Get radical by getting simple: a 32kWh LFP house energy system

[nebster]

Hey Folks,

After lots of lurking and now some occasional arguing posting in other threads, I've decided it's a good time to start my own thread documenting my own house power system.

I have benefited a lot from the experience of other cruisers posting here, plus of course other cruisers' postings on some of their own sites, plus a lot of information gleaned from other venues and sources. My own take on how to build a large mobile ESS (energy storage system) is somewhat different from others, and the combination of the scale, recency, and weird choices applied to my installation surely makes it unique. And maybe I am crazy, but you can argue post below and we can sort that out down thread.

I am going to spread out the description over several posts that I'll make over several days, so please read beyond this OP, and bear with me.

Let's start with...

Background

I moved into my new, old RV full time this spring. In preparation for doing that, I renovated the interior of it and redesigned the whole ESS and power delivery system.

Why am I even posting this here on a boat forum, then, you might ask?

First of all, there's immense overlap in functionality and design consideration. My RV has most of the same charge sources (solar, shore, diesel engine). We spend long periods of time off-grid. Much more so than not, the fundamentals are the same. I'll try to elaborate on some of the areas where I see differences, though, when we get into the details, below.

Second of all, you guys are much more DIY-inclined and self-sufficient, out of necessity if not also desire, than the RV community. The latter is much larger, but there is more motivation and a long history of pushing the boundaries here.

Third of all, we are planning to sail away on a cat in a handful of years as well, and so this installation is like a beta test for the next one.

Anyway, I began the design process in the summer of 2017, used last winter to bench test and validate various aspects of the system in my garage (yes, a great luxury that many of you don't have, I know), and installed everything this March. As of today, I have just about five months of "production uptime," and I'm happy to report that there have as yet been no unplanned outages.

Motivation

This is our second full-time RV, and we have an indefinite timeline to be on the road, so I wanted to make a substantial investment in getting a system that would provide for our "luxury camping" lifestyle for years to come. I also had a break between jobs, I am a software guy in real life who cherishes any chance to be more hands-on, and I decided it would be a fun hobby.

But, also, equally seriously: there aren't very many package options out there at the scale I was shooting for, and although I did find a couple promising ones, if the 3x cost savings wasn't enough to scare me straight, there were usually other compromises that further pointed me towards just going full on DIY.

The final bit of motivation ties back to moving onto the boat one day: I want to know exactly how everything works before we are far afield and I am left to my own devices.

[continued...]

[nebster]

System Overview

The heart of this system is a ~32kWh, 52VDC-nominal LiFePO4 battery bank. The bank is inverted up to 240VAC single phase by an inverter/charger and then split into traditional USA "split-phase" with an autotransformer, yielding two 120VAC legs 180 degrees out of phase.

From there, house line-level distribution proceeds through traditional residential panels with standard circuit breakers, in-wall wiring, and so on. We have both 120VAC and 240VAC loads in the RV. The 240VAC loads include a microwave, a clothes dryer, and a water heater that provides continuous domestic hot water as well as hydronic radiator heating.

We also have a substantial set of 12VDC house loads. These are powered via a pair of redundant 1500W AC-DC converters.

We have a baseline demand of roughly 300W. The two largest baseline consumers are the inverter itself and*our fairly old and inefficient residential refrigerator.

We have 1600W of rigid solar panels on our roof, ganged 4s4p for a nominal 80VDC feed to the basement charge controller.

We have a shore supply cord that allows us to access the traditional 240VAC/50A 12kW power pole common in many RV parks. Our inverter/charger is capable of charging with 200A at 52V, or roughly 10kW.

Because we also frequent locations with only a basic 120VAC supply, we have a separate, smaller charger capable of delivering 35A at 52V.

Finally, we have a 20kW diesel genset that also provides 240VAC.

Our inverter provides up to 15kVA continuous power and can spike to 30kVA. The remainder of the system is engineered for battery delivery of 15kW continuous and 30kW for 5 seconds. In practice, we rarely exceed 6kW continuous, although I have verified 14kW for a period of several hours a few times during testing.

[nebster]

10 (Maybe) Radical Things

Here is a list of some of the crazy/stupid things I have done, that maybe you should think about doing, too!

Rationale will be provided later; I'll take votes now for which one to provide first.

1) Cells procured from a no-name small-scale LFP manufacturer in China, directly, via Skype IM and video calls. Funds wired in advance. Cells shipped straight to me, via an airplane. Not very expensive!

2) Cells are a new aluminum-shell design with slightly conductive metal skin (!) and a mostly unknown track record.

3) Battery pack is comprised of cells in series first, and then those strings placed in parallel. OMG.

4) No "BMS", no automated balancing. Heck, no balancing planned at all, unless something weird happens.

5) No top-balancing. Bottom-balance! Yes, even for an off-grid partial-SOC multi-charger scenario. Crazy, right?

6) Only one discharge source (the inverter). Nobody else tapped into the DC bank directly! All discharge mediated through a single device.

7) Roundtrip DC-AC-DC for 12VDC house loads. (That must be so inefficient...)

8) No charging to 100% SOC. Not even close. Get over it. 92% is the new 100%.

9) Autotransformer, creating a derived neutral, in a moving vehicle. The NEC will be very unhappy with me, if they ever find me. I can't wait to find out what ABYC thinks!

10) Solar panels completely not ground-referenced to anything. Floating 80VDC. Battery pack completely not ground-referenced to anything. A separate, floating 52VDC circuit. Horrors upon horrors.

Okay, if that's not enough to whet your appetite, I'm sure I'll come up with some more later.

Before I go, seriously: all of these have been pretty carefully considered, of course. And I know some of these will ring at least somewhat practical with many of you, based on seeing your comments on other posts. I'm being a trifle tongue-in-cheek. But I do think it's important to challenge conventional wisdom on some of the topics, because it feels to me like it's holding people back or driving people to overcomplicate or overbuy their solutions.

[nebster]

Components

Here's a high level BOM for the project. Total project cost (so far) is about $25k, including $6k of RV prep necessary to retrofit the new system, plus my time valued at precisely $0/hour. Hey, you get what you pay for.

Cells: 96, 100Ah nominal LFP aluminum-shell prismatics. Actual initial capacity was 105-107Ah. 112 cells ordered, to provide for spares and/or a sacrificial 16s1p test string. (Turns out I didn't nuke the test string, but there were 2 cells that were basically DOA. I am carrying 4 spare cells on board.)

Interconnects: mostly Bluesea busses and fuses for 52VDC. 8 AWG to 4/0 AWG welding wire, because the DC wiring runs in my RV are not subject to oil, greases, or moisture. (Not acceptable in a marine environment, but oh so much easier to work with on land.) FTZ heavy duty lugs crimped on, always.

Protection: Bluesea MRBF fuses. Gigavac contactors for emergency and maintenance disconnect, at string-level and pack-level. Both sides protected in both floated DC systems. Thermal fuses for high temp disconnect (77C).

Chargers: Victron Quattro. One 48/15000/240V, one 48/3000/120V. Victron MPPT for PV.

Inverted supply: above 15KVA Quattro, plus Victron autotransformer 100A/32A.

Thermal management: numerous thermostatic cooling fans, dedicated HVAC feed to insulated battery storage bay, cell-level thermal fusing, bluetooth thermal probes and networked monitoring system.

Solar: Renogy 100W rigid panels, roof offset mounts/spacers, MC4 wiring. All probably irrelevant for a boat, but these panels fit better on a fragmented RV roof.

Control: Victron Venus for overall system monitoring and most control aspects. Interface relays for genset autostart. Victron BMV for coulomb counting/SOC estimation. Zeva BMM v2, plus custom firmware, for cell monitoring and LVD/HVD.

[offpist]

Can we see some pictures on these aluminum-shell cells?
How do you plan to isolate them from each-other?

I have some experience with Lithium-Ion cells from VW E-UP and they are also aluminum-shell but covered in shrink plastic from the factory for isolation.

Once I tried to make an 14S battery by removing shrink plastic and gluing all cells together in series using Sikaflex.. did not go to well. The glue conducted current and the cells gassed inside making them double size within a few days..!

[SailingSisu]

Love all of your setup!

Just ordered my solar + LFP set up. Also Victron, but including Victron batteries, which is balancing. I have flown racing drones before using similar LiPO batteries and they did need balancing hence, me going that route. Would be great to hear from you about this balancing issue!

How much did your batteries weigh?

[upston]

This sounds fantastic, i am sure the " That will never work crowd " is gasping for air. Please elaborate on the lack of BMS. It is expensive but we are told necessary on L batteries ??

I also have a floating ground, no ground and it seems to be fine, please elaborate on that as well.

Finaly why do you need so much power, what the ell are you running

[john61ct]

Source of the cells please.

[Dadrock33]

Following . . . . this could get interesting . . . if for no other reasons than some of the concepts (and maybe some of the responses!) could become "normal" . . . who knows? I'll await a bit more presentation and some discussion before I think we'll see some interesting "critiques" of your system; especially since yours is being installed in an RV and, in the future, a boat.


Heck, get some vertical wind vane generators and put in some more batteries and solar panels and make the whole thing electric!!

[mbullock]

What's the model of 12V converter please? How efficient is it? Just comparing it to Victron 48-12 converter.

The idea of going LFP to 240/120V I like. Are you going to run your cooking off electric?

[nebster]

Originally Posted by offpist

Can we see some pictures on these aluminum-shell cells?

Yes, I'll post some images in an update soon.

How do you plan to isolate them from each-other?

In my setup, they are isolated with hard plastic spacers that enclose each cell. The spacers provide the electrical isolation that is required, but they also create air channels for cooling/warming. (And they lower the effective energy density of the pack. I'll share more details on the pack soon.)

[nebster]

Originally Posted by SailingSisu

Love all of your setup!

Just ordered my solar + LFP set up. Also Victron, but including Victron batteries, which is balancing. I have flown racing drones before using similar LiPO batteries and they did need balancing hence, me going that route. Would be great to hear from you about this balancing issue!

Thanks! Victron's offering seems really nice. Their packs would not fit into my space, an issue I ran into with several of the commercial offerings.

Note that the batteries/BMS are actually made by another company, MG Energy. MG actually makes two different chemistries for Victron. The "HE" packs are not LiFePO4 chemistry, have fans for cooling built in, and require more space around them for that airflow. I would not recommend the "HE" packs on a boat.

I'll go into more detail about balancing soon, but in a nutshell, rebalancing is generally not needed if you're okay staying away a 100% charge. And there are several very good, other reasons to stay away from 100% anyway, so... Just Don't Do It, as Nike would say.

How much did your batteries weigh?

About 300kg.

[nebster]

Originally Posted by upston

This sounds fantastic, i am sure the " That will never work crowd " is gasping for air. Please elaborate on the lack of BMS. It is expensive but we are told necessary on L batteries ??

I'm going to write a long entry on BMS. A big part of the problem is that "BMS" is an overloaded term, which leads to confusion in short posts on forums. But, yes, I think it's reasonable to build a pack with a very lightweight and simple BMS. I even think it's preferable!

I also have a floating ground, no ground and it seems to be fine, please elaborate on that as well.

Will do.

Finaly why do you need so much power, what the ell are you running

Far and way, air-conditioning*dominates when it's warm. But we do have an electric kitchen, a regular washer/dryer, fridge, and so on. Our summer power consumption looks similar to a small house; maybe 30-40kWh/day.

[nebster]

Originally Posted by john61ct

Source of the cells please.

They are made by a company called Frey.

[nebster]

Originally Posted by Dadrock33

Following . . . . this could get interesting . . . if for no other reasons than some of the concepts (and maybe some of the responses!) could become "normal" . . . who knows? I'll await a bit more presentation and some discussion before I think we'll see some interesting "critiques" of your system; especially since yours is being installed in an RV and, in the future, a boat.

I think it's unlikely they'll all make sense for any other individual system, but I think a FEW of them might make sense on almost EVERY system. Especially if it's a DIY build where you have control over each layer of the design.

Heck, get some vertical wind vane generators and put in some more batteries and solar panels and make the whole thing electric!!

Yeah, might as well go crazy, eh?!