Brainstorming about adding a 48V house bank

[codingparadox]

It looks like we're purchasing a boat with a largely untouched stock electrical system from 2008, so it's a pretty blank slate for us to work on. We're fulltime great loop liveaboard tech workers, and go through lots of daily power with computers and monitors. The boat has around a 1000Ah AGM 12V bank, a small inverter, a dumb AC-DC charger, ACRs, a tiny solar setup, etc. Standard 2008 setup.

I basically have two options to proceed. I was originally leaning toward doing something similar to Slowboat's recent post (https://slowboat.com/2020/12/airships-new-power-system/), involving dual 5KVA Quattro inverter/chargers, adding in a pile of solar. When I looked into everything, though, I was going to need at least 2 and possibly 3 solar charge controllers, since with >3000W of solar on the roof, at 12V, you theoretically need over the top of the 250/100 Victron charge controllers. So your setup ends up being two 3k$ Quattros and 2x big solar charge controllers, and a whole pile of 4/0 cables to keep it fed.

But the more I thought about it and looked into what's available, the more I started wondering if it actually made sense to bring up a second 48V setup. It means you end up only needing a single 10KVA Quattro, a single charge controller, much smaller cables, and lots of the 48V Lithium battery banks are significantly cheaper and smaller form factors than getting a ton of 12V battery packs, for the same KWH capacity. So I started drawing up what I'd need to do, and, well, it's actually really not that bad, and you end up with a ton of advantages. Except for one piece -- the charger setup to get from the 48V primary bank to the 12V "daily small loads" bank.

I've attached a screenshot of my preliminary diagram to show what I'm thinking about. I was assuming there would be some easy high-efficiency 48V->12V 3-stage chargers, but I actually can't find a single thing. There's fixed voltage converters like the Orion and a bunch of others, but they'll all use a bunch of extra power basically attempting to float charge the 12V bank, which isn't what we want.

So, I'm wondering if anyone here has any other ideas for how to efficiently get power from 48V to 12V. I could obviously do crappy things like run 120VAC->12VDC chargers through the inverter, but that's a bunch of different pipeline stages converting battery power to heat. I could obviously use 12VDC->DC chargers, or just ACRs, from the 12V bank to the port/starboard/genset batteries as well, but that doesn't solve that first main link problem.

[GoneDiving]

Have you considered using a programmable 12v solar charger? Input is your 48v bank rather than panels. Programmable so that you can set the max 48v draw and 12v charge rates.

Keep us updated as I'm also fitting a 48v house system:

48v 840ah Lifepo4 house bank
24v 200ah start bank
20x250w solar panels to 48v house
2x200w solar panels to 24v start
48v to 12v 60a dcdc converter
48v to 24v 30a dcdc converter
4kva single phase generator
2x5kw single phase, 60a solar charge, 80a utility charge, 48v inverters
240v AC to 12/24v 10a DC charger

Cheers

[codingparadox]

That's an interesting idea!

[RaymondR]

Originally Posted by codingparadox

That's an interesting idea!

It's not only interesting, It's an excellent idea.

I've been a fan of an increase in the standard voltage on boats for years. Happened on my previous boat when the cable conduits all started to fill up. The other scheme I dreamed up was to run separate house and nav ring mains and switch with MOSFETS at the appliance using a multi strand data cable.

I am installing a completely separate 24 volt system to run my fridges on since the compressors are 12V/24V and I suspect the electronic control units will run cooler on 24V. I'm rebuilding as eutectic and will be running the compressors all day to better exploit the solar power.

[codingparadox]

Yeah, I actually emailed Victron earlier about it to see if they could think of any reason why it wouldn't work. I'm mostly wondering if the MPPT aspect is going to have it trying to hunt for the maximum power point, when really we just want it using what it's handed without thinking about it.

I'm still waiting for someone to productize a plug-and-play big-fat-power-cable with MOSFETs/relays controlled by a tiny canbus that you can just run as a backbone down the boat for all of your power needs.

[smac999]

Originally Posted by codingparadox

I'm still waiting for someone to productize a plug-and-play big-fat-power-cable with MOSFETs/relays controlled by a tiny canbus that you can just run as a backbone down the boat for all of your power needs.

lots of those out there. c zone is probbaly the current common one.

why do you need 10,000w of inverter?.. that is massive.

most boats are fine with 2-3000w

also not sure why the 1000ah 12v bank. 100 would likly be fine. as long as the dc to dc could handle most loads. it's just a tempory buffer for the windlass and large draws. make the 48v one big enough to handle all storage needs.

the only advantage of the large 12v bank is if / when the dc to dc fails you still have power for a while. you'd have to carry a spare dc or dc. or run 2 in parallel for redundancy.

if this is a 120/240v boat you'll also need an autotransformer after the quattro.

I put a 5k quattro and 100a auto-transformer on a boat a few weeks ago and it worked out well.

[codingparadox]

The idea is to run 100% of AC loads off the inverter, so I can take advantage of PowerAssist to manage everything. Also run the water heater and some AC off batteries and not have to disturb everyone in the anchorage with a generator.

I realize this is not what others are doing. I'm okay with that.

[GoneDiving]

Originally Posted by codingparadox

The idea is to run 100% of AC loads off the inverter, so I can take advantage of PowerAssist to manage everything. Also run the water heater and some AC off batteries and not have to disturb everyone in the anchorage with a generator.

I realize this is not what others are doing. I'm okay with that.

Others also have to allow for motor in rush currents, prefer not to run inverters at full load, like the idea of having an additional redundant unit, have admirals who won't hear of playing tetris with electrical loads, have power hungry hobbies or have a pathological hatred of petrol/gas powered scuba compressors.

Of course, this is all hypothetical as none of the above apply to me. 😎

[codingparadox]

Yeah, I was actually thinking that I might use this as an excuse to pick up dive gear, since the new boat has _plenty_ of space to store it, and spend some time getting some certs. 48V makes a lot of these things that are usually prohibitively high load suddenly tractable. I'm pretty excited about the flexibility it's gonna give me over the coming years for projects.

[codingparadox]

Oh and I just saw the edit with more questions about the 12V bank.

The boat already has the 1000 Ah 12V bank. Once I prove out the 48V system I'd probably sell off/recycle most of that bank, and replace it all with a single battery like a high-current start battery.

[CarlF]

I think the 48v is a great idea but you lose a lot of efficiency in your plan converting voltages and going from one battery bank to another. I would charge the 12V directly - maybe use one alternator and 1/3rd of the solar panels. For the 1/3rd of the solar panels still wire the solar panels as 48v so you can use smaller wire gauge but have the solar controller then output be 12v.

Keep a 48-12 charger but use it only to balance rather than for all charging of the 12v bank.

If you expect to use your genset a lot have both a 12v and 48v charger connected to it.

Also have the 12v starting batteries charge from the main 12v banks with a simple VSR bank combiner. Since the start batteries are always mostly charged they don't need perfect charging technique.

[codingparadox]

Oh yeah I didn't update the chart after I realized the 4 DC-DC charger idea was stupid. Attached the updated schematic.

So it's 98% efficient to translate from the 48V bank to the 12V bank, minus another few percent for the AGM replenish inefficiency. But most of the time it'll be live-refilling the current being drawn by the house, so it'll just be the 98% efficiency swap. But much of those loads won't be terrible over time. Fridges and such can be AC powered, so it'll come right from the 96% efficiency inversion out of the 48V bank.

I'm probably going to leave at least 1 of the 12V alternators on the engines and add the 48V one onto it, so I have a 12V backup for absolute emergencies, but it shouldn't be used unless the 48V setup goes down for some awful reason.

The key to keep in mind here is that with how fast the charging is on the 48V bank, the few percent inefficiency is overwhelmed by how much faster and more efficiently I can charge all the banks with the 48V charging, and how much more efficiently the genset, main engine alternators, and solar can be converted and stored.

[smac999]

I never put ACR's on gen batteries. as then you end up with the little gen alternator trying to charge the house bank. let the AC chargers do that not the gen alt. just give it a little 5-10a ac charger to keep it maintained and leave it separate.

you'd almost want a 12-48v charger to charge the 48v bank from the engines too?..

[GoneDiving]

Originally Posted by codingparadox

So it's 98% efficient to translate from the 48V bank to the 12V bank, minus another few percent for the AGM replenish inefficiency. But most of the time it'll be live-refilling the current being drawn by the house, so it'll just be the 98% efficiency swap. But much of those loads won't be terrible over time. Fridges and such can be AC powered, so it'll come right from the 96% efficiency inversion out of the 48V bank

There are numerous inefficiencies to consider.

I'd regard round trip lead acid (dis)charging as the most inefficient. That's why I intend to use this only for starting and have a direct dc to dc charge path (predominantly solar).

DC to AC phase change (and wide voltage increase) is also inefficient so I'll only use it for power hungry or AC only equipment. I'd expect 48vdc to 230vac to be more efficient that from 12vdc.

I'd expect dc to dc converters and lithium round trip to be the most efficient so will power most of the continuous and low draw (but still high daily ah) equipment. I like the redundancy of having a large dividable bank rather than multiple banks of different voltages.

I currently have 415v 3 phase, 230v single phase, 24vdc and 12vdc onboard. I'm actively trying to standardise to 240vac and 12vdc loads with the conversion being via 48vdc.

Of course, I do have wildly inefficient paths too (48vdc to 230vac to 24vdc battery charger) but these are only intended as backup supplies. Famous last words!!

One thing I do enjoy about lithium is that it is very easy to engineer out perceived problems. With the power density per unit size, weight and now cost, it's a simple matter to size a system so that cloudy days, consumption spikes or cell degradation are not even noticed.

I converted a dive scooter to 50v lithium and I'm never going to use the 9 hour run time but "one more quick dive" for the day, no time for a recharge, 10 year old cells haven't even been noticed. 😉

[codingparadox]

If you have the dc-dc converter going full time on the 48->12v bank, it's going to produce the current that your 12V house loads actually use, and it won't pull it out of the larger-resistance path from the (lead acid) batteries. So your 12V batteries will just stay full unless you briefly pull more current from it than the converter can supply (windlass, etc.), then will charge back up and be full again, while the ongoing load pulls off the "charge" current.

So in theory, with the 98% efficient dc-dc load, you should only be getting roughly 2% + epsilon loss between the 48V bank and your house loads, virtually all of the time.