Ideas for Charging My Dinghy

[Peregrine1983]

Hi all,

Curious to hear ideas for charging our dinghy from the mothership while we are at anchor and/or underway.

Our dinghy has a trolling motor, 100AH AGM battery, and 50w solar panel. The panel does an OK job in good sun, but if I am using the dinghy often I want the ability to charge the battery from the mothership.

The electrical setup on the mothership is a start battery, 4 golf cart house batts fed by 300w of solar through a Victron charge controller and an additional 150amp alternator on the motor. There’s plenty of juice to go around - especially when motoring.

I would like to be able to tow the dinghy and charge it while underway from the mothership. Some sort of quick disconnect would be ideal.

I am also concerned about “confusing” the software on the Victron unit by essentially constantly removing and adding a battery to its bank every time I hook up the dinghy.

Any thoughts appreciated.

[john61ct]

No need to worry, a SC won't get "confused", but you need to test endAmps to ensure Absorb hold time is long enough with the greater AH load.

Make sure the two batts have similar charge voltage specs, and size the wiring for minimum voltage drop -actually measure it to check.

If the dinghy batt is a very expensive one, or if the distance is long enough (wire so thin) that voltage drop is an issue,

you may want to use a DC-DC charger at the target batt to give it its own accurate charge profile.

Otherwise, a combiner/VSR will help make sure the batt is paralleled to your house bank only when charging is active, better than a simple battery isolator switch.

I like Anderson connections, but there are marine-specific high-amp trolling motor plugs might be better if the location is wet.

Don't forget the fuse at the source end

[seandepagnier]

Simplest way is to connect it directly and it will probably do what you want, try this first. Consider that AGM can't replace water like flooded, so prolonged high voltages is not recommended.

If you want to charge fast, or a full charge when the main bank is drained, you need a way to boost the voltage. Simplest way would be a common battery charger from an inverter, but not efficient.

You can build a battery charger that can run from 9-16 volts, and fully charge a 12v battery (while maybe killing the main bank), there are schematics.

[Peregrine1983]

Awesome info. Thank you both.

The dinghy battery is a standard West Marine 100ah AGM. The distance for the charging wire would probably total about 20 feet. I will size appropriately - probably using a duplex wire to keep things more tidy. I have some research to do about charge voltage specs to make sure they are similar. I like the idea of using a DC charge controller in the battery box for the dinghy battery.

This is the dinghy battery -
https://www.westmarine.com/buy/west-...ours--15020258

These are the house batteries (x4) - https://www.samsclub.com/sams/durace...prod3590228.ip

[Adelie]

I would not charge while towing unless the dinghy was tight to the stern. Even then I’d think twice. Dinghy on davits or on deck no problem.

You think it’s bad when a painter goes thru the prop? What’s going to happen when charging cables go thru the prop attached to a battery that can put out 600+ amps?

[Peregrine1983]

Thought about this. That would certainly suck. Although I suspect my biggest issue would be the wire around the prop, not the current as it would be fused.

[john61ct]

Yes you want to avoid that 8-)

But a good fast fuse will help prevent a fire

long as fuel isn't around sparks 8-O

Since the batt's nothing special, consider investing at some point in a 100AH LFP, higher usable AH but much lighter and smaller.

Carry it between the charge source and the dinghy, use as a backup while you wait for the lead one to die.

[Fuss]

Put Andersen connect on dinghy battery. Fuse cabling. Put Andersen connect on transom cable connected to house bank. Fuse this cable. Make cable to connect the 2 Anderson connectors. Make sure both connectors never see water. Monitor voltage at dinghy battery to be sure not over bulk charging specs.
And as a small backup....get yourself a 10a lfp battery, wire it with Anderson connect and fuse cabling. This can be also be charged in a more convenient place and taken in dinghy for emergencys.
Change solar to 100w.
Put dinghy battery at bow if not already there.

[mbartosch]

We use an electric dinghy trolling motor with a DIY 12 V/40 Ah LiFePO4 battery in a waterproof suitcase. The battery pack contains a BMS, a relay for HVC/LVC cutoff and a Victron BMS 700 coulomb counter so the dinghy pilot knows about the remaining SoC while using the dinghy.

The battery is connected to the trolling motor with Anderson plugs. (The Anderson plugs work fine, but I found that after a while the tinned contact area becomes eroded and the bare copper appears - which is prone to corrosion.)

For redundancy and flexibility reasons we have two identical battery suitcases. When not in use both of them are kept in a "charging station" in one of the locker boxes of the boat. Anderson connectors are installed there which connect the two batteries to the charge bus of the boat. The solar chargers are attached to this bus.

With a "1 - 2 - Both" switch it is possible to divert the solar charging, either all charging to the house bank, all charging to the dinghy batteries, or to both the LiFePO4 house bank and dinghy batteries.
In the "Both" position the two dinghy batteries essentially extend the usable house bank capacity by an additional 80 Ah. A very nice feature on long passages.

In the "1" position the solar panels are directly and quickly charging the dinghy batteries (about 20 A charge current on a moderately sunny day), while in the "2" position the dinghy batteries are kept at full charge while the solar panels continue to charge the house bank.

Note that this is only possible because the house bank has identical chemistry as the dinghy banks (otherwise one would use a DC-DC charger, e. g. a Sterling Battery to Battery charger).

[john61ct]

Excellent setup

[Fuss]

Originally Posted by john61ct

Excellent setup

Yes, sounds very good to me too....any chance of any photos, that would be nice.

[Dockhead]

Originally Posted by john61ct

Excellent setup

+1

A very well thought-out set up If I ever have an electric dinghy motor, I'll copy it.

In any case, for the OP -- two batteries in their own swappable boxes is the hot tip. Charge one on board (DON'T run a charging cable to a dinghy under tow! ) and you'll always have a fresh battery. For a long dingy trip, you can carry both batteries

I would probably just permanently install a B2B charger and create a charging station somewhere in the mother ship. That way you are completely independent of whatever charging is being done to the house bank, or its chemistry, etc. So you can choose LiFePo or FLA or whatever you want, without regard to what your house bank is.

[Peregrine1983]

Thanks all. Great tips! I LOVE the idea of DIY LiFePo4. The whole reason I’m using AGM is because of the high cost of LiFePo4. As stated, with two lightweight batts I can just swap them out to a charging station on board and not have to worry about this whole charging tether thing.

This also solves the issue of having a really heavy battery (even though it’s strapped into a battery box) on the dink under tow. If the dink ever flips, it’s going to suck.

I believe I can use my AGM that’s currently in the dink to replace my windlass batt that’s getting old.

So one more question - what’s the best resource you have found for how to build my own LiFePo4s? I’m currently on YouTube...

Thanks again

[mbartosch]

Attached to this post is a picture of our two battery suitcases, a little worn after one year of heavy use during our cruise last year. They are powered down for the winter season, so the displays are off. Otherwise they normally show SoC of the pack.

When using the battery in the dinghy we keep them out of sight for the casual observer with a ragged old blastic bag. No need to raise interest, or worse yet, panic. Some people might have seen too many movies
This brings me to the downside: it might be a bad idea to carry the suitcase battery around in public. I once intended to ask in a bar for a wall outlet to recharge the battery with a battery charger. My wife reminded me that this might be a bad idea in these times.

However, the concept works fine so far. But of course the battery is also locked to dinghy with the steel wire.

Unfortunately I don't seem to have a photo of the "battery charging station" in the boat locker box with the 1-2-Both switch, but I guess you can imagine that.

The design of the battery pack is described here. Please note that the BMS I used is no longer available, but it's not the only BMS on the market.

These battery packs have other uses as well:
This post describes a very early stage in which we actually temporarily replaced the old lead acid house bank (before we had the LiFePO4 house bank installed) with the suitcase battery on a vacation trip. It was right after we bought the boat and I wanted to measure real world energy consumption. Totally killed the old batteries, but with the portable battery pack we were able to finish our vacation.

[Peregrine1983]

Originally Posted by mbartosch

Attached to this post is a picture of our two battery suitcases, a little worn after one year of heavy use during our cruise last year. They are powered down for the winter season, so the displays are off. Otherwise they normally show SoC of the pack.

When using the battery in the dinghy we keep them out of sight for the casual observer with a ragged old blastic bag. No need to raise interest, or worse yet, panic. Some people might have seen too many movies
This brings me to the downside: it might be a bad idea to carry the suitcase battery around in public. I once intended to ask in a bar for a wall outlet to recharge the battery with a battery charger. My wife reminded me that this might be a bad idea in these times.

However, the concept works fine so far. But of course the battery is also locked to dinghy with the steel wire.

Unfortunately I don't seem to have a photo of the "battery charging station" in the boat locker box with the 1-2-Both switch, but I guess you can imagine that.

The design of the battery pack is described here. Please note that the BMS I used is no longer available, but it's not the only BMS on the market.

These battery packs have other uses as well:
This post describes a very early stage in which we actually temporarily replaced the old lead acid house bank (before we had the LiFePO4 house bank installed) with the suitcase battery on a vacation trip. It was right after we bought the boat and I wanted to measure real world energy consumption. Totally killed the old batteries, but with the portable battery pack we were able to finish our vacation.

Fantastic. Thank you. BatterySpace.com/AA Portable Power Corp. Tel: 510-525-2328 - Powerizer Battery Official Site seems to have decent prices on the cells, BMS, etc.