Charging dinghy from mothership

[sparrowhawk1]

Think I've already said this but a 30 lb thrust trolling motor will push a Walker Bay 8 at 3 knots, 2.5 knots into 15 to 20 knot winds. Edit. I just read the last post, I would be happy with a converter that will charge my trolling battery to 13.8 and hold it there overnight so if anyone knows where I can get one for around 20 bucks let me know

[sparrowhawk1]

the nights I'll want to charge my dinghy from the house are when I'm going to be off the boat all day the next day so my house will be topped up for me to do it again the next night. I have found many converters for under 20 bucks, I just don't know which one to get.

[john61ct]

Link to your batteries and a few example high-enough amp converters for feedback.

You will be doing the start and stopping of the charge cycle yourself.

Not sure I understand the $20 thing when just the wiring will cost many times that, if done right.

[sparrowhawk1]

Originally Posted by john61ct

Link to your batteries and a few example high-enough amp converters for feedback.

You will be doing the start and stopping of the charge cycle yourself.

Not sure I understand the $20 thing when just the wiring will cost many times that, if done right.

I already have a fused extension cord that reaches my dinghy all I need is a converter to boost from say 12.2 -12.4 to between 13.8 and14.2. There are many Converters on Amazon that will do this I just don't know which is the right one.
PS. If it says 4 amps Max is it self-limiting?

[john61ct]

sorry if I wasn't clear

I meant, please post links to the ones you think may be suitable, and perhaps you'll get feedback here.

And no, you cannot assume decent overcurrent protection, especially with cheap devices.

In fact, best to assume you must buy a device rated (in theory) for 3-5x the maximum continuous actual current it will see. And carry spares,

with known good devices you can be a bit less cautious, and English-speaking tech support can answer detailed questions like that, if not already spelled out in their documentation.

That is also why I asked for a link to your batteries, both source and target bank in the dinghy.

Are you only charging the target while a charge source is active? That would be smart, unless your source bank is constantly at high SoC or very large compared to the dinghy's.

And again, please acknowledge you understand will be doing the start and stopping of the charge cycle yourself, as opposed to using a proper DCDC charger.

Detailed specs on your "extension cord" would also be useful.

[sparrowhawk1]

Thanks for trying to help me. But a couple of things were not seeing eye-to-eye on. First I operate boats all day long that have a regulator with a fixed voltage anywhere from 13.8 to 14 .6. So I don't understand how you see bringing it to 13 8 or so and holding it there is going to be a problem. There are literally hundreds of DC to DC Converters on Amazon and I don't know where to start in finding which one is the best for the job. I'm sure there's features I don't know about,( they all basically seem to do the same thing but some are junk for sure) My extension cord is 12 gauge rated at 20 amps with a 15 amp circuit breaker close to the house battery. My house is almost never below 12.4 volts in the morning and the fact that I only charge the dinghy during the night when I know I have to leave early in the morning which means I'm not going to be using the house that day except for a super efficient refrigerator so it will be topped up for the next nights dinghy charging. Last night it was gusting to 12 knots when I walked by the amp meter the highest amps out where 2.1 lowest was less than 1. on a windless night it doesn't get much above 3 amps when the fridge is running and maybe 7 amps Max when I have a bunch of lights on which is not very often. so there's no way I'm hurting a 400+ amp hour battery by taking less than 30 additional amps out.

[john61ct]

Well we could spend lots of time on details wrt to safety and ensuring longevity of expensive banks.

For example no wire can be "rated at 20A" without spec'ing voltage, length, temp rating of insulation etc.

And, very few lead battery chemistries will charge to full at such low voltage, and going higher risks damage to the target battery from overcharging.

But I get the feeling you prefer a more rough and ready approach, and perhaps are willing to just replace the banks more frequently.

I doubt if anyone here can recommend a **super cheap** converter for charging a battery, since that is not how boat owners usually work, for good reason,

not what what these devices are designed for.

Basically you are being a pioneer, and may suffer some arrows in the back as a result.

Do please report back here once you've figured things out, may be very helpful to fellow like,minded members, especially if you can measure V, A, and aH results of your experiments.

Personally I'd go with Blue Sea, Victron, MasterVolt or maybe Samlex, "features" like decent overcurrent protection means lots pricier.

For the super cheap stuff, forget Amazon marketplace, even eBay would be better looking at seller rating, better gaurantees. Maybe if Amazon directly, at least Digikey and Mouser, maybe even Banggood must do **some** vetting.

[senormechanico]

Originally Posted by 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.

Iggzactly.
And all your wants for $20 guaranteed? Good luck with that.

[sparrowhawk1]

Hmm my extension cord had a sticker on it saying rated for 20 amps. You haven't explained how holding a battery at 13.8 is going to hurt it or how taking electricity out of my house Bank to charge it is any different than running another refrigerator. Most true boaters are pioneers, how many land dwellers supply one hundred percent of their electricity with "alternative" sources? As far as cheap Chinese stuff not working I have a 40 amp mppt and a 1500 w inverter they both cost one quarter of the price of the high-quality stuff and work just fine for over 5 years. yes it's a Gamble that's why I'm asking for assistance in picking out a converter that has worked for someone. And I agree Amazon ratings are not always accurate. PS over the years I've gone through many high quality multimeters I currently own a Sperry that's giving me accuracy issues. My point is even expensive stuff fails in this rough environment.

[sparrowhawk1]

I understand the advantages of bulk absorb and float. None of the vessels I operate professionally have ever had that kind of regulator. The alternator or Magneto would put out maximum amperage until it reaches a set voltage and hold it there. the reason they do that is because it's cheaper, less complicated and works. Yes you have to add water more often. edit. over the years I've known several fellow Cruisers that have had to replace their "smart" expensive regulator.

[sparrowhawk1]

And I understand it's your opinion that holding a battery at a fixed voltage will either under or over charge so we can drop that one.[emoji48]

[noelex 77]

My thoughts:

First make sure you are producing enough power to charge the dinghy battery. If your dinghy battery is using for example 40 Ahrs a day, that is about 50 Ahrs (more to allow for some inefficiencies) a day that the mothership must be capable of generating. It does not matter if the mothership batteries are normally full, the question is will they still be full after the extra 50 Ahrs (or whatever the number is) draw.

If you have enough charging capacity in reserve, there are several methods of charging the dinghy battery from the house battery bank. The best way is a 12v battery to battery charger. These are produced by Mastervolt, Sterling and many other suppliers. Unfortunately, they are reasonably expensive. The second way is a simple cable to connect the house bank to the dinghy battery. This will not harm the house bank, but the dinghy battery will not be charged optimally. The dinghy battery will have a shorter life, especially if it an AGM type battery, but given the relatively low cost this may be a good solution. If you do this, make sure the cable is fused and has a reasonable thickness. The connectors should be capable of managing a high initial current and have low resistance. The 50A Anderson plugs would be a good choice. Don’t use cigarette lighter plugs.

[sparrowhawk1]

Okay let me go over this one more time to keep everything clear. I have a solar panel( actually I have 2 if necessary) in my dinghy with a charge controller so if I go in early and it sits in the sun all day the battery will be topped up when I come home after Dark. the battery will be at the most requiring 30 amps but usually will only need 15 which I understand means I have to put 20 back in, also I do not need to top the battery up I just need to put enough in it so that I will not discharge my battery past 50% when I go to shore.. I have a 15 amp fuse 12 gauge wire to charge from house to trolling. I plan on having the controller/ converter in the dinghy at all times so it must be cheap.( I also use the extension cord to power my 12 volt drill and spotlight) I really appreciate everyone's help but at this point the only thing I'm interested in is how to cheaply bring my trolling battery voltage up to between 13.8 and 14.2
or put a constant 2 amp current in. I don't need you to endorse it, just tell me how to do it at my own risk. Thanks again for everyone's help.

[john61ct]

OK.

When your dink batt is depleted, say to 50%

Get the highest-current charge source going through your cable into the battery, measure the amps being accepted / demanded by the batt* in the first few minutes.

The fuse hopefully preventing the cable from getting too hot*.

Let's say you measure a peak current of 12A as an example.

So buy a converter that is "nominally rated" for 30A, and it will probably work. You might need to provide more cooling via a small fan or a bigger heat sink if you notice it getting hot.

That number will increase if you put in a different battery, either going up to a higher CAR chemistry, or bigger AH capacity*. Therefore maybe requiring a higher ampacity converter.

That converter should of course be on the dink side of your wire.

As for Voltage, you should presumably use 14.4-14.8V, see the mfg specs*,

for **charging** the batt when it is depleted, then stop once it is Full, as determined by current dropping down below say half an Amp, takes around 6-7 hours.

Completely different Voltage from what is needed to **maintain after** the battery is Full aka Float.

If the battery is depleted, and you are only giving its Float V, then it will **never** get to 100% Full, and might even take many days just to get to 85-90%.

* Since you didn't provide any info to help on these specific issues, this is all on you, I am not saying any of this is safe, and again it is definitely not treating the batts properly.

[sparrowhawk1]

And I just reread the last post I have to say my experience is the voltage differential between directly connecting a fully charged house and a fully discharged( yes I try never to go lower than 50% but stuff happens) trolling battery through a 25 ft 12 gauge wire is not enough to cause high amperage flow. With the house at 13.
8( being charged with solar) and the trolling at 12.4 before charging, I saw less than 2 amps going in. I just connected with the house at 14 and the trolling motor at 13.2, less than 1 amp flow. With a meter that I tested with a fluorescent light that I've used other meters on so I know it's accurate.