FWIW: My LiFePO4 sailboat upgrade

[madmike]

Our lead acid batteries have finally bit the dust so we're upgrading to LFP. Here's a description of the system design. It doesn't follow all the ABYC recommendations or all the best practices from this thread, but it seems like the right compromise of safety, cost, and convenience for our boat. The house batteries will be DIY 12V using 302 Ah CATL cells and Overkill Solar BMS. They'll be mounted in a box under the quarter berth, so they'll be well above any likely bilge water. I plan to secure the batteries against any motion but not really attempting to maintain compression against cell swelling. Engine starting is with a new AGM battery.

This electrical system is divided into the lithium-powered "house side" (red and black in the schematic) and the AGM-powered "engine side" (orange and purple in the schematic). This division allows connecting the dumb chargers (alternator and wind turbine, which both have simple automotive style regulators for lead acid) and heavy loads (windlass and starter) to the AGM battery. The smart chargers (solar and shore power, both configured with Lithium profiles) and our modest domestic loads (biggest items are fridge and autopilot) to the lithium. A Sterling BB1240 DC-DC charger provides house charging from the alternator or wind turbine. Our alternator is the stock 70A unit that came on the diesel engine, so 40A max seems like an adequate charging current. This DC-DC charger also has a nifty "reverse charging" feature that provides float charging of the AGM battery from solar or shore power. Of course the two negative busses are connected at one place (the SmartShunt) and also have a single-point connection to the bonding system.

We already have a Cerbo GX to collect data from the SmartShunt and MPPT for battery monitoring. I'm installing BlueSea 5191 fuses where any smaller gauge wires connect to the positive bus bars. Both battery banks have 200A primary fuses (class T for the Lithium and AMG for the start battery) and battery isolation switches.

For convenience, I'm keeping the already-existing 1-2-Both switch. I know ABYC doesn't like the Both setting that can parallel the two different battery chemistries. Under normal use we'll always have this switch connecting the alternator and starter to the AGM start battery. In an emergency (such as a battery bank failure), we can disconnect one bank using its isolation switch, then move the 1-2-Both switch to Both to power everything off the remaining battery. The disadvantage of this choice is the risk of mistakenly connecting the two batteries together; the advantage is in an emergency we don't have to mess around with a jumper cable.

I'm sure everyone else will end up with different decisions and compromises. Hopefully this configuration can give you something to think about.
Thanks everyone, for all the helpful suggestions and information here on the forum. We appreciate it.

 

[AnarchyJet]

First thing that jumped out at me was a 50a CB on a 10 awg wire feed by a 15a SCC. It is not needed, but if you really want protection on that put the proper size for the wire.
Second, I wouldn't risk the chance that the two banks get paralleled. Why not put the feed for the 12v panel on a 1-2 switch. Then you can safely pick which bank is powering it.

 

[madmike]

Both of those items (the solar circuit breaker and the 1-2-Both switch) are legacy items that came with the boat. The solar breaker is less "wrong" than it looks; we actually have two sets of 200W solar panels with two MPPT 75/15 units in parallel through the same breaker. I left the second system out of the schematic for simplicity. Protection of the 10 AWG wire is provided by the 60A fuse on the positive busbar, since that's the potential source of a dangerous short-circuit current.

If we replaced the 1-2-Both switch with a 1-2-off switch, besides the extra work involved (a bunch of other wiring and stuff needs to come out before we can access the switch), we'd lose the ability to power the house electrical panel from the AGM start battery.

 

[willo]

OK, let's talk about this a bit.

• I don't know what's being added vs augmented here. It would be a good idea to identify existing/new so that you can determine if something should be replaced or re-sized.
• A 50A breaker behind a 15 amp MPPT is wrong. You should have a breaker that matches the load. If that MPPT ever dumps out more than 15A, it'll catch fire long before the breaker trips. I'm guessing you sized the breaker to the wire or it's just one you already have?
• Make sure that wires to/from the lifepo4 packs are equal in length. The diagram implies otherwise. (just add a note to your diagram)
  • Same for the BMS to Shunt.
• 1 AWG to your house panel is insane. Right size that. I like to use table references and then upsize a little for extra margin. (like if 6awg is fine, maybe run 4.)
  • The blue seas panels are built in switch groups. You may want to run smaller individual wires to each group.
  • Is there some bus bar/wiring that you're just not showing?
• Bridge those ground bus bars to ensure you have a unified ground.
• You may want some form of disconnect on the windmill - I like the DC din breakers for things like that (and my solar panel disco breakers)
• I would run the smart shunt aux input to the engine battery so you can see it's status even when the isolate is disconnected. There's nearly 0 load via that wire.
• If you want the alternator to survive, you should make sure that the isolation switches are never operated when the engine is running. I'd clearly label that if it's not already.
• Are you replacing the ground wires or just noting that it already has 1/0 wire?
• No inverter loads? Just 12v systems?
• The JBD/Overkill BMS is decent. You'll be running multiple smaller ground wires from the pack to the BMS and the BMS to the shunt. You should identify the sizing here. I suggest the version with bolt terminals on the BMS board.

After all that...
Go through and mark where you need terminals. Mark the ring sizes. Mark wire lengths. Put together a BOM for the components and wire sizes and lengths. I did my re-wiring on the hard where it was easy to run down the street for additional parts. However I know for a fact that I paid some premium because of it. I did pre-order most of my parts but I kept running into oddities that required additional tweaks.

 

[willo]

I see you already mentioned the solar breaker... so let's say that you need to consider your schematic the source of truth. Mark that you have mulitple, etc. Note that things are simplified or whatever or we can't really give you complete feedback.

 

[AnarchyJet]

Crude mockup of what I meant. Blue box on the left would be an added 1-2 switch to the supply of the 12 v panel. Old 1-2-both switch is not really needed how it is set up, but if it is already there just leave it. This would be the safe way to run the panel off the agm bank if the lfp bank was down.