Good Morning Mainesail,
I know you don't do any DIY long range consulting anymore on lithium battery banks but this is more of a philosophic question and I guess applies to any type of battery bank.
In your
article on LiFePo4 batteries you wrote the following:
Now this argument makes sense for a typical weekend boater. But what about a cruiser. I am not thinking about the getting me into harbor (I plan to have a reserve lead acid battery for that). But if I am in say the San Blas Islands in Panama where there is nothing around, I could take the time at anchor to reconfigure and re-balance the bank.
Curious on your thoughts in that application.
Thanks,
Jesse
There is really nothing wrong with a parallel then series cell bank but just not what I would choose on my own boat with regard to LFP, if the cells exist to do it in series.. Some things to think about...
#1 How do you fuse protect parallel balanced cells to prevent a thermal runaway/meltdown event should one cell fail internally? The correct answer to this is that
you really can't. Any fuse large enough to run the loads you expect, starter motor, windlass etc., without nuisance tripping, will rarely if ever trip into an internal cell failure unless it is a very "sudden" and very high current failure such as a nail puncture. Failures of the seperator that lead to cell shorts are most often caused by
dendrites. These failures are almost always from mischarging/over-discharging/cold weather charging or other owner absuse. They start out as small shorts that are simply not enough to trip a fuse but are large enough to heat the shorted cell.
#2 Now that you're in the middle of nowhere, & rebuilding the pack, and you already know one cell has failed, how will you "
match" the other
used cells to build a
half capacity bank from? In order to build your own LFP bank you first need to match the cells and far too many DIY's ignore this critical step.
#3 Are you going to carry the test equipment on-board to match cells? These days it has become more & more difficult to find well matched cells,, as compared to a number of years ago, due to the fact that most "A" graded cells are staying in China and the low grade cells are being exported. Unless you have a very large commercial contract, Lithionics, Trojan, Mastervolt, Victron etc. you'll need to choose your cell supplier very, very carefully and even then you'll still need to confirm how well the cells match.
#3 If you start with premium good quality cells, and charge them correctly, you should not need to worry about premature failures. Every failure I know of, except for a few that were mismatched cells to begin with, have been user/owner induced. Even the
mismatched cells are "
owner induced" as every LFP pack build needs to be done correctly and the only way to do this correctly is to physically capacity test each and every cell in the pack before assembly. Top balancing is just part of the process but, not the whole process.. Manufacturers such as Lithionics, Mastervolt and Victron use parallel cells because it costs them a lot less to build varying pack sizes than it does to stock numerous cells sizes for builds that are series only.
#4 If you've had an issue in one cell it is likely the other cells are not in perfect shape either. If cruisng in warmer climates, eg: Panama etc., your cells will also degrade/lose capacity more rapidly than in a cooler climate so any replacement cell being brought into the existing pack won't be an optimal fit. Rebuilding a pack that has already experienced one cell failure is a
questionable at best practice. Unless you have the capability to test each cell, and build the remaining pack only from the cells that match clossest, this is really a practice that essentially remains in the
nice thought category.
