My 300Ah Victron Smart Lithium battery has been in use on my boat for 7 years. Recently the BMS has been shutting the system down at around 48% SOC. Today I took some data and note that the voltage of this battery starts to plummet at around 12.75v, 60% SOC. This pretty much halves my battery capacity. I also see a big cell imbalance at around 50% SOC.
Would it fair to assume that this battery has come to the end of its useful life?
Unknown.
It would be best to fully change the battery until Victron Connect reports the battery has been “balanced”.
I have a self assembled 300Ah Winston battery @ 6 years that the BMV reports these figures.
The battery is getting fully charged most days and reports itself as balanced at 100%. My problem is what I would expect to happened around 5-10% SOC is happening around 50% SOC. Put another way I would expect to take about 280Ah from the battery without charging but I can only take about 150Ah.
Pls post your smartshunt settings, the battery posts last full charge 8 days ago…
Imho your battery is too small, you have over 1000 full cycles in 7 years, @water_rat has 85 in 6 years, my batteries have 45 in 4 years.
The Victron Datasheet specifies a 5000 cycle life for their Smart Batteries
The reason why the last full charge is 8 days ago is because the boat is out of commission and I am doing a slow discharge to get to the bottom of this problem.
Your screenshot says “Last full charge 8d 18h”.
Make a full charge cycle, if possible with shore power and reduced current until you reach 14,2V.
Thank you. I have done this now with 2 hours absorption at 14.2v and am now again monitoring a slow discharge.
This is not a new problem, it has been apparent for a number of months with many full charge cycles during this period.
At the current rate of discharge it will be about a week to get to 50% when I expect the imbalance and subsequent BMS shutdown to re-occur.
Your discharge floor is at 35%, so 0..100% SoC equals 0..200Ah charge. If your SoC is at 50% you have 200Ah left in your battery, not 150.
But your tailcurrent is much too high, should be 1%. Peukert should be 1.
Pls post the charger data also.
Do the victron batteries have 5 or 10 years warranty ?
I don’t think you are right about this. The discharge floor is simply used for the ‘Time to Go’ feature of the BMV battery monitor and also in some relay settings. It is also the point where the BMV monitor will sound an audible alarm.
The tail current setting bears no relation to the reported state of charge, it is used to tell the BMV when the battery SOC can be reset to 100%. 4% is considered an acceptable for setting for these batteries.
Of course I stand to be corrected.
I got my BMV settings data from one of my suppliers, one of the main Victron dealers in the USA. How to set up your battery monitor correctly with Lithium Batteries
That’s not correct.
The shown SOC is always the full capacity.
The “discharge floor” is only used for the “time-to-go” calculation.
So with a “discharge floor” set to 35% the “time-to-go” shows 0 minutes if you hit 35% SOC.
100% is 300Ah and 0% is 0Ah
Lithium isn’t 100% efficient → peukert isn’t 1
Victron recommends 1,05.
Batteries have 3 years warranty.
At 7 years old battery, so out of any warranty, you can be bold and open the battery and measure the individual cell voltages.
My hunch is that one cell is out of balance - yes, I know, it says balanced at 100%, but still - and that cell is dragging down the total voltage.
If it’s possible, make a top end balance manually, as indicated here: Top Balancing "How to" | DIY Solar Power Forum
No need to open the battery, VC reports the cell voltages.
Dunno.
The math works out to 130Ah drawn a day.
On average.
Better…
Sorry, no experience with such batteries. 
For OP:
Which are the individual cell voltages when at 100% and says balanced?
Because 0.2V difference before plummeting is BIG when you think that between 0% and 100% you have only 0.5V difference…
One interesting thing about your BMV history.
Your battery round trip efficiency (charge vs discharge energy) is very high at 95.4%.
Could be a load bypassing the shunt.?
2 things that degrade lithium batteries.
High charge/discharge currents.
And even worse, high ambient temps.
I did consider loads bypassing the shunt but the only thing connected to the batter’s neg post is the shunt.
High Ambient temp for sure, The boat has been in the tropics all of those 7 years, now in Malaysia where the battery compartment temp regularly approaches 40C. I have a fan installed but not much more I can do about that.
Maybe i shouldn’t write in the middle of the night 
You could, for a small increase in electricity for a fluid pump and the initial cost for a saltwater/air heat exchanger to be in front of the fan you already added, use the surrounding water to cool down the battery compartment a little.