Sailboat 48v + 12v System Design Question

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lahma asked Jan 4, '24

Sailboat 48v + 12v System Design Question

I'm looking for some advice regarding a 48v + 12v electrical refit for my sailboat. My design tentatively will be a 48v house bank (~400ah), and the existing 12v system will be powered by 48vDC-12vDC chargers with a single 12v-100ah LFP battery as a buffer. Solar will be in the mix here too, likely with the MPPTs connected to both banks (flexible 12v panels to the 12v system; large ridged panels to the 48v side). Alternator will be upgraded to a 48v-130a with either Wakespeed or the new Arco regulator.

My question is regarding the charging of the 12v bank from the 48v bank - what is efficient enough that it's not going to be a battery drain and is reliable. I have 2 solutions available today - 1. Victron MPPT controllers to do the DC-DC conversion, but they must have current limits below their rated output for longevity since they're not really designed for this. 2. The new Mastervolt 48/12-50a battery charger, with the downside being it's twice as expensive as Victron MPPTs and no integration with Victron monitoring (Cerbo, etc). 3. Just to cover my bases here, I am aware of the Victron 48/12-30a DC-DC *converter*, but this is not a battery *charger* as they have no 3-stage charging algorithm (so it's not really going to work properly with a 12v LFP battery) - this is a mysterious gap in Victrons product lineup, I would think they would want to promote 48v mobile systems.

I'm trying to figure out if either of these charging solutions will be an annoying constant power drain on the 48v bank. When the chargers are in Float mode, what sort of power consumption are they utilizing? When running small 12v loads (LED lights, 12v fridge, etc) will loads be pulling amps from the chargers or the 12v LFP? When running a large 12v load (windlass, electric winch) that will require some of the load to pull off the battery, will the chargers also feed what they can to this load, or do they only kick on once the battery voltage sags enough?

Thanks!

48v battery 48volt charging

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pwfarnell commented · Jan 04 at 11:11 AM

Do you keep your inverter on all the time, if so a small ac charger could be used to keep your 12V batteries topped up. I am not sure about the efficiency compared to the DC to DC conversion route or the idle current, but just a thought.

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lahma pwfarnell commented · Jan 04 at 06:29 PM

I'll probably use this option as a cold standby backup. The problem I have with it under normal circumstances is the inverter is another point of failure in the 12v charging chain. If it goes out, no more 12v charging (other than solar), which isn't viable for an ocean crossing vessel.

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Answer 1 · 2024-01-04T08:00:40Z

JohnC answered · Jan 04 at 08:00 AM

Hi @Lahma

I have an all-Victron system like this on my boat, and it works very well.

I started off with an Orion 48/12-20 *converter* to serve 12v loads directly, now mostly lighting as I retired most 12V appliances in favour of ac. Then I converted it to charging some old 12V batts at a flat ~13.8V. Not all the time though, it's Remote Switched in (using a string of Assistants in a Multiplus) when the 48V bank is above a set V and SOC (so when the solar is working and in Absorb state), OR when the genset is running. Manual override possible with switches, but rarely used, only when a heavy 12V load is expected.

The old 12V fla batts have held up for maybe 5 years longer than I expected. And the last place I'd spend big money on LFP is in an application like that. But that's just me.

The thing that eliminates most of the issues you're concerned about is the switching. This is mine today, a by-difference plot (the Orion doesn't talk to the system)..

My main is 225Ah fla @48V, the buffer 450Ah @12V (but very old & degraded).

Hope this helps. You're not alone. Choose well..

1704354604062.png (9.2 KiB)

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Answer 2 · 2024-01-04T08:49:06Z

lahma answered · Jan 04 at 08:49 AM

Thanks John, interesting setup you have there. Yea my planned 12v LFP 'buffer' (the about-to-be released Epoch 12v/120ah that is marketed also as starter capable) is a bit overkill, but my design decision behind this is I want a comfortable amount of capacity to run 12v loads for a period if the whole 48v bank goes down (critical navigation electronics, VHF radio, etc). When Epoch announced this new LFP that can handle large surges to 800amps, I figure might as well take advantage of LFP benefits for the buffer too - not that much more $$ in the grand scheme of this project.

I have seen mention of your method as well, using a converter to 'flat charge' the buffer battery, but I worry about the robustness of this on a LFP battery and just in general it seems like a bad idea going against mfg charge recommendations. Hard to dispute your success with it thus far though! The power utilization graph looks pretty normal, at least compared to my boat's 12v usage, and I assume this data is including the loss in those DC-DC converters which isn't tiny at 87% efficiency. Thanks again for the info!

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