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tanglewood 04-06-2018 18:26

LFP system design approach - comments welcome
In this thread I asked about sources for CALB CA 180 batteries for a system I'm planning/building. Here's some more context and details about what I'm planning, and why. Comments are welcome.

First a little background. I'm a retired EE, computer science, turned exec, but still a geek at heart. Our boating is long distance/remote cruising with extended periods of time away from civilization, so dependable systems are vital, and repair/workaround essential when the inevitable issues arise. We are building a boat, and the plan of record is to use LFP, but before pulling the trigger on LFP for the boat, I'm building a proof project in a slightly less demanding environment to test and assess LFP viability for the boat.

The test project, and the subject of this thread, is an LFP system for an off-grid house. In nearly every way the requirements are the same as the boat, but less consequential if there is a failure, and a lot easier to get parts etc when needed. If nothing else, the shipping address isn't changing all the time :-)

The current system is principally solar powered with about 3.2kw of solar with MPPT controllers. Then an inverter/charger that powers everything, and charges via a generator with auto start. Batteries are 4V, 1300AH FLA, strung together for 48V. That's about 32kw of usable storage, and will run the house for about 3 days with no sun. Solar covers all our needs except for Nov through about Jan. Total generator run time is about 100 hrs per year. This system has been on-line for 10 years, and the batteries are reaching their end of life, so the time seems right to switch to LFP.

With that preamble, here's my current thinking for the LFP sub-system:

- I'm going to build the system myself. It's not that I object to the available pre-engineered BMS systems, but I'm very concerned about the long term viability of nearly all of them. When some part fails in 3 or 5 or 10 years, I think there is a good chance that any given company I might choose will be gone, and I'll have to buy a whole new BSM. Victron is about the only company that I think has long term staying power, but the cost puts LFP into a price range where I don't think it's worth it. I want something that is long term repairable and supportable and will live the life of the batteries. So the plan is to an industrial PLC with off-the-shelf modules for monitoring and control.

- The PLC "BMS" will be a monitoring and fail safe system, but not a balancing system. Instead, I'll follow the approach of operating the batteries away from their SOC end points, or "knees" in the voltage curves. When monitoring suggests an imbalance, I will have to respond manually to re-balance.

- The plan for batteries is to use CALB CA180s. CALB seems to be the most reliable vendor, but it's admittedly a low bar, and I am open to alternatives. The 180 cells seem to be the largest that are readily available. Bigger cells means less paralleling, few interconnections, etc. The 400s are appealing, but don't seem to be available

- I'm planning on 360AH of LFP capacity, so about half of the FLA capacity that I currently have. That will last me 1.5 to 2 days without sun. If the batteries get too low, the generator will start and recharge in a little over 2 hrs. The generator may run more frequently, but for a lot less time in each run, and less in total. Right now, a full recharge of the FLA batteries takes just under 6 hrs.

- The battery config will either be 1) paralleled pairs of cells, wired in series for 320ah @ 48V. That's 16 pairs of batteries, for 32 total. the other option is 2) two 48V strings of single batteries, with isolation switches for each string. So two strings, each 180ah @ 48V. This later arrangement would be to provide for string isolation for diagnostics, and to be able to run on one string if the other is out of service.

- For monitoring, I plan to instrument voltage and temperature on each battery, or pair of batteries if they are paralleled. Absolute values, and differences in values will lead to various warnings, alarms, and fail safe actions.

- I only have two charge sources to contend with, and both are programmable. I plan to program each to essentially be a two stage charger, with a bulk voltage to perform charging, followed by a float voltage down at the battery resting voltage. There will be no absorb stage, with an immediate transition from bulk to float. I expect float will require some tweaking to prevent excess battery cycling and get solar to carry the loads once batteries are charged, but while solar power is still available.

- The only fail safe I'm currently planning is a main disconnect contactor that will be controlled by the PLC.

- I'll also have to adjust the AGS stop/start

There must be more, but that's all I can think of right now.


john61ct 04-06-2018 18:37

Re: LFP system design approach - comments welcome
Very interesting. Please document here as you go.

Check out Controllino, Arduino based PLC.

Consider both LVD and OVD along with temp cutoffs.

Based on "worst cell" maybe, but personally I think per-pack may be granular enough.

If you do P before S, Victron 712-BMV can also monitor 50/50 imbalances.

tanglewood 04-06-2018 19:20

Re: LFP system design approach - comments welcome
After the proof system, assuming success, the boat system would be very similar with only a few adjustments.

- The boat system will be 24V @ 720Ah. Same kWh capacity as the house, but in 24V rather than 48V. Thee was an interesting discussion a while back about 48V systems. I had considered 48V, but rejected it for a number of reasons. a) a lot of gear is 12V or 24V, and if I went with 48V it would push all my electronics and other DC gear to 12V. In my opinion moving more stuff to 12V is moving backwards, not forwards. b) 48V inverters and chargers are very available, but main engine alternators, and especially regulators become very specialized/unique. I'd rather keep the alternators and regulators more mainstream. 3) if the house bank were 48V, I wouldn't be able to parallel in the start back as a backup/recovery/work around measure. So I decided to stick with 24V.

- There will definitely be two battery strings with manual disconnects for maintenance, diags, and to work around problems.

- There will be switching (manual) to disconnect the LFP batteries and switch in one of the AGM start banks as an additional backup/workaround.

- Charge control is a bit more complex with several additional charge sources. There will be inverter chargers. stand alone AC chargers, dual main engine 5kw alternators, and solar panels.

But in all other ways it will be the same, so you can see that the house system is a good proxy for the boat system

Oh, and as an ultimate backup, I'm going to build the battery shelves to accommodate the standard complement of AGMs, and then organize the LFP cells in that space. Then if for whatever reason LFP emerges as having been a bad choice, I can pretty easily revert back to AGMs.

tanglewood 04-06-2018 19:22

Re: LFP system design approach - comments welcome

Originally Posted by john61ct (Post 2645282)
If you do P before S, Victron 712-BMV can also monitor 50/50 imbalances.

As in they can individually monitor the two cells that are bolted together in parallel?

Bleemus 04-06-2018 19:36

Re: LFP system design approach - comments welcome
Leave it to an EE to make storing power on a boat ten times more complex than it needs to be.

john61ct 05-06-2018 04:35

Re: LFP system design approach - comments welcome

Originally Posted by tanglewood (Post 2645308)
As in they can individually monitor the two cells that are bolted together in parallel?

Two halves of a string, any AH size or number of cells per string, within the voltage limits.

Manual is online.

CatNewBee 05-06-2018 06:14

Re: LFP system design approach - comments welcome
Nice project. Good luck. I would go for a BMS, that balances, but your approach is doable, little more manual monitoring and engagement.

ullar 05-06-2018 06:58

Re: LFP system design approach - comments welcome
Interesting project. Keep us posted please.

KeelMe 05-06-2018 07:07

Re: LFP system design approach - comments welcome
There is some good information here.

john61ct 05-06-2018 08:02

Re: LFP system design approach - comments welcome
Here's my "boilerplate" LFP summary, mostly from marine electrics discussion forums involving long-term users and professionals, with special thanks to Maine Sail (see below).

Any and all feedback is welcome, especially if more "canonical" information from the links cited conflict with my summary.

Systems: OceanPlanet (Lithionics), Victron, MasterVolt, Redarc (Oz specific?)

Bare cells: ​Winston/Voltronix, CALB, GBS, A123 & Sinopoly

Best to size your cells for two parallel strings for redundancy, unless you have a separate reserve/backup bank. Don't go past three, or you may see balancing issues that affect long-term longevity, maybe four in a pinch.

Note nearly **every** vendor, also those of ancillary hardware touted as "LFP ready", gives charging voltages **way too high** for longevity.

My (conspiracy) theory is that manufacturers would prefer their cells get burned out in under 10 years.

EV usage is very different from much gentler House bank cycling. Most EV people talking "lithium-ion" mean other chemistries not as safe as LFP, much shorter lifetimes, and with completely different setpoints and behaviors.

My charge settings for LFP: 3.45Vpc, which = 13.8V max for 4S "12V".

The point is to look at the SoC vs Voltage chart, and avoid the "shoulders" at both ends, stay in the smooth parts of the curve.

Either "just stop" charging when voltage is hit, or if you want another couple % SoC capacity, stop when trailing amps **at your spec'd voltage** hits endAmps of .02C, or 2A per 100AH.*

Note even at the "low" max charge voltage, letting the charge source continue to "push" even low currents long past the endAmps point is **over-charging, and will** greatly reduce lifecycles.

So if you can't then "just stop", set Float well below resting Full voltage, at say 13.1V, but that is a compromise, and *may* shorten life cycles.

With LFP, you don't need to fill up all the way at all, as far as the cells are concerned. In fact, it is bad for them to sit there more than a few minutes. Therefore only "fill up" if consumer loads are present, ready to start discharging, ideally right away.

Many sources claim there is a "memory effect" from keeping charge voltage and ending point exactly the same every time lower than manufacturer specs, that can apparently over time lead to apparent lower capacity. The recommended fix is to "go higher, into the shoulder" every so often, similar to "conditioning" a FLA bank monthly. To prevent the issue, vary your setpoints a bit, sometimes go a point or two higher or lower, vary Absorb time a bit etc. There is no consensus just how serious the problem is.

Store the bank as cool as possible and at 10-20% SoC, or maybe higher to compensate for self-discharge, if not getting topped up regularly (I would at least monthly).*

Letting the batts go "dead flat" = instant **permanent unrecoverable** damage.

Same with charging in below 32F / 0C freezing temps.

Persistent high temps also drastically shortens life.

Charging at 1C or even higher is no problem, as long as your wiring is that robust, vendors may spec lower out of legal caution.

Again, going above 14V won't add much AH capacity, but will shorten life cycles dramatically.

And of course, we're talking about gentle "partial C" House bank discharge rates, size appropriately and be careful feeding heavy loads like a winch or windlass.

Following these tips, letting the BMS do active balancing is unnecessary and potentially harmful, just look for LVD / OVD and temp protection. Multiple layers of protection are advised if it is a very expensive bank, so you don't rely on any one device to keep working.

Check cell-level voltage balance say monthly to start, then quarterly, finally every six months if there are no imbalance issues, but only if that seems safe to you.

This thread is long but informative

, make sure to give both Maine Sail and Ocean Planet your close attention.

Also MS' summary notes here

**Everything** at that site is worth reading, very valuable. He also has great articles in Practical Sailor. His new site under development transitioning the pbase content is here, feel free to make a donation to help with those expenses.

Best of luck, and do please report back here!

CatNewBee 05-06-2018 11:59

Re: LFP system design approach - comments welcome
Good advise and lots of information.
As always, the statements are relative and not absolute.

It depends on your use pattern, budget and loads. Usually you have an agenda when switching to LFP, you dont do it simply to replace the FLA, no, you do it on purpose. You chose LFP for more capacity, higher efficiency, longer life time and of course to run easily higher loads for more comfort.

You usually buy LFP to use them daily and not to store them long time unused at 30% charge. LFP are expensive, you do not buy excess capacity to just extend the life expectation of the bank by not charging to full and not draining to empty. FLP - especially the yellow LiFeYPO4 Winston cells are overprovisoned by the manufacturer to guarantie 80% of capacity after 5000 cycles. It is common to have 20-30% more USABLE capacity than the nominal cells capacity, it is safe to use over 80% of the nominal capacity before hitting dangerous voltages. My 1000Ah cells can discharge from 3.65V (full charge) up to 1300Ah before hitting the 2.9V cell voltage.

The voltage is between 30% ant 80% almost constant, only on lower and higher SOC there are significant jumps. That said, you CANNOT tell if the cells are not balanced as long as you not charge to full or drain to empty. You will not recognise cell issues if you just stay in the middle.

A BMS with single cell monitoring and reacting on dangerous voltages, temperatures and imbalances, cell resitance etc. is a must, otherwise you system will one day run away and die. A good BMS does balance at least when charging over 80% SOC. It will also recognise the SOH ( state of health ) and will inform you early about upcomming cell issues.

is it a good thing to use only 50-60% of the capacity between 30 a d 90% for longitivity? Yes and no. If you use the full potential including overprovisioning, you have 100% energy for the buck, maybe your system lives 4000 heavy cycles. If you use 50 - 60% you pay 40-50% more than needed to just reach 5000 cycles. So you dont save anything, you just end up with an old battery. Given the 4000 cycles - its about 11 years daily using the full battery capacity - you can buy then a better cheaper and more advanced technology, with the 5000 cycles you have paid more for the energy stored and used and stick 3 more years to the now old bulky system.

Just saying. This batteries are made to be used and are too expensive to not take as much advantage as possible out of them.

The most frequent use pattern are liveaboards, where you rely on the battery and use it in a wide range of loads, small currents all day long for fridges and electronics, variable charge currents from solar, wind and water, heavy loads from watermaker, waterheater, washer/dryer, galley usage, A/C, diving compressors etc...

There is no need to stop charging before full, because you cycle the battery all the time, latest during the night. When you really use the battery, there is no need to stay in the middle all the time. This is fot long term storage only.

Chose a good BMS and do not rely on manual monitoring or balancing. Check from time to time your cell voltages and compare them to the BMS readings, also do some stress tests once a year at least to check if the BMS really switches off the chargers or the loads on the upper ang lower end, also check the capacity and cell balance / health. This may save you the day before longer passages or staying in remote places with limited access to spare parts.

You usually do not need 2 banks for redundancy, LFP are far more reliable tha lead acid0 batteries, you always can hook up a lead battery if necessary or run the LFP without the BMS-if it fails somewhere in the wild, also you have still your start battery as redundancy and can run the engine to genetate power, if you dont have a generator, also your solar system can supply you during the day. You have to understand your system and to know what to do, then you dont need 2 banks at all.

Morrissey 05-06-2018 15:52

Re: LFP system design approach - comments welcome

A large Chinese supplier you missed, that is making inroads for home and commercial use here, is BYD.
Warren Buffett's Chinese battery giant BYD to take on Tesla in Australia |

I quite like their B-Box LV system. I note Tanglewood's assessment of a 48 V system, but would still choose it for the Inverter bank for a new boat. Most of the charging would be from solar, and my 345 W Sunpower panels run at over 60V. I have 2000W at present, but for a new boat I would find a way to have a minimum of 5000w. Most solar controllers are limited by amperage output eg Outback FlexMax80. So the higher voltage your inverter bank, the better. Because my house bank at present is 12V I need 2 FlexMax's for my 2000W of panels . If I were to run a 48V bank I could have 1 FlexMax only at 48V.

In addition to an inverter bank I would also run a 12 or 24 V bank of modest size for DC boat loads. Engine alternators would be of that voltage. Then, have either a DC generator or a hydraulic (from propulsion engines) generator to provide 48V charging. You could have a 48V inverter charger if you wished to have 48V shore charge capability, but I think I would just use inverter only and not have a shore charge capability.

john61ct 05-06-2018 16:39

Re: LFP system design approach - comments welcome
Not in line with my needs / preferences for marine use, but yes interesting.

Do you know which of the many lithium-ion chemistries they're using?

Morrissey 05-06-2018 17:03

Re: LFP system design approach - comments welcome

Originally Posted by john61ct (Post 2645917)
Not in line with my needs / preferences for marine use, but yes interesting.

Do you know which of the many lithium-ion chemistries they're using?

BYD's b-box uses LFP.

CatNewBee 05-06-2018 22:39

Re: LFP system design approach - comments welcome
How about a Tesla PowerWall and Inline solar controller? You can then step down to tne 12V for yout electronics by a regular charger and a small buffer battery. Just saying...

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