motor?

I ran a 28lb trolling motor on a 16ft canoe with a 50a flooded lead acid, it worked a treat, I was often on the "plane" and I never ran out of charge while going for a couple of hours at a time.

To answer your question, yes you can bypass the BMS as often the cells will do better than what the BMS will allow. You may have to monitor the voltage while you're holding it flat out so you don't risk over-discharging the cells. Keep them above 12.0v

Also in my experience the drop-on lithium deep cycles aren't always waterproof, I've lost two BMS's from water ingression. Something to be mindful of.
 
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A battery's capacity decreases as the rate of discharge increases. The Amp Hour rating of a deep cycle battery (going back to Flooded lead acid days) is stated at a 20 hour rate. So, for example, a 12 volt, 100AH rated storage battery can deliver 100 amps if discharged uniformly over a 20 hour period. So you divide 100 Amps by 20 hours, and you get a discharge rate of 5 amps continuously to realize the 100 Amp Hours of your battery's rating. The rate of discharge is not linear, so it is not logical to believe that I could get an hour out of my 100 AH battery if I discharged it at a 100 amp rate.

There are plenty of discharge curve graphs which can be found with a little googling, so I'll leave that to you if you care to research.
 
A battery's capacity decreases as the rate of discharge increases. The Amp Hour rating of a deep cycle battery (going back to Flooded lead acid days) is stated at a 20 hour rate. So, for example, a 12 volt, 100AH rated storage battery can deliver 100 amps if discharged uniformly over a 20 hour period. So you divide 100 Amps by 20 hours, and you get a discharge rate of 5 amps continuously to realize the 100 Amp Hours of your battery's rating. The rate of discharge is not linear, so it is not logical to believe that I could get an hour out of my 100 AH battery if I discharged it at a 100 amp rate.

There are plenty of discharge curve graphs which can be found with a little googling, so I'll leave that to you if you care to research.
Yup I'm pretty familiar with the discharge graphs for FLA, usually you get C5, C10, C20, C100 etc figures. But I thought that was a peculiarity of lead-acid (Peukert effect?) and didn't apply to lithium chemistries.

Going FLA wouldn't be the worst idea in the world, and would obviously be much cheaper, at least initially. To get my half hour endurance without going below 50% DOD I would need a 64Ah+ battery, which should cost comfortably under £100 and weigh about 16kg. This weight and the limited cycle life would be the downsides.

I think I have a few different options for LFP:
- the above linked 40Ah pack plus charger, this would come to about £280 delivered and give me the half hour endurance with a little room to spare. I'd also expect higher real world performance due to constant voltage of the battery. Weight would be about half the FLA, and I'd expect several thousand cycles so it would eventually pay for itself compared to FLA. I'd need to manually disconnect before the voltage got too low.
- a pair of very cheap (and nasty?) 35Ah 'drop in' batteries run in parallel. These, at eBay prices, would come to about £320 and give me significantly more endurance- over an hour- and the advertised weight is only 3.5kg per battery.Not sure I believe these numbers.
- a single cheap drop-in of 64Ah or more, in practise the cheapest I can find that fits the bill would be an Ultramax, 75Ah and costs £350. Weight just under 10kg. Theoretical range of 1hr 10min.

I still am not sure about the wisdom of going with no BMS at all. For the Winston pack this would mean running directly from a set of balanced cells, and then using their recommended charger. Is this a good idea?
 
How big is your trolling motor?

I ran a 28lb trolling motor on a 16ft canoe with a 50a flooded lead acid, it worked a treat, I was often on the "plane" and I never ran out of charge while going for a couple of hours at a time.

To answer your question, yes you can bypass the BMS as often the cells will do better than what the BMS will allow. You may have to monitor the voltage while you're holding it flat out so you don't risk over-discharging the cells. Keep them above 12.0v

Also in my experience the drop-on lithium deep cycles aren't always waterproof, I've lost two BMS's from water ingression. Something to be mindful of.

It's a Bison 68lb. Max draw is 64A.
If 12v is a sensible low voltage cutoff, it sounds as though I shouldn't be relying on the inbuilt BMS anyway- most of them are set to 10v.
 
Yup I'm pretty familiar with the discharge graphs for FLA, usually you get C5, C10, C20, C100 etc figures. But I thought that was a peculiarity of lead-acid (Peukert effect?) and didn't apply to lithium chemistries.

Going FLA wouldn't be the worst idea in the world, and would obviously be much cheaper, at least initially. To get my half hour endurance without going below 50% DOD I would need a 64Ah+ battery, which should cost comfortably under £100 and weigh about 16kg. This weight and the limited cycle life would be the downsides.

I think I have a few different options for LFP:
- the above linked 40Ah pack plus charger, this would come to about £280 delivered and give me the half hour endurance with a little room to spare. I'd also expect higher real world performance due to constant voltage of the battery. Weight would be about half the FLA, and I'd expect several thousand cycles so it would eventually pay for itself compared to FLA. I'd need to manually disconnect before the voltage got too low.
- a pair of very cheap (and nasty?) 35Ah 'drop in' batteries run in parallel. These, at eBay prices, would come to about £320 and give me significantly more endurance- over an hour- and the advertised weight is only 3.5kg per battery.Not sure I believe these numbers.
- a single cheap drop-in of 64Ah or more, in practise the cheapest I can find that fits the bill would be an Ultramax, 75Ah and costs £350. Weight just under 10kg. Theoretical range of 1hr 10min.

I still am not sure about the wisdom of going with no BMS at all. For the Winston pack this would mean running directly from a set of balanced cells, and then using their recommended charger. Is this a good idea?
You actually gave me a good lockdown research topic! Thanks! I found some research from a university in Belgium on the Peukert effect, and how to modify the equations to determine state of discharge given a load, time, and a Peukert constant for the appropriate Lithium chemistry. So, while the discharge curves are much flatter, there is a bit of the Peukert effect still. I need to digest this a lot further.

But you will get pretty close to your hour when draining your 64AH Lithium at 64 amps.

As for no BMS at all, I suppose it depends on how intimately you want to be involved in your discharging and charging process.
 
So after a bit more research, I'm thinking about putting together a 60Ah pack in a rugged waterproof case, with external stud terminals.
The motor draws a bit less than I thought-it's 696w, so only goes above 60A if the voltage is down at 11.6v

For a BMS, I'm pretty confused. I know I will need a 4S 12v unit rated at 60A or more. Maybe a Daly ? Even cheaper ones are available.

What else do I need to consider? I'd be most likely charging it on the bench from a 240v source, although hooking it up to the solar system on my yacht would be very useful too.
 
I just picked up the no frills 60A Daly for $19 via Amazon. 30A charge limit.

BMS leads are 10awg w/200°C rated insulation. That's marginal for 60A. Since you KNOW you're going to deal with sustained high current, probably worth targeting a 100A unit for margin.
 
Still mulling over options for this project.
One very easy route would be to buy a ready-to-go battery like this
Hard to beat on price - 60Ah, some sort of BMS built in, charge indicator, and it comes in a waterproof enclosure and has a charger- all for £234.
I'm sure the BMS will be pretty poor, 10v LVD and maybe only whole pack monitoring. And the charge indicator probably next to useless.

Whether I buy this or build something to do the same job, I have to charge it somehow on the yacht. I'll have a large PV array and Epever MPPT charge controller. Am I likely to encounter any issues charging using this? Can I adequately rely on the charge controller's settings to deliver a proper charge?
 
Had a play around with my trolling motor today for the first time. It took a fair dent out of a brand new and fully charged 110Ah leisure SLA. Would really like to go lithium as soon as possible, as I think the voltage will hold up much better.

So here's my latest idea- remember this will never, ever, be left running a load unattended, so I think it's feasible to rely on manual intervention for low voltage cutoff.

I can source 4x80Ah cells for about £100 delivered. Mount them in a suitable case, wire up four mini digital volt meters so I can actually see how each cell is doing. And off I go. Total cost should be under £130 by the time I've bought the case, connectors, etc. Having cell level information would be better than using some ready-made thing with a junk BMS, no?

(I haven't figured out how to charge this yet... I have a benchtop PSU that I can use to bulk charge the whole pack, and then to balance as required. Remember this will only be seeing occasional light use, not daily cycling).
 
What do we think of this deal?
They're only 32A cells but supposedly rated for "3-5C" discharge so 53A on the trolling motor should be no sweat... if you believe the numbers.
Seller seems pretty good, 98% feedback and has satisfied customers on this forum.
 
I still am not sure about the wisdom of going with no BMS at all. For the Winston pack this would mean running directly from a set of balanced cells, and then using their recommended charger. Is this a good idea?

Why don't you connect the cell positive terminal to a relay (bypassing BMS), and have the BMS control the relay?
That should give you low-cell-voltage shutdown but no current limit.

With AGM, even if you reach low-voltage "50% DoD" due to Peukert, you could probably go lower (to the extent that was just IR drop), or continue operating at lower speed. Even 70% to 100% DoD with reduced cycle life may not be an issue if you only use it once per week for 6 years. Daily, might only give you 18 months. But up to 80% DoD you probably aren't killing Ah cycle capacity, you're just using up lifetime Ah capacity faster (getting your money's worth sooner.)

If you want more than 1000 cycles in the next decade, then lithium has the potential to save you money (if you can buy it right).
 
The larger 280Ah cells have internal resistances below 0.2mΩ.

The picture in that product with the same tester shows 1.28mΩ

The voltage drop due to IR of those 32Ah cells will be pretty typical of a lead-acid battery with those kinds of IR numbers. I would be very dubious of any 3-5C claims.
 
Interesting.
At 100A draw, 321W delivered and 12.8W dissipated in cell resistance, 4%
At 150A, 481W delivered and 28.8W dissipated, 6%

Next I would consider thermal mass, see how long to heat cell to maximum temperature.
My guess is it could deliver 300W to a motor for a reasonable length of time.

"Weight: about 631g" so 2/3 of a liter of water, or 20 ounces. Calculate how long to heat that with 13W (water similar heat capacity as lithium.)

"Internal resistance: 3" Is that milliohm? If so a couple times higher than 1.28
 
Why don't you connect the cell positive terminal to a relay (bypassing BMS), and have the BMS control the relay?
That should give you low-cell-voltage shutdown but no current limit.

With AGM, even if you reach low-voltage "50% DoD" due to Peukert, you could probably go lower (to the extent that was just IR drop), or continue operating at lower speed. Even 70% to 100% DoD with reduced cycle life may not be an issue if you only use it once per week for 6 years. Daily, might only give you 18 months. But up to 80% DoD you probably aren't killing Ah cycle capacity, you're just using up lifetime Ah capacity faster (getting your money's worth sooner.)

If you want more than 1000 cycles in the next decade, then lithium has the potential to save you money (if you can buy it right).

Thanks, valid points. But a very large part of my thinking is that lugging a lead-acid on and off the dinghy regularly to is going to get pretty boring pretty quickly.

My absolute ideal outcome for this project would be a tiny but powerful battery pack that be attached to the motor itself, and easily taken home for charging.
Since the cheap BMSs sound like they're barely worth having, what I'd really like to do is fit four individual voltmeters on the battery case, for real time cell level monitoring. Maybe if I found the right components it could be as simple as four green lights, and if one turns orange it's time to go home.

E-bike packs seem much more appropriate in many ways- designed for short bursts of high current- but they are invariably higher voltage.
 
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