Charging dinghy from mothership

[john61ct]

Yes, but (see above). Far from optimal for either side.

[sparrowhawk1]

Optimal can be measured in different ways. It uses the least amount from my house Bank and if you don't leave too early in the morning it will Top the battery up. We're only talking about replacing 20 at the most 30 amp hours and usually less with my setup.
And once the dinghy battery is charged the dinghy dedicated panels may even contribute to charging the house

[john61ct]

I would minimize that voltage drop. Probably via a DCDC charger so I can control the direction of flow.

And unless the source bank is LFP, only when a charge source is active.

[sparrowhawk1]

So I went to shore half hour ride I parked my dinghy in a place that's shaded. Got back voltage is at 12.3 measured After an hour from when I got back. Plugged into house .7 amps going in. House of 12.4 measured couple minutes after I hooked up. Refrigerator running full blast several fans running.

[john61ct]

And? Are you saying that is good?

Charge voltage should be well north of 14V.

Not getting to 100% Full will murder the batt get a fraction of its life.

By going into one batt then another you are needlessly wearing out two, twice as fast.

And losing 60-80% of the energy from the original charge source.

Sure can be done but far from best practices.

[sparrowhawk1]

That's not giving any opinions just stating facts. Like I said before I have two solar panels that will Top it up in the morning. Which I believe both house and trolling batteries will be at 12.4. Which doesn't hurt my house and helps my trolling battery.
At 9 a.m. this morning my house voltage was at 13.6 I don't know how long it will take for a higher voltage or to charge my dinghy battery with just the house but it would do it. Just a matter of having the dinghy at the house during the day. Edit my dinghy voltage is at 12.3 it's not going to bring my house down below that and the point that the voltage is that low it's an extra battery in the house.

[sparrowhawk1]

The Gage with my solar panels(12.4 is what it reads this morning which is what it was when I hooked up the trolling batt. last night) does not quite match my Sperry multimeter. anyway 7 a.m. this morning house is 12.54 and dinghy 12.48 according to Sperry. And the 12.3 voltage of my dinghy last night last night was from Sperry. According to the solar panel gauge a little over an amp is going in.

[Jammer]

Originally Posted by Peregrine1983

I have a solar/electric dink setup that I love, but I want to find a way to also charge it from the mothership’s bigger solar system and battery bank when needed. I have an auto battery charger/maintainer, but this seems it would be inefficient as it would have to run on AC 110v off the mothership’s inverter.

Modern inverters are pretty good. Of the solar power you start with, you'll get maybe 75-80% of those watt-hours into the dinghy battery going through an inverter and charger.


If you use a DC-DC charger, and no inverter, you'll get maybe 85-90% of the watt-hours you started with.

What is the best way to hook up the dinghy batt to the mothership if I want to charge?

The best way is to use a DC powered battery charger designed for the purpose, permanently installed below decks, with well-thought out cabling and connectors.

Originally Posted by Peregrine1983

I assume it's not as simple as just creating a connection to my house battery bank. I'm thinking of something that will top it up overnight at anchor.

Correct. Creating a connection to your house battery bank will not provide reliable charging at night. (It could provide good charging during the daytime, as the solar panels on the big boat would end up charging both the house bank and the dinghy)

Originally Posted by john61ct

It is **very** inefficient going from batt to batt,

you could easily be reducing House lifespan by a lot if this is a regular thing

I don't believe there's anything to suggest that to be the case. A dinghy charging load is a load like any other, and will not reduce the lifespan of the house bank any more than, e.g. running the chartplotter or a vent fan or autopilot. Most house banks never get enough cycles to cause failure. They fail for other reasons.

Originally Posted by john61ct

Not a matter of degree, the concept is flawed from the get go.

Only a tiny fraction of input power makes it into storage.

Well, around 90% of the input power.

Moving it from one storage to another is triply wasteful.

And every cycle is a little slice of bank death, you only have so many.

Good quality house banks, properly sized, in a cruising boat, don't ever reach their cycle life. They fail for other reasons. Age, sulfation due to being left, inadvertently, in a discharged state, shorted cells due to low electrolyte levels and overheating, etc.


Whether it is worth the added complexity is something the OP will have to decide.

[sparrowhawk1]

Well I was just rereading and I thought wow that's really high voltage I had early in the morning for my house Bank and then it dawned on me that I have wind generators. Bang head.

[sparrowhawk1]

it would be worth having a dc-to-dc converter if I could find one for about 20 bucks, it would have to be self-limiting say maybe 2-10 amps. Any recommendations? And for the naysayers I agree that this is not for everyone but I have a large solar array and wind generators, my batteries are topped up almost everyday and Breezy nights my wind generators keep up with my night demands. So taking 10 to 20 amp hrs from a 400 amp hour battery Bank is more cost-effective than having a little outboard engine that I have to buy fuel for and maintain. Not to mention reliability. Quiet, odorless operation . time saved not buying, transporting, mixing fuel. I am not recommending anything just stating what I have done, do so at your own risk. I have found the small voltage differential between the two batteries and the resistance in the long extension cord keeps the amperage very low. That is why I'm considering getting a dc-to-dc converter.

[sparrowhawk1]

And I just Googled little Outboard prices and a Mercury 2.5 horsepower goes for $900 which equals a trolling motor, battery, solar pannel, controller and one-and-a-half house battery banks. And before anyone says... yes I know the gas engine will push me much faster much farther but for small distances it's a no-brainer to me.
And if you want more speed instead of one and a half houses have one house one more battery for the dinghy and a higher thrust trolling motor.

[Peregrine1983]

Figured I'd chime in here as I started this thread.

While the trolling motor setup was fine for short runs to and from our mooring, we've since set it aside. We've been cruising further afield and anchoring far our more often and using the dinghy in varied conditions. The trolling motor became tough going upwind and we found it was basically unusable motoring into chop.

I didn't want to drop the cash on a Torqueedo, so I very reluctantly decided we needed the safety, speed, and power a gas motor provides. I picked up a 2001 Merc 5hp 4 stroke for $500. It's served me very well. I feed it only ethanol free gas and it's been excellent. My last motor (Tohatsu 9.8) was nothing but issues, but this one has been good ***knock on wood***. It's much lighter than the Tohatsu 9.8 and it's actually reliable... imagine that. Ethanol free fuel is the only way to keep one of these things running reliably IMHO. The additives didn't work for me.

I've even dropped the money and bought $20 a can TruFuel a few times when ethanol free wasn't available... that's how much I don't want to put ethanol fuel in it.

[john61ct]

Originally Posted by sparrowhawk1

it would be worth having a dc-to-dc converter if I could find one for about 20 bucks, it would have to be self-limiting say maybe 2-10 amps

Don't you need a proper charger?

Or just move the batt to the source?

Nothing decent is going to be that cheap unless you get lucky at an estate sale or flea market

[sparrowhawk1]

In order. First I have 2 dinghys, different tools for different jobs.
For the Walker Bay you can only put a small engine on it anyway and I can duplicate that with a higher thrust trolling motor and two batteries. But that's not what I wanted, I wanted a cheap, reliable way to travel short distances where I wouldn't have to worry so much about theft. john 61ct why isn't a dc-to-dc converter a proper charger? The battery stays in the Walker Bay. I want to be able to put about 15 amps into it overnight from my sailboat house batteries if I have to leave early in the morning the next day. Otherwise the solar panels dedicated to it keeps it topped up. I was hoping dc-to-dc would be as cheap as AC to DC Chargers but I guess not.

[john61ct]

If your target bank is of any quality, thus not dirt cheap, you want a charger that will use the proper spec Absorb voltage and maintain it only long enough to get it to 100% Full (Hold Absorb Time) and then stop charging (drop to Float).

Too low an Absorb V or too long hold time there will both shorten batt lifetime.

If you already have properly regulated charge sources, just carrying the dinghy batt to connect it closer, or maybe just using heavier gauge wires, will reduce the voltage drop.

Or buy a proper DCDC charger, hundred$

Or replace the dinghy batts much more frequently if they're not getting properly charged.