I agree with everything that Terry has written regarding the charging.
As I've written before, I don't think individual cell BMS is needed for a house bank, given the low cell count, charging rates, and discharge rates. BMS control was developed for the EV market where the batteries have many cells in series and parallel, and both charge and discharge rates are very high. We come no-where near to any of those conditions.
In general, our pack is still working to our expectations. We've actually been away from the boat for almost 3 months now, without it being connected to any power supply, so it'll be interesting to see what has happened when we're next there in a few weeks. Based on previous experience, the battery will still be at the same state of charge (within measureable tolerances), and all will be fine.
We have had a problem with our new alternator regulator
not putting much charge into the cells, but I haven't trouble-shooted that fully yet, and I suspect it's a regulator installation
issue, and isn't related at all to the cells (this is backed up by the fact that the shore power
charger works very well with them).
On the initial balancing: I had one cell that was charged higher than the others, and it took a while to bring it into the same charge range. I used a high powered (160W) resistor and connected it to the cell using car jump leads. It pulls out up to 60A, so gets the SOC down very quickly (we have 400Ahr cells), but you have to pull the voltage down below the desired voltage because the high current flow understates the cell voltage. (But it's really quick - about 10 min.s does the job). I found that I needed to play with the cell over about 5 cycles to get it into line.
I've said it before, but I'll repeat it: I think that every yacht should have low-voltage and high-voltage cutoff devices on at least the whole battery pack. That's simply because EVERY battery chemistry can be damaged by significant over-discharge and over-charge situations, and I agree with OceanPlanet that even the most diligent of us sometimes forget. But, I agree with Terry that the rates and cell numbers that we use for house banks means that cells don't really get driven out of balance, and in my opinion that means that battery level cutoffs are all that is required, not cell level.
We had high and low voltage cutoff mechanisms on our lead-acid cells, and we've kept them for our LiFePO4
cells. The same devices can be used for both chemistries if their parameters can be changed (which most can these days). We have a Vetus low voltage monitor
and cut-off, and our charge devices (alternator regulator & shore charger) are smart and have charge voltage cutoff.
Following discussions on this thread (mostly with Terry), combined with experimentation, I have gradually wound the charge termination voltage down without seeing any decrease in performance. It is now at about 2.4V/cell, and that has to be better for the pack than a higher voltage. Make sure that the 2.4v is measured at the battery, not the alternator/charger.
I do think that many sailors who go to LiFePO4
cells at the moment will want some "safety insurance" because they are a less-known technology, and thus would prefer the "no-see-um, no-touch-em" approach, which OceanPlanet talks about. Most wouldn't want to go near a power resistor clamped onto one cell; in fact, most wouldn't know what it is. And that's ok in my book. So, although a number of us (few or many, who knows) are just fine without a cell-BMS, I don't think we should criticize those who use them. I personally would worry about BMS failure mechanisms as brought up by one post.
Anyway, so far I've seen nothing in our particular implementation of these cells as a house bank which would cause me to change what we've done if I had to do it again - and I'm really glad that we made the move from lead-acid technology.
I hope this helps.