Lithium battery for trolling motor

Kelpie

Kelpie

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Yup here I am starting yet another thread on batteries. SWMBO says I need help:D

I've recently bought a 12v trolling motor and I and I want to try using it on the dinghy- expecting it to be more of a backup to the oars than a real replacement for the outboard. For the short runs I have in mind I don't need a huge battery, and I'd love to pair it with a small LiFePO4 to keep everything light and manageable, especially as the battery will have to be carted on and off the dinghy for charging.

Max current draw is 68A, and this is where my plan comes unstuck. A small "drop in" battery of e.g. 20Ah usually only has a maximum discharge current of 20A. It seems to be the rule that for these types of battery, a 1C discharge rate is normal, so I'd be looking at a 70Ah battery minimum. I don't really expect to need an hour of full throttle performance, so it seems a shame to have to fork out £400 just in order to have the discharge capacity. And this problems stems from the cheap BMS that is fitted to these batteries- the cells themselves can manage.

I suppose I could go DIY, with a small battery pack and a beefy BMS... and then I have the headache of putting it into a suitable case for portability. But I'd rather just go for an off the shelf solution, if one exists.
 
My crew gets very satisfactory performance from a Yuasa 36Ah deep-cycle AGM battery (£68 from Tayna) which I think is intended for golf caddies. The trolling motor draws 58A maximum (iirc) and one up in a dinghy the battery is good for at least 90 minutes of playing around. Lithium would be nice, but the prices are still horrifying.
 
JumbleDuck said:
My crew gets very satisfactory performance from a Yuasa 36Ah deep-cycle AGM battery (£68 from Tayna) which I think is intended for golf caddies. The trolling motor draws 58A maximum (iirc) and one up in a dinghy the battery is good for at least 90 minutes of playing around. Lithium would be nice, but the prices are still horrifying.
Click to expand...
I can get a 35Ah LifePO4 battery with built in BMS and a mains charger for £160. Nearly bought it, until I realised that the max current draw is 35A, so I could only use it at half throttle. I'm tempted to get a pair of them and run them in parallel...

I'm investigating ebike packs just now, they are small and need to handle fairly high currents, but not many of them are at 12v.

Daft thought off the top of my head: make a miniature hybrid system, as discussed on the other threads. A small and cheap lead-acid battery, primarily used as a way of handling the high discharge currents; couple this with a cheap drop-in LFP, which provides most of the actual storage. Hmmm.
 
Sounds as if things are moving the right way - that's a lot better than anything we found, though still not good enough. Give it a year or two ...
 
What options are there for trolling motors these days. The proper e-outboards cost more than I would pay.
 
dgadee said:
What options are there for trolling motors these days. The proper e-outboards cost more than I would pay.
Click to expand...

I just went for the most powerful 12v Bison- 68lb thrust. Supposedly saltwater rated, they cost about £175 new although I got mine secondhand for a good bit less. At that price it doesn't have to be amazing, just better than rowing.

On our last cruise I hardly ever used my 2.5hp 2 stroke at anything above idle, because when you do it just makes you wet and/or makes the Avon start to fold in half. On that basis I reckon we would be better with electric. Silent, it has reverse, very SMWBO-friendly, and it will always start instantly regardless of whehter it's hot or cold.

One of these motors plus a cheap LiFePO4 battery ought to cost about £300, and give half an hour at full throttle. Very competitive with a 2-stroke outboard. But as I've discovered, these little lithium batteries can't handle that sort of current. You basically have to buy a battery that can go full throttle for a whole hour... which is a heck of a lot, and fairly whacks up the price. Someone needs to pair smaller (cheaper) cells with a beefier BMS to make the ideal battery for a trolling motor.
 
Kelpie said:
Max current draw is 68A, and this is where my plan comes unstuck. A small "drop in" battery of e.g. 20Ah usually only has a maximum discharge current of 20A. It seems to be the rule that for these types of battery, a 1C discharge rate is normal, so I'd be looking at a 70Ah battery minimum. I don't really expect to need an hour of full throttle performance, so it seems a shame to have to fork out £400 just in order to have the discharge capacity. And this problems stems from the cheap BMS that is fitted to these batteries- the cells themselves can manage.
Click to expand...

Have a look at this LiFePO4 battery, 20Ah with no internal BMS electronics. Max discharge 200A. Max continuous discharge 60A. Weighs 3.5kg. Sounds like it might do the job. And probably about £150 with shipping.

Shop GWL | Lithium Battery LiFePO4 (12V/20Ah)
 
pvb said:
Have a look at this LiFePO4 battery, 20Ah with no internal BMS electronics. Max discharge 200A. Max continuous discharge 60A. Weighs 3.5kg. Sounds like it might do the job. And probably about £150 with shipping.

Shop GWL | Lithium Battery LiFePO4 (12V/20Ah)
Click to expand...
Great mind think alike, I've been eyeing that up already. But I think I may have to pair it with some sort of electronics... it can't be as simple as just hooking it up and turning on the motor... can it?
With no BMS I also don't know how to go about charging it. I'm pretty new to all of this.
 
As it's for a trolling motor then all concerns about risk of drop-ins vanish. If it stops working you just pick up the oars or a replacement. It it were me and I was interested in exploring further I'd write to GWL and ask them to recommend what low and high voltage figures they recommend for 80% depth of discharge and hence for long life. (They only provide max and minimum for 100% in the tech sheet.) As there is no balancing possible the cells (cylindrical presumably) rely on a good initial balance. By keeping away from the upper charging knee they should stay in balance for longer but if they don't stay in balance there is nothing you can do but accept the reduced capacity.

You could charge to say 14.6volts (or whatever GWL recommend) and then immediately stop and then use. Just bulk charging, no absorption or float. Keeping the battery around at 80% is not too bad for longevity but better to arrange to take it to that level just before you use it whenever possible.

It may be a bit more tricky to decide when to stop drawing a load - some way of monitoring voltage during use but taking into account the considerable load of the trolling motor. Accidental over-discharge is another life-shortening issue.

So, overall, it might be worth a punt but the absence of a BMS for balancing could ultimately be a problem for getting maximum value from it. GWL are an excellent firm and nothing like dealing with grey imports from China. In my few dealings with them I have found them very helpful in answering these kind of questions.
 
Last edited:
Kelpie said:
I just went for the most powerful 12v Bison- 68lb thrust. Supposedly saltwater rated, they cost about £175 new although I got mine secondhand for a good bit less. At that price it doesn't have to be amazing, just better than rowing.
Click to expand...
We have the 54 lb one, bought almost new and cheap from Gumtree. I have modified it by cutting down the shaft - it's just a GRP tube - and have started replacing all the mild steel fastenings with stainless. All quite easy, though it turned out out to have a pingf***it as a detente for extending the tiller..
 
Poey50 said:
As it's for a trolling motor then all concerns about risk of drop-ins vanish. If it stops working you just pick up the oars or a replacement. It it were me and I was interested in exploring further I'd write to GWL and ask them to recommend what low and high voltage figures they recommend for 80% depth of discharge and hence for long life. (They only provide max and minimum for 100% in the tech sheet.) As there is no balancing possible the cells (cylindrical presumably) rely on a good initial balance. By keeping away from the upper charging knee they should stay in balance for longer but if they don't stay in balance there is nothing you can do but accept the reduced capacity.

You could charge to say 14.6volts (or whatever GWL recommend) and then immediately stop and then use. Just bulk charging, no absorption or float. Keeping the battery around at 80% is not too bad for longevity but better to arrange to take it to that level just before you use it whenever possible.

It may be a bit more tricky to decide when to stop drawing a load - some way of monitoring voltage during use but taking into account the considerable load of the trolling motor. Accidental over-discharge is another life-shortening issue.

So, overall, it might be worth a punt but the absence of a BMS for balancing could ultimately be a problem for getting maximum value from it. GWL are an excellent firm and nothing like dealing with grey imports from China. In my few dealings with them I have found them very helpful in answering these kind of questions.
Click to expand...

Thanks once again for an excellent and informative post.
What about adding this:
Shop GWL | Victron Energy Battery Protect BP-65 12V/24V 65A

Is one amp above the required current enough... and could you us ea VSR instead...?
 
Kelpie said:
Thanks once again for an excellent and informative post.
What about adding this:
Shop GWL | Victron Energy Battery Protect BP-65 12V/24V 65A

Is one amp above the required current enough... and could you us ea VSR instead...?
Click to expand...

I've not used one but it looks like it would be OK for low voltage disconnect. You would need to carefully work out which voltage to use. You could do that by running the engine at normal running speed while carefully monitoring the voltage under that load. As you probably know the flat discharge curve of LFP means that voltage holds up very well until near the end so as soon as the voltage starts to drop and you are at the start of the lower knee then note the voltage-under-load and immediately stop. Use that voltage for the low voltage disconnect on the battery protect. Check again a couple of times - varying the revs (load) a bit monitoring the voltage to ensure that the low voltage disconnect cuts in soon enough but not so soon that you overly limit the capacity. The alarm warning will be useful.
 
Thanks. The trolling motor itself has some sort of display on it, obviously this will be designed with lead-acid in mind. I'll need to play around with it to see how the power indicators correspond to battery voltage.

Would there be any reason not to just use a VSR to disconnect the battery?

And if I bought a cheap drop-in battery with integral BMS, could I hack in to it to draw power directly from the cells, then charge it via the BMS... or would that do bad things...?

(Just going to register on the DIY Solar forum to take all my inane questions over there instead :) )
 
Kelpie said:
Thanks. The trolling motor itself has some sort of display on it, obviously this will be designed with lead-acid in mind. I'll need to play around with it to see how the power indicators correspond to battery voltage.

Would there be any reason not to just use a VSR to disconnect the battery?

And if I bought a cheap drop-in battery with integral BMS, could I hack in to it to draw power directly from the cells, then charge it via the BMS... or would that do bad things...?

(Just going to register on the DIY Solar forum to take all my inane questions over there instead :) )
Click to expand...

I'm in danger of over-reach for those questions. See what the good folks of DIY Solar have to say. There's an awful lot of them.
 
JumbleDuck said:
We have the 54 lb one, bought almost new and cheap from Gumtree. I have modified it by cutting down the shaft - it's just a GRP tube - and have started replacing all the mild steel fastenings with stainless. All quite easy, though it turned out out to have a pingf***it as a detente for extending the tiller..
Click to expand...

What speed from what dinghy? I am right off outboards - crew can't start it and a nuisance to lift on/off. I usually just row.
 
I have the same trolling motor paired with 100ah lifepo4, the most I seen it draw is 53amps. This is one person in 8ft dinghy. Using a 60amp breaker from ebay, no problem. The run to my mooring is about 600metres.
 
andyj301 said:
I have the same trolling motor paired with 100ah lifepo4, the most I seen it draw is 53amps. This is one person in 8ft dinghy. Using a 60amp breaker from ebay, no problem. The run to my mooring is about 600metres.
Click to expand...
Same as Jumbleduck's (54lb) or same as mine (68lb)?
 
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