Re: Converting my Amel 54 to lithium batteries: what I did, what I like and what I don't like (after one year of full time live aboard use)
Scott SV Tengah
Glad you found some value in it. I went through a lot of brain damage to get to where I am today, so no need for others to repeat my mistakes.
Answers to your questions:
1) That is correct, 1 fuse per 25.8v pair.
2) You are correct - no worries, if I'm wrong about something, I'd rather have someone point it out than for me to remain blissfully ignorant about my ignorance. :) They are daisy chained. I tried to use the BP-220 but that failed due to overcurrent. I think the Victron system responds to over/under voltage in a very blunt manner - I don't believe it shuts down nor disconnects individual batteries. One of the cells in one of the batteries is over voltage and the all of the chargers stop charging. In the under voltage situation, it just cuts off your load to prevent battery damage until the charger(s) have raised voltage back above the lower limit.
Each battery can be monitored (temp and cell voltages) via bluetooth. I surmise if your BMS cut everything off, you could check individually and then remove the battery (or more accurately the battery and the one it's serially connected to) from the bank and continue on.
3) My understanding of the low voltage cutoff is that it's "global" - so the VE.BUS BMS can't instruct a specific BP-220 to load disconnect. I actually think your idea of putting three BP-220s in the system, one for each battery pair MIGHT work to protect the batteries, but as I said, the load disconnect signal is global. Nowhere in the documentation does it show the capacity to assign specific a specific load disconnector to specific batteries. I'm loathe to spend more money on BP-220s when a single one should have been able to handle the passarelle.
4) The height on the 25.6/200 is a problem. Even with my 12.8/150 model, I had to trim a bit of the bracing on the lid. I just looked and noted the 12.8/200 is reported to be the same size as the 12.8/150. I'm not sure this model existed at the time I installed my system AND it seems a bit odd that you get 33% more capacity with the same size and only 10% more weight. So it may be a typo. But if not, it's an easy way to get more capacity.
6) Great idea in general. I will say that my Volvo isn't ground isolated. A previous owner replaced the 12v alternator and didn't put the isolated version in, so the negative is connected to the bonding system. I've been told by very knowledgable Amel reps that it's not ideal to have the 12v negative connected, but it's not a huge problem. Hell, most boats are setup that way. I'm careful, so we installed an additional "main battery switch" whereby I can disconnect the 12v negative when I'm anchored for a while or tied up, so as to eliminate the connection between 12v negative and bonding. This also prevents the genset/engine from starting, obviously.
But your idea is very good.
2007 A54 #69