LiFePO4: charge bus, load bus, buffer battery

[s/v Jedi]

I’m coming up to a decision I need to make on how to do the final setup of large Lithium banks aboard. I have been going back and forth the idea of just having an AGM battery on the load bus at all times as a buffer... in case LFP packs decide to disconnect from the buss etc.

I would love to read your ideas over how to do this: simple one bus for charge and load, separate busses, or adding a LA battery to deal with events.

In case you use multiple banks: charge until HVC? Which one(s) on the load bus?

[mbartosch]

This has been a topic of various previous discussions. You might get some ideas on one of the many possible solution from our design described at
https://www.entropypool.de/2015/05/1...system-design/

[s/v Jedi]

Originally Posted by mbartosch

This has been a topic of various previous discussions. You might get some ideas on one of the many possible solution from our design described at
https://www.entropypool.de/2015/05/1...system-design/

Thank you. Your schematic shows real world solutions where you used workarounds that seem reliable to me. That said, you only have one lfp bank and it seems you made many compromises to avoid a LA buffer battery, like the charge bus only having solar controllers while alternator and inverter/charger are connected elsewhere, at different points.

I understand the decisions you made and I’m looking for a wider, general discussion rather than a solution tailored for one boat

[gpeacock]

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 or Solar 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.