My solution was to use as many parts
that I had before LiFePO4
which included a Bluesea ACR. If I'd started from scratch I might have done it more fancy with a Victron or Mastervolt integrated bus system. It's been many many iterations but I am finally happy with my setup.
Here's my criteria:
1: Must have a battery connection when motoring to protect Alternator
in case of BMS disconnect.
2: Prefer not to use LiFePO4
bank when plugged in at a dock
3: Need a backup start battery in case LiFePO4 bank goes offline
4: Need a backup battery for critical systems in case LiFePO4 bank goes offline
As you can imagine, these points all led to having a "buffer battery". Which in my case is a cheap
FLA big enough to start my engine
if need be.
While off the dock
, the Bluesea ACR is wired so that it only automatically connects the two banks only while the engine
is running. My engine normally starts and runs everything off the LiFePO4 bank. The FLA really does nothing and just gets slightly topped up everytime the engine runs. My Alternator regulator
is set for LiFePO4 so yes, it doesn't get fully charged until I get to a dock but since its not doing anything I dont care. In this use case it's just there to keep my alternator happy if the BMS disconnects my LiFePO4 bank.
When on shorepower I manually switch off the LiFePO4 bank and run everything off my charger/buffer battery. This keeps the expensive bank happy and allows me to leave the boat
for 6 months without worrying about something happening to it in the super hot or freezing cold conditions.
If the BMS shuts down my LiFePO4 bank I can sort out why and then take the appropriate action. It will not turn back on by itself. I think this is a good thing because I don't have to worry about some crazy condition that hasn't been thought of. While I am sorting out what action to take I run off my buffer bank so Im never without power on board.
Neither my AC charger
controller care about disconnecting under load so my setup without a Charge bus/Load bus works fine this way.
Ive now had the LiFePO4 bank for 6 years. It took me 4 years to get it to a point where Im happy, but over the last two years its worked exactly as I planned.
As an aside. MY BMS has only disconnected my bank twice in the 6 years. Once in the first year where I had a bad cell and once about a year ago when I was working on my engine while running it and my Alternator sense wire got caught in the belt and the voltage went sky high. It hit 25 volts in the 30 seconds it took for me to run to the cockpit
and shut it off. Those who claim you don't need a BMS with the capibility to protect the bank just haven't had their battery bank long enough.