DC/DC - 48v to 24v conversion + buffer battery?

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hoeken asked Oct 5, '20

DC/DC - 48v to 24v conversion + buffer battery?

I am in the process of spec'ing out the power electronics for a large sailboat. For various reasons, it will have a 48v LiFePo4 bank where the majority of charging sources come in, as well as the inverter/charger, and then a 24v buffer battery where most of the DC loads will draw from. It looks like the total 24v DC power usage will be around 10kwh per day, but with loads like electric winches or windlass potentially spiking temporarily to 3.6kw (~150A)

My first question is this - what is the proper way to handle this conversion? The only 48/24v dc/dc converters I see on the site are the 'dumb' Orion DC/DC chargers that go up to 400W. It looks like we'll need about 500W/hr of charging to keep up with demands, and an extra converter for redundancy (absolutely cannot have this fail at sea!) So it looks like 3 x Orion 48/24v converters in parallel?

Secondly - the buffer battery will be a Victron 24v 200ah LiFePo4 smart battery. The DC/DC converters seem to be configured to just output a steady voltage. Does this buffer battery need to be supplied with a 'smart' charger to give it exactly the right voltage to fully charge, or can the converters be set to a safe voltage and just keep it mostly charged?

Orion DC-DC Converters not smart 48v battery

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Answer 1 · 2021-06-13T14:08:37Z

hoeken answered · Jun 13, 2021 at 02:08 PM

Its been some months now, anyone have any comments? @Guy Stewart (Victron Community Manager)

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Answer 2 · 2021-06-13T14:53:22Z

rona answered · Jun 13, 2021 at 02:53 PM

You could look at wakespeed ws 3000 talk to Al .

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hoeken commented · Jun 14, 2021 at 10:36 PM

That is very interesting. Im meeting with our electrical engineer tomorrow to go over all this and will bring this up if they dont have a solution. 3000w is a bit much, i really just need 2-3 units @ 400W for redundancy.

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Answer 3 · 2021-06-13T16:11:45Z

Vance Mitchell answered · Jun 13, 2021 at 04:11 PM

Out of curiosity, what charge sources do you have for the 48v battery?

It might be possible to redirect some of the charge directly to the 24v system.

For example if you if you had 4 x 150/35 MPPT controllers it might be worth splitting some of your panels into a fifth 150/35 to assist the 24v.

This would help with redundancy as if one fails you have options.

If you are using a generator, then you might be best with a charger or inverter/charger which would give another option to supply AC power if the 48v system ever fails.

If you can tell us more about your charge sources it will be easier to suggest options.

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hoeken commented · Jun 14, 2021 at 10:31 PM

The reason for the 48v system is driven by using a dual Integrel (9kw alternator) setup, one on each motor. Want to get that sweet sweet max charge rate without installing a generator.

We will also have 9 x 400w solar panels with individual MPPT controllers that will feed into the 48v bank, but could also be connected to the 24v bank. (thats one of my oh shit scenario plans)

As designed, everything is charging the 48v bank with some 48v to 24v converters to the 24v bus, and a 24v battery on that bus for spike loads like motors (like if we cant source 48v winches or a windlass)

The 24v buffer battery should really be sitting mostly full it's entire life, and only used for peak loads the converters cant handle. That and a backup battery to give us time if that 48-24v conversion link fails.

My main question is how important would it be to get a 3 stage charger on there vs just using the dc to dc chargers set to a safe voltage to keep it at 80-90% SoC.

Seems like there might be some gaps in the 48v range of intelligent battery to battery chargers.

If im missing something obvious here, please let me know. Just seems weird to provide a static voltage to the battery and whole 24v bus.

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Answer 4 · 2022-03-21T03:28:53Z

jimthesoundman answered · Mar 21, 2022 at 03:28 AM

If you have 4 twelve volt batteries in series, that would give you 48 volts for your charge setup. Then you could have a switch to disconnect that charge setup and change it over from 4 twelves in series, to two twelves in series, twice. Then if those are in parallel, you could have double the amps, at half the voltage which would work out for your heavy spike loads.

But if you needed to charge at 48, and discharge at 24, that is also possible, as long as your charge setup is four twelves or two 24's. You would just keep the four twelves in series for charging, and then tap off of two of them for the 24 volt loads. There are many different ways you could set it up to come to the same result, some would give you more amperage but also be more of a hassle, some would be much less amperage and be much less of a hassle.

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Answer 5 · 2022-03-21T04:19:58Z

Kevin Windrem answered · Mar 21, 2022 at 04:19 AM

Would it make sense to use a larger DC-DC converter and not have a 24 volt battery at all? You could split your 24 volt loads over 2-3 48 to 24 volt converters, each able to handle the peak current of their loads. Might be less expensive.

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Answer 6 · 2022-03-21T06:14:32Z

bathnm answered · Mar 21, 2022 at 06:14 AM

Take a look at Safiery and their Scotty. Depending on the type of sailing you do I would personally keep a 24v buffer battery just in case. That way if the lithium has a problem you still have some power for critical loads.

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