DC - DC Charger Placement in Sailboat LFP setup

jc1409

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I've spent many hours learning and scouring this great forum and the web, but still puzzling over best setup for my sailboat solar/LFP upgrade i'm planning early spring:

Here's is a list of my planned equipment - all 12v:
- Single diesel engine, 125A 'dumb' alternator
- 1 110A AGM starter battery
- 2 60A AGM bow thruster batteries
- 6 100A LFP house bank batteries (new)
- 1,155W solar via Victron 150v/100 MPPT (new)
- 2 40A battery chargers (with LIFEPO4 charge settings)
- 1 Battery isolator with 1 input (alternator) and 3 outputs
- 2000w inverter

I'm adding the LFP house bank (replacing AGM bank) and adding the solar, and trying to use as much of my current system (chargers/isolator) as possible. The solar and house battery bank are sized based on our usage plans.

My plan to bring the system together:
- Keep the Isolator, with the alternator feeding into it and passing on charge to the (1) starter battery and (2) the bow thruster battery.
- Solar MPPT charges the LFP bank
- The 2 existing 40A battery chargers are used to charge the LFP bank

My main struggles to figure this out how to keep the Bow Thruster and Starter AGM batteries topped up and 100% charged. Options:
  1. Instead of placing both 40A chargers to the LFP bank, use one of the 40A chargers to charge the Bow Thruster and Starter batteries, and add a DC - DC charger to take the excess to the LFP bank. This seems overkill and inefficient as I would expect the DC-DC charger to be in heavy use (while plugged into shore power) because much of the current from the charger would not be needed by the bow thruster/starter batteries, and be passed on to the LFP bank. The bow thruster/starter batteries need to be topped up and ready all the time, but only get relatively limited use in boating operations.
  2. Keep both 40A chargers charging the LFP bank, and add a DC - DC charger, charging from the LFP bank over to the Bow Thruster/Starter batteries to keep them topped up. This seems like most efficient.
    • If I did this, could I place the output of the DC-DC charger (from the LFP bank) onto the battery isolator input (sharing that input with the Alternator), so that the charge current gets split between the bow thruster and starter batteries? I don't think so as I have a feeling the output from the DC - DC charger may cause risk to the alternator, but this would make things easier if possible.
    • If not, what's the easiest way to 'split' the DC - DC charger output between the bow thruster and starter batteries? Would like to avoid adding another battery isolator / piece of equipment if possible.
Thanks
Jeff
 

jc1409

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Sorry now realized I should have possibly placed this in Marine, but was also thinking it was a pretty general question. I can't find the ability to move the thread from the edit feature.
 

Shinebox

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I use a DC-DC charger in the manner that you describe to charge my starter battery. Most folks seem to go through the LA battery for use as a capacitor of sorts and then the dc-dc charger to the LFP bank. In my setup, I have my alternator controlled by a wakespeed regulator that is itself controlled via CANbus by a REC active bms. As I said, the starter (a firefly oasis group 32) is charged via a victron non isolated dc-dc charger, which is set to only run when the input voltage is above a certain point… I.e when the engine is running. This has worked quite well for me.
 

jc1409

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Thanks Shinebox - sounds like you have a nice setup.
I'm still struggling over how to set this up some. Part of my plan was to keep my 'dumb' alternator charging the start/bow thruster batteries. My question then is how do I 'supplement' charge to the start/bow thruster batteries above what the alternator supplies, to keep them topped up.
- put one of my 40A chargers to the start/BT batteries, and then use DC-DC charger to send excess current to LFP bank? DC-DC would switch on when alternator running and when charger is running from shore power.
- put both 40A chargers to LFP bank, and then use DC-DC to top up start/BT batteries. Based on input voltage trigger, I guess the DC-DC would kick on in various scenarios to charge the start battery (shore power, solar/mppt), but would then turn off once start battery full?

I wouldn't expect much current needed to keep those batteries topped up, so leaning towards second option.
 
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jc1409

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Each charging source (MPPT and the two 40ah chargers) use specific LIFEPO4 charging configuration
 

jc1409

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Here's the charging setup:

The LFP batteries get charged from:
- solar (100a MPPT charger from 1155 watts of solar)
- Two 40a chargers powered by Shore Power

The starter and bow thruster batteries get charged from:
- 125a alternator

I could either add a DC-DC charger from the LFP batteries to the starter/bow thruster batteries, to keep them topped up.

Or, I could move one of the 40a chargers (shore power) to charge the starter/bow thruster batteries, and if I did this I would place a DC - DC charger from the starter/BT batteries to the LFP batteries, to take the excess from the 40a charger to the LFP batteries.
 

Shinebox

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Got it. I was turned around with you not having any alternator charging for your LFP bank. If your thruster batteries only get limited use, I imagine your alternator hookup should be more than sufficient. You could always place a dc-dc charger into the system and use it as you needed to top up the batteries. I’m assuming that your engine is running when you’re using the thruster, and if that’s the case, those batteries are getting juice from the alternator. Again, with normal thruster use (short deliberate bursts) you won’t be drawing down those batteries much.
 

jc1409

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Thanks Shinebox - makes sense, and you are right about short use of thrusters and only when engine is running. I think I'm chasing a non-issue with this, worried about the possibility that those batteries don't get fully topped up from engine use (or drain after sitting for period of time without use - anchor, dock, etc.). Your idea of Dc-DC to top them up from the LFP seems easiest.
 

Shinebox

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I honestly wouldn't bother with a DC-DC charger in this setup. You will not be discharging those batteries appreciably whenever you use the thruster (unless you're one of those people constantly hammering on it.... which will destroy your thruster anyways), and a 120 amp alternator should take care car of any deficit pretty quickly.
 

jc1409

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Makes sense - I may implement without a DC-DC charger and do some close monitoring to see if I need to add one. If I added the DC-DC charger in this way, I would also need to add something to split the charge between starter and bow thruster battery (an isolator?) - any easy ways to split charge current from dc-dc like that?

I know the draw from the thruster is pretty high - but I'm pretty light with the bow thruster, unless some kind of against the wind maneuver is needed in close quarters - have twin rudders and no usable prop wash and limited prop walk. On a longer cruise this would be fine because would expect to be using engine more, but for normal 'day sailing' usage, would probably end up with deficit with only ~ 20-30 minutes of motor use max.
 

Shinebox

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20-30 minutes of engine use is going to be more than enough to keep those batteries happy.

If you were to put in the dc to dc charger and want to split the charge between your starter and your thruster batteries, the simplest way would be to have a switch where you would select what you wanted the dc-dc charger to top up. Again, much like your thruster (even more so in fact) your starter battery, despite having to put out +/-100 amps per start is actually drawing down the battery a tiny amount. Your alternator, despite being 120 amps, has likely never had to deliver that kind of current.

What kind of boat are you sailing?
 

jc1409

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It's a Beneteau Oceanis 46.1.
Another thought leading me in the opposite direction and would be interested in any thougts - this setup only let's me charge my house bank by solar or shore power - not from the engine.
If out on extended cruise without good sun (and no shore power), I haven't left myself any options to charge the house bank. Option 1 from my first post gives me ability to use engine to charge house bank (via DC-DC charger). No generator on board.

Much of this I know is just based on preference for how I want charging to work. I assume many have 3 different battery banks with one Lithium - surprised there isn't more of a standard approach to this, but maybe not so common. thanks
 

Shinebox

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Most people have a house bank and a starter battery. Some boats have batteries dedicated for a windlass or thruster as you do, but there is debate about this. Yes, you save on running a long length of thick cable, but instead you’ve put a heavy battery right where you really don’t want it… all the way in the bow.

I’m ok with running heavier gauge wire, as I’m against putting weight into the ends if I can avoid it. This makes a big difference performance wise, and also with sea keeping ability when it starts to get choppy. System complexity is also reduced. That’s just my preference, but I will say it comes from a background in yacht design, building and extensive racing and cruising.

As for charging your house bank with some other means beyond shore power or solar, the DC-DC charger is certainly route, and it seems to be the method most people use, mainly as a way to protect the alternator from killing itself in the event that the BMS shuts off the charging when it detects a problem with the bank. You would need to parallel Several of them to get anything close to the full rated output of your alternator during bulk charging.

In my system, which I mentioned before, I use the DC-DC charger just to float the starter battery. I can tell from the victron app that it’s never putting out anything more than an amp or two, as that battery never gets discharged. For protecting alternator, I use a Sterling alternator protection device:



If you were to go this route, you would want a decent alternator regulator. Balmar MC14 and the Wakespeed are both programmable regulators that can be set to charge LFP banks. The Wakespeed has several nice features that the Balmar lacks, including CANBus support 0 current output and RPM input, which is handy for tweaking your output to match engine RPM which can reduce mechanical wear and tear.
 

wholybee

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I am generally in favor of all or as many charging sources as possible going directly to the house bank, and charging other batteries from there. As mentioned by Shinebox an external alternator regulator is best for this. I'd pass on the balmar and get the wakespeed. Some people do make do with an internal regulator, and some leave the alternator on the start battery.

However you do that, for charging the remaining batteries. You have the isolator, which is probably almost enough. Try it and see, the fully charged and resting voltage of lfp is about the same as float voltage of the lead. So they would always be floating. You could (and probably should) put a dcdc charger between the lfp bank and the isolator if the isolator alone doesn't work as you want.
 

jc1409

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Thanks for all the help everyone - It's not how I would set it up from scratch, but here's what I've decided in order to keep changes to existing system/wiring to a minimum:
- using existing isolator (which currently splits alternator 3-ways to: house bank, starter, and bow thruster (all AGM now)
- my current setup includes 2 40a chargers that are connected to the output terminals on the isolator for all 3 battery banks
- when I replace the AGM house bank with Lithium, I am simply going to add a DC-DC charger between the lithium bank and the isolator output to the house bank. This way, I get the alternator and charger to the Lithiums (through the DC-DC), and I keep the alternator and chargers being split also between the starter battery and bow thruster battery

1640284560408.png
 

Browneye

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Nice boat, I like the Beneteaus. I sold a 40' John Alden yawl, all wood, when we left the PNW 25 years ago. Wow.

With a similar setup in my RV, obviously no second bank for the bow-thruster, but house lfp and fla starter battery yes, I added a single 100w solar panel and controller just for the starting back. The alternator keeps them charged when used, and the panel maintains them.

The starter/chassis batts run all of the 'to-go' electrics - power step and awning, slide-out motors, leveling jacks. My guess on the original concept was you start the engine and then run all the power items in preparation for travel. The house side runs all the lighting, furnace blower, fridge board, water heater controller, water pump, and new inverter.

Note that a 2000W inverter could draw 200A from your house batteries if fully utilized. That's a lot of 12V energy!
 

jc1409

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Wow - talk about nice bluewater boats! I've only seen them in pictures and read on internet, but beautiful!

I love the Beneteau and seems like I have been in electrical upgrade 'planning' mode for quite awhile. I'm wiring for the max loads, but in reality would not expect to get near full utilization on the inverter. Inverter powers only outlets now, and microwave would be our big power draw.
My growing interest in all things electrical (due largely to this site...) is leading me to consider replacing current Mastervolt 2000w inverter for a Multiplus ii 3000. I've got 50a shorepower input, and may even consider adding a 'slow start' device to one of my aircons and give me some ability to run that for a short time, but that's a vague future project now.

I've changed my current setup idea from the above based on all the input I received, and am now planning to (a) move the two 40a chargers to charge direclty to my LFP bank, and (b) adding a 10a dual output shore power charger, to maintain my start/bow thruster battery. Lots of input said I would not need to maintain them and that the alternator would be enough, but I'm always thinking worst case I guess. Also, didn't like the idea of relying so much on the DCDC to pass through the shore power charging current to my LFPs (heat/efficiency/etc.).
thanks
Jeff

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