Best way to manage this LFP house battery

[nebster]

Wire each set of 7 in parallel then just use one BMS with 16 channels. Ideally take the alarm output of the BMS and use it to both sound an alarm and disable the inverter.

I really want to avoid a single 16s pack so that I can isolate individual strings or cells without shutting down the system. With 100 cells, the odds of a cell (or bus bar, or lug, or whatever) needing work are pretty high.

[Mike90250]

Sorry nebster.
These are good questions you are asking, but I think you need a lot more study before you are ready to buy and build, one of these time bombs.
Each group you parallel, should be matched capacity, so you do not have a lo cap cell bundled with a high cap cell, which would set you up for a certain fire in a couple weeks.
You CANNOT trust the mfg or vendor to do this for you.

[max2k]

Hey guys,

I'm assembling a new LFP for my RV out of Chinese cells. These are 100Ah prismatics, and I have 112 of them for 16s7p. My plan is to add isolators for each 16s string so they can be pulled out for service or disconnected automatically if something is amiss.

Meanwhile, I was hoping to manage this bank passively (and conservatively) just using pack-level voltages for LVD and charge termination. I'm using a large Victron Quattro inverter that somehow seems to have a lot more limitations on how it can be set up than I expected. Here are the issues I am observing:

1. The inverter can be set to alarm (A), shutdown (B), and restart (C) at voltages A/B/C respectively. However, C must be 4.0v > B. For now I am setting B to 47.0v and C to 51.0v. In normal use the pack should never get here, because I will add logic to start the charger (and genset if needed) much earlier.

2. The charger has a CC limit (X), a voltage (Y) to transition to CV/absorption, and then a "max absorption length" time (Z) with increments of one hour and a minimum of 1.0h.

On my test run so far with a bottom-balanced, smaller 16s1p pack, I've charged at X=50A to Y=55.2V, but I had to manually stop the charge after only 20 minutes of CV because a cell hit 3.6v (actually, 3.65v before I caught it). I will try again at 54.8v tonight, hoping that that leads to a longer absorb window. But the 1.0h timer limitation seems arbitrary and problematic.

Victron actually documents a method to set a tail charge current termination percentage, which is what I think we really want, but somehow that feature has disappeared from the latest firmware. I've asked them where that feature went, but I haven't received any response yet.

I can buy or build a cell-level voltage-monitoring system and use that to control the charge, but I was hoping to avoid having to wire 112 sensors and all the complexity/extra cost that that presents. If I'm willing to use 80% of the pack capacity, do you guys see a way to pull this off without going this route given the above?

Disclaimer: I have 0 practical experience with LFP so the following is from recent discussions here and my overall engineering common sense point of view. It sounds like you can manage your initially manually balanced bank just using total battery voltage, especially if you're willing to sacrifice a little of capacity. I would never use timer to control charging process. Conservatively the bottom knee in voltage to watch for during discharge is around 3.0V /cell while the upper to watch for during charge is 3.6V / cell. You need to make sure your DMM has enough accuracy (better than 0.5%) to perform these measurements. Between those voltages LFP has very flat charge/discharge curve making it difficult to guess its SOC using voltage.

I think your arrangement of 16s7p would be preferable from overall safety point of view- you can fuse each string individually and in case of a cell developing short in one of them string fuse would blow preventing other cells killing entire string and starting fire. The cell itself has quite a bit of energy stored so if it suddenly discharges on itself internally it's already quite an event: 100Ah x 3V = 300Wh meaning it can produce 10kW for 100+ seconds which is enough to physically burn it out. You'd need to mitigate this by putting some separators between them and providing ways to vent out smoke in case this ever happens so single cell failure doesn't start small Chernobyl in there as total stored energy you'd have there: 0.3 x 16 x 7 = 33.6 kWh, enough to power sizeable house for 24 hrs.

Can you share where did you source your cells from and @ what price?

[jflorey2]

I can buy or build a cell-level voltage-monitoring system and use that to control the charge, but I was hoping to avoid having to wire 112 sensors and all the complexity/extra cost that that presents. If I'm willing to use 80% of the pack capacity, do you guys see a way to pull this off without going this route given the above?

Wire each set of 7 in parallel then just use one BMS with 16 channels. Ideally take the alarm output of the BMS and use it to both sound an alarm and disable the inverter.

[nebster]

Hey guys,

I'm assembling a new LFP for my RV out of Chinese cells. These are 100Ah prismatics, and I have 112 of them for 16s7p. My plan is to add isolators for each 16s string so they can be pulled out for service or disconnected automatically if something is amiss.

Meanwhile, I was hoping to manage this bank passively (and conservatively) just using pack-level voltages for LVD and charge termination. I'm using a large Victron Quattro inverter that somehow seems to have a lot more limitations on how it can be set up than I expected. Here are the issues I am observing:

1. The inverter can be set to alarm (A), shutdown (B), and restart (C) at voltages A/B/C respectively. However, C must be 4.0v > B. For now I am setting B to 47.0v and C to 51.0v. In normal use the pack should never get here, because I will add logic to start the charger (and genset if needed) much earlier.

2. The charger has a CC limit (X), a voltage (Y) to transition to CV/absorption, and then a "max absorption length" time (Z) with increments of one hour and a minimum of 1.0h.

On my test run so far with a bottom-balanced, smaller 16s1p pack, I've charged at X=50A to Y=55.2V, but I had to manually stop the charge after only 20 minutes of CV because a cell hit 3.6v (actually, 3.65v before I caught it). I will try again at 54.8v tonight, hoping that that leads to a longer absorb window. But the 1.0h timer limitation seems arbitrary and problematic.

Victron actually documents a method to set a tail charge current termination percentage, which is what I think we really want, but somehow that feature has disappeared from the latest firmware. I've asked them where that feature went, but I haven't received any response yet.

I can buy or build a cell-level voltage-monitoring system and use that to control the charge, but I was hoping to avoid having to wire 112 sensors and all the complexity/extra cost that that presents. If I'm willing to use 80% of the pack capacity, do you guys see a way to pull this off without going this route given the above?