LFP Bank Sizing + Solar Charging Regime

[mcswain29]

Rounding the bend on a DIY build using some used (purportedly in a robotics research application) CALB CA100 cells. All have NSA serial number stickers and are all from 2020 (so 3 years since DOM, and were sold new to the North American market back then).

Individual charging to 3.600vpc, 24h rest, and subsequent capacity testing stopping at 3.000vpc yields me a bank of 8 cells with capacities between 99 and 102ah, resting voltages of 3.21x, and IR's between 0.23 and 0.25 milliohms. Currently underway with the (long) process of parallel top balancing before assembling into a pack

Boat is a smaller sailboat, used for great lakes weekending and holiday stints of 2 weeks+. Minimal loads: LED lights, stereo, some scirocco cabin fans, etc. My main load is refrigeration which can use between 180 and 500wh per day depending on ambient conditions and what BRR (beer rotation rate) is...

Planning on solar only for the LFP house bank - currently 220w of sunpower panels thru a Victron smartsolar 75/15. This manages to keep things going, surprisingly, see below. Have the option to add another 100w and a second smartsolar 75/15.

Currently the fridge is wired to the "load" output of the solar controller as I otherwise had no way of measuring it's consumption, and I do wonder if I should just leave that as-is or run everything through the victron 712 thats going in with this install as well

Engine start is and will remain a boring old FLA, and planning no engine charging for the LFP housebank (18hp diesel with a puny alternator, not worth the $ to upgrade)

Building a 2P4S bank of nominal 200ah capacity almost seems overkill to me, as my current housebank is a Costco special "not so deep cycle deep cycle" g27. I will capacity test this once the Lifepo4 bank is installed - but for now I have nothing other than a best guess of approx 40ah of useable storage at the moment. That might be optimistic.

There's no space constraint for either 4 or 8 cells, so I am thinking best to go with the bigger bank and not have to push it into the upper knee regularly? I.E. charging to 13.6-13.65v as an upper limit?

What are some others out there using for solar charging profiles for DIY LFP banks? I was thinking to have a 2 or 3 different charge profiles including a "balance" regime to drive voltage up into the balancing range as required (JK bms, so balancing voltage is programmable as well and no need to take it to 14.XX) and maybe a "cloudy days coming up" profile to top things off... I am also thinking I can get away with a rebulk offset >0.2V as I will have quite a bit of storage capacity compared to daily use

IF you've made it this far, thanks for reading the novel and toss some thoughts/experience back
Thanks!

[donradcliffe]

Your plan sounds good. You could put two 100ahr batteries each with 100 amp bms for reliability/cost reasons. More solar would be nice if you can do it.

No need to overthink balancing. Just install a $10 active balancer on each pack, and unplug it within a day or two. I haven't needed to plug mine back in in 3 years.

I assume you have a separate engine start battery. If you have a one-two-all switch, you can use your puny alternator to charge the house bank after 3 days of rain.

Don't buy another victron controller unless it has a display or link to your phone. For the same kind of money you can get an Epever controller with remote display and keep a constant eye on your battery's state of health.

One of the modes you need to come up with is a storage or maintenance mode when you are not using the boat. If you have parasitic loads, you need to drop the solar charger settings down to 3.3vpc, with minimum or no bulk or adsorption.

The LFP batteries really don't do with temperatures below freezing. Plan on taking them home for the winter or moving south.

[Statistical]

There's no space constraint for either 4 or 8 cells, so I am thinking best to go with the bigger bank and not have to push it into the upper knee regularly? I.E. charging to 13.6-13.65v as an upper limit?

There is not a lot of value charging that high. Resting voltage for LFP is ~3.4v. Yes if you charge it to 3.6 or 3.65V it will temporarily be higher but even disconnected from all loads it will decay back to 3.4V.

The only thing charging at 3.65V does is allow the battery to accept* higher current for longer. You will stay in bulk longer charging at a higher rate. However in most cases unless you are using some massive (like 200A+) shorepower charger you don't need that higher voltage. If your solar charge controller is putting out 30A max the battery will soak all that up even at 3.5V or possibly even 3.45V and it is a lot gentler on the battery.

As for different charging profiles you can certainly make it as complicated as you want but most likely the fiddling about won't do much of anything. LFP is very accepting of a wide variety of charging rates and handle staying at a wide variety of SoC so you don't really need to do anything but set it and forget it.

As pointed above if you will leave the boat unattended with minimal loads for long periods of time you may want to drop the charging voltage. This is simply because the minimal parasitic loads won't be enough to discharge the battery relative to the solar charger output so the battery will remain at a near constant 100% SoC which is harder on battery calendar life. Despite all the freaking out about cycles unless you are living aboard calender aging not cycle aging will dominate simply because you will have so few cycles per year.

As an example you sail 3 times a month and rarely discharge the battery below 50% SoC. From a cycle point of view the batteries will probably last 6,000+ cycles. At 3 times a month that is 2000 months or 160 years. Will the battery last 160 years? No because in 10 to 20 years depending on calender aging regardless of usage the batteries will need to be replaced.

I would recommend staying with victron if you already have one. The "smart" chargers can coordinate charging using bluetooth (ve.smart) or if connected to a central manager like cerbo. As you suggested I would keep the chargers the same as if the one connected to the larger array fails you can always swap them.

[Bill O]

Would probably recommend 2-6v golf cart (~200+ah) batteries instead of lithium. Not as sexy, but functional for the few short trips you are planning.

You generally have minimal systems (except for the fridge), so the golf carts would easily handle the draw. For the fridge, would suggest freezing gallon jugs of water and add them to your box to keep your food/beer cold overnight. Turning off the the fridge at night will help reduce your power load, then fire it up again in the morning w/the solar.