locked Re: Batteries starting to get weak - MMM - What to do -
Drop in replacement have build in BMS-es that is correct. The quality of those can be from excellent to abysmal and you have very little clues to determine this from the outside.
I do think that assembling your own battery pack is currently the best way forward if you have technical skills. It's cheaper and you determine for yourself which BMS system gives you the best facilities for your usage case.
It also enables you to make better use of available space. An example of company that facilitates this route is https://shop.gwl.eu/index.php?force_sid=nc4jtsqa8h85c6d1m7j3t8ncnj& (I have no relation with them and never bought anything there)
Your assumption about 15-30 year investment is highly optimistic at best. Personally I would assume a lifespan of 8 years simply as a result of lack of evidence that these things last longer. Your best bet is looking at the way Tesla's from 2013 are behaving and those use different chemistries (Li-Ion instead of LiFePO)
As said before, my main beef with this stuff is that to get the best life expectancy from these things you need to treat them carefully, meaning not fully empty or fully charged for extended periods and keeping them balanced. Most systems can only balance them at 100% SOC. This means you will need to think of the SOC of your batteries in relation to your cruising plans and modify the configuration accordingly. The latter is can be quite the hassle as you need to "tell" all your charging sources to keep a certain maximum float-voltage (more accurately maximum SOC). Alternatively you can also make them stop charging, but that means you will be short-cycling the batteries needlessly because of the never ending consumption by systems on the boat. The effects of this are unknown to me and I cannot find clear information about it. It also means your battery bank is not "set and forget" as many supplies claim is some form or shape.
My point is that it is not so hard to put a well working system together but designing one that minimizes stress on the battery system and maximizes longevity is quite the challenge. It is also depending on your use-case provided you can predict that for the coming 8-15 years (I can't). The Battle Born batteries give you very little to control the SOC and don't provide you with information on what's going on inside the black box. Their 10-year warranty doesn't mean much to me. Who knows if the company is still around in 10 years and what they will tell you then.
So in the end, I think there are some good reasons to switch to Lithium but life-span discussions in relation to total cost of ownership and comparing them to alternatives are risky at best. My thought is that the charge/discharge curve of lithium and the lack of need to charge them to 100% every time are the VERY big plus arguments. All other points are secondary and maybe even debatable.