Preface: Please understand that this article is in no way meant to attack or disparage Mastervolt. Mastervolt is a good company but like every company out there they may not always be perfect in every way. This ruined $10,000.00 LiFePO4 battery (pricing when it was purchased) just happens to be a great case study to discuss LiFePO4 design short-comings and how they can affect the cells, even up to the point of destruction. If you are a DIY thinking about building your own LFP batteries, please read this and think about how confident you are that you could do better than the engineers at Mastervolt.


Sadly LiFePO4 failings are seldom talked about on the web. I think folks don't want to admit they have made mistakes but we can learn from those mistakes. I know of numerous DIY LiFePO4 banks destroyed (only two of those discussed publicly) This is THOUSANDS and THOUSANDS and THOUSANDS of dollars in ruined batteries. I also know of far too many "factory integrated" banks that have been ruined.


It should be noted that none of these destroyed banks has started a single fire, just put massive dents into the owners wallet.



The Battery:


This is a 24V nominal LiFePo4 battery. 6.117V is BAD NEWS!


This battery was brought to me by an individual in the industry so I could try and figure out what went wrong. I really wanted to get to the meat and bones of how a BMS protected factory made system could actually fail like this?


The owner noted the battery no longer performing, as it had been, and capacity was not as it had been originally. When the boat was decommissioned at the end of the season the battery was taken off-line per Mastervolt instructions. Despite following the instructions the 24V 160Ah battery had been allowed to become grossly over-discharged with a total pack voltage of just 6.117V for what should be a 26.5V pack. Why? Read on and you'll see.


The resting voltage of this pack should be well above 26.5V. The 8 prismatic cells measured from as low as 0.760V to as high as 0.813V when it arrived here.


There is a reason I suggest that; care/use, system installation, proper charging, proper discharging & system integration all matter equally.


Despite having a very expensive, complex and permanently integrated BMS, there were two major failings:


#1 The battery became grossly over-discharged despite being physically disconnected from the vessel when it did.


#2 The battery had also been grossly over-charged and is what led to the capacity demise and cell swelling, which is partly a design flaw. A proper "system installation", using a solid BMS design, safe & sane LiFePO4 charging voltages & CV DURATION, could have prevented this $10,000.00 failure, but it did not, because unnecessarily high charging voltages were both indicated and designed for in this model of Mastervolt LiFePO4.


The BMS system never should have allowed either of these two major events to occur, but it did because the design and implementation was flawed. Perhaps this was based on a lack of, or just poor information & guidance from the cell maker or both parties are to blame, impossible to really say other than this is now a 10K pile-o-scrap at nowhere near 2000 cycles....