Tender trolling motor charging?

I live on a trawler on the hook. 500w of solar on the roof + a diesel generator & an inverter. I pull a skiff behind it with a late 90s 50hp Honda 4 stroke. I have 1 battery in the bow for my electric trolling motor (12v system). My current solution to charging my trolling motor battery is a 6amp charger mounted on the skiff + an extension cord (slightly longer than my skiffs painter line). So ive got DC coming out my battery bank, converted to AC by trawlers inverter, then converted back to DC by charger on the skiff.

I'd like to think there's a more efficient way to do this. I know the inverter draws 1.7amps just being on, so i usually leave it off unless I want to watch TV or use the microwave -- but obviously if I need to charge my trolling motor battery, it needs to stay on.

I fish 3-5 days a week. But the trolling motor, doesnt get that much use. Mostly for netting bait or to throw a few casts at a docklight. So its not like the battery is discharged everytime I use it. On the otherhand, my outboard gets alot of hours, whether idling in the inlet or trolling offshore.

As for alternative charging methods:
A) I see minnkota makes a product that sends excess power (from the engine alternator) to a spare battery once the starting battery is full. But would my little outboard be putting out enough excess to have much effect?
B) 20W solar panel with charge controller mounted on skiff. I've read these put out about 1.5amps at full midday sun. Not alot, but I live in Miami, so it would add up to something. Biggest problem with that is not really a good spot to mount it.

Any insight on either of those solutions? Or another alternative?

It doesnt need to be a 100% solution. No big deal to hook up a charger when troller battery gets used alot in a day. I just want a solution that can recharge after the typical minor usage, so that I don't need to plug the damn thing into AC power constantly and end up wasting more amps in the conversion process than im actually putting into the battery.

Also, if plugging in is inevitable, could I at least eliminate the inverter/conversion loss and run an extension cord from my trawler DC outlet to a dc charge controller at the trolling motor battery on the skiff?

The Minn Kotta device you mentioned will work as stated in your last paragraph, it doesn't need to get its power from the O/B alternator, it can get it from your trawler's dc system or either. Charge from the trawler's DC system while tied up and from the O/B when underway in the skiff.


Use a DC plug on a cord and sockets on the other end (Minn Kota makes these) so that you can plug the DC cord to the trawler or the O/B.



David

Just to be clear...this is the minnkota product you are referring to?




https://minnkotamotors.johnsonoutdoors.com/battery-chargers/board-alternator?id=13891

I'm just asking because in the description it states "after your main battery is fully charged, it will send excess power to your trolling motor battery".

I can see how that would work on the skiff (if the outboard makes significant enough power that the starting battery is always full).

But how would it work when routed from the trawler? Would the trawler bank need to be 100% (because obviously then it would not work)?

How big is the trolling motor battery? How much do you generally drain it in a day of fishing?

I think I would like the idea of a DC-DC charger from your Honda as you mentioned. If you ran the OB engine enough running to and from your fishing location, it might keep the trolling motor charged. Of course it might also burn out the alternator? I don’t know. Check with a Honda mechanic.

If that isn’t going to work, if you have a Bimini on the skiff, could you mount a single solar panel on its frame? A simple 100W package with panel, controller, connecting wires and mounting hardware can be found for <$200. I think a rough estimate would be that a 100W panel would give you 30 Ah a day. More or less depending on the charge controller efficiency, whether the panel is angled toward the sun, clouds, shade etc... So that would give you about 210 Ah a week. So figure out how many Ah you draw from your battery during a weeks use. If the 100W panel won’t be enough, it is easy to get larger panels. Mine is a 360W panel that with its controller can put out a lot Ah over a day when it is sunny. Right now it is very cloudy and it is producing 70W and putting 5 amps into the batteries.

So figure out how many Ah you need to replace into that trolling battery and go from there.

I have a bimini on the skiff, but its down 98% of the time because you cant really fish with it up.

Troller battery is 122ah. Like I said, it rarely gets extended use, but trolling motors do draw alot. No idea how much I draw it down on an avg day. But more often than not, I dont even actaully use it.

ERTF makes a good point. I had assumed that when the DC source (O/B, trawler system or whatever) gets above a certain fixed voltage, maybe 13.5 like most combiners, then it starts charging the trolling motor battery by boosting the voltage (using DC to DC technology) to charge the trolling battery. That way the trawler battery would only supply power when it was at least being charged by a source like the solar panel.



But maybe it does and maybe it doesn't. Need to investigate the specs and operations manual and/or contract Minn Kota.


David

I have a Newport NV-55 on a utility Livingston I use around my dock. It draws as much as 52 amps. I use a 7w solar charger (about $40). It seems to keep it up, even in coastal Oregon.
Another way is to run dc wires with the bow line and have the charger on board. You can wire tie the wires to the line or if you're handy, weave them into the line. Or use a battery isolator that will charge when an alternator is running. Or buy a DC to DC charger.

I would think that a solar panel would be the best way to go, just have to think out of the box a bit to figure out a way to mount it. With charging via the main boat the voltage drop on the really long wires will probably be an issue. There has to be a way to find a mount for solar. How about the flexible solar panels? They aren’t as efficient as rigid but you could accept a bit less efficiency if you can get it mounted. How about a temporary mount and put it away when you are using the dinghy? Maybe put the bimini up when you are not using the dinghy and use a flexible panel on top of it. Then stow both when using the dinghy.

ERTF said:

I have a bimini on the skiff, but its down 98% of the time because you cant really fish with it up.



Troller battery is 122ah. Like I said, it rarely gets extended use, but trolling motors do draw alot. No idea how much I draw it down on an avg day. But more often than not, I dont even actaully use it.

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OK. I have a small Torqeedo OB that I am using on my RIB. I also have a flexible solar panel that will charge the battery. I don’t use the panel often, but if I am out on a 2 week summer cruise with lots of dink usage, I unroll the solar panel and tie it down. It keeps the Torqeedo battery charged up nicely. So, a rigid, fixed mount may not work for you but maybe a flexible panel would?

However, given your use, I’m thinking that a DC-DC charger or combiner like you mentioned may be your best option.

If it was me, I’d really try to do something to get away from running a 120 extension down your skiff’s painter.

Comodave said:

I would think that a solar panel would be the best way to go, just have to think out of the box a bit to figure out a way to mount it. With charging via the main boat the voltage drop on the really long wires will probably be an issue. There has to be a way to find a mount for solar. How about the flexible solar panels? They aren’t as efficient as rigid but you could accept a bit less efficiency if you can get it mounted. How about a temporary mount and put it away when you are using the dinghy? Maybe put the bimini up when you are not using the dinghy and use a flexible panel on top of it. Then stow both when using the dinghy.

Click to expand...

The bimini just isnt an option, I'd have to assemble and disassemble the solar panels almost daily. Really any solar panel on the skiff that I have to move/stow when I use it, is a non-starter. The best permanent placement option I could think of is a flexible panel on my cowling then run a wire all the way up to the bow. Would have to be a pretty small panel & still not a very elegant solution. But thay was my original idea before I came looking for a better one.

dhays said:

If it was me, I’d really try to do something to get away from running a 120 extension down your skiff’s painter.

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Yeah its not ideal, but I do have it setup so the end of the extension can't fall in the water. And its plugged into an external GFCI on the trawler. However that adds another flaw to the sysyem, because when it rains, sometimes it trips the GFCI, so then I awake the next AM to find my battery didnt get charged.

dhays said:

OK. I have a small Torqeedo OB that I am using on my RIB. I also have a flexible solar panel that will charge the battery. I don’t use the panel often, but if I am out on a 2 week summer cruise with lots of dink usage, I unroll the solar panel and tie it down. It keeps the Torqeedo battery charged up nicely. So, a rigid, fixed mount may not work for you but maybe a flexible panel would?

However, given your use, I’m thinking that a DC-DC charger or combiner like you mentioned may be your best option.

If it was me, I’d really try to do something to get away from running a 120 extension down your skiff’s painter.

Click to expand...

Curious to know about your torqeedo and rib. What size rib and Torqueedo? how long does a charge last etc?

If I understand correctly, you run your dink 3-5 days/week and the dink OB alternator charges your dink 12V start battery during each use. Your trolling motor 12V battery is independent of the dink electrical system and needs periodic charging which you provide via electrical cord and charger.

If this is correct, how about adding a Yandina combiner between the dink start battery and the TM battery to share the charge after the start battery reaches ~13+V?

FlyWright said:

If I understand correctly, you run your dink 3-5 days/week and the dink OB alternator charges your dink 12V start battery during each use. Your trolling motor 12V battery is independent of the dink electrical system and needs periodic charging which you provide via electrical cord and charger.

If this is correct, how about adding a Yandina combiner between the dink start battery and the TM battery to share the charge after the start battery reaches ~13+V?

Click to expand...

Yes that is the situation.

The combiner may be a solution. But I really need to figure out how much excess charge my OB is putting out.

Sailor of Fortune said:

Curious to know about your torqeedo and rib. What size rib and Torqueedo? how long does a charge last etc?

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I’ve got a very old Walker Bay Genesis 310 Deluxe open RIB. All that means is that the RIB itself is reasonably heavy at 135 lbs. I also have an old Torqeedo Travel 1003. The way I use it, it will give me a range of about 4 nm while pushing the RIB at about 3 knots. It works great for getting back and forth from boat to shore, cruising around the harbor etc.... not something you want for exploring or traveling any kind of distance. For that I use it for, it is great. I bought it initially to use on my inflatable that I used with my sailboat. I like it because it is easy to move, lightweight, quiet, and I’ve done absolutely not maintenance on it despite its being used in salt water.

There are times when a larger OB would be nice and when I bought this boat the RIB came with a Honda 8hp 4 cycle with electric start and power tilt. The motor ran great, but was too heavy and didn’t have enough power to get the RIB on plane. So I took it off and put my Torqeedo on.

I have a Striker 60lb thrust on my 10ft ally dinghy.It pushes me along at 3kn which is great to get me to shore or puttering around.I have just upgraded my main solar to 2x 325w from 3x130w and so have mounted one of the 130s to a storage box I have on the aft deck and run a wire to the dinghy battery to charge it. Works great and with a pair of Anderson plugs easy to switch from charge to motor. Not a particularly long run as I tie the dinghy to the side of the boat to get our dog on and off.
Works for me. Oh and I sold the other two as well so offset the cost

John

Same idea as the combiner, Rebel, but for less than $100 US, it eliminates the need for switch throwing or plug switching. Great for up to 100A.

Yandina 100A Combiner
https://www.yandina.com/c100InfoR3.htm

I ordered a Yandina combiner, to recharge after minimal uses. Not sure how much I will get from that because I think my outboard only puts out 10amps max.

For when i run it down more, what do you guys think of this DC to DC charger (from trawler / eliminate inverter): Amazon: https://www.amazon.com/Tecmate-Opti...ocphy=1015116&hvtargid=pla-624926126847&psc=1

I think that small 2 amp DC to DC charger will only supply a fixed 13.5 V output. If you leave it connected for a couple of days it will ultimately charge up your trolling motor, but it won't be fast.


Sterling makes real DC to DC three step chargers, but for a lot more money.


David