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diy solar

What if I don't use a BMS at all?

you could run large loads and critical loads strait to battery then every thing else thur a BMS... so you lose you "lights" you know your about to destroy $$$$ battery...
 
Folks I agree with all of the above posts that a BMS is essential to protect the health of a battery, but ...

I do think in marine applications there is a serious tradeoff between protecting the health of the battery and the health of the boat and crew. There are numerous situations where any boat owner would gladly sacrifice a battery if it means saving the boat. If there is even 1 Ah left and it helps me get the anchor up while dragging onto a lee shore, or get out one last MAYDAY call, then please let me use it. I really can't have the BMS deciding that the health of 1 cell dropping below 2.5 V is more important. So yes, monitor it (perhaps at cell level) and feed it with high quality components (Victron), actively balance it, but ask yourself what your risk tolerance is to the various modes of failure and which failures leave you in more dangerous situations.

One alternative is using a low current BMS to control a contactor's coil voltage and adding a readily accessible switch to bypass the BMS. But I must be able to bypass it without getting out wrenches and diving into the battery compartment.
Adding a bypass for this case may give you the extra 1 Ah you need and sacrifice the cells, but I think you have been reading too many accident reports which are always rife with revisionist history/survivorship bias. I think a better option in this case is to have multiple batteries packs each with their own BMS connected in parallel and each with their own breaker. That way, if you think you might get into trouble, you can throw one of the breakers to keep it as a backup. If one battery dies, the others keep working.

There are multiple benefits of parallel batteries:
  1. Smaller batteries are typically more robust in vibration environments
  2. A single bad cell will in a pack that causes a BMS disconnect means that you still have the other battery(ies)
  3. The option to do lower-current BMSs, breakers, and wiring which are more readily available
You could also use a separate-port BMS which has separate charge and discharge ports. That way you could keep all batteries charged, but disconnect the load on the critical battery, but I think that is just another set of power switches in the BMS that can go bad. LiFePO4 batteries do not need to be kept topped up like lead-acid batteries, so you can charge one bank to 90%+ and then disconnect it and keep it as your emergency battery for weeks without any harm.

I know the other option is tot buy a 150amp BMS, but those are bulky and don't come cheap
A 200A BMS will cost you US$120 if you shop around. You could also do two smaller BMSs in parallel.

I am considering adding only active cell balancing and /or a volt-meter that gives reading for all four cells.
Adding a 4-input volt meter is probably going to cost you as much as a BMS.
 
On my boat I run a 100ah gel battery in parallel with the LFP house battery. The LFP has the protections, the gel has just a fuse. A gel, floated at LFP voltages can live 15-20 years.
 
you could also set your BMS to 3.0 cell volt (about 10%) then but a bypass relay with a switch so when you lose power flip emergency power big red light comes on.. you have power for emergency or start generator or radio
 
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