locked Re: Batteries starting to get weak - MMM - What to do -


Scott SV Tengah
 

Mark,

I looked at his first post and a few things he stated that is not consistent with the known characteristics of lithium:

1) Take a look at the voltage vs. SOC graph for a Lifepo4 battery. 
https://www.powerstream.com/lithium-phosphate-charge-voltage.htm

At 3.2v per cell or (3.2*8 = 25.8v) you are at around 3-8% state of charge.
At 3.3v per cell or (3.3*8 = 26.4v) you are at around 20-31% state of charge.
At 3.4v per cell or (3.4*8= 27.2v) you are at around 96-99% SOC. 

But it's not just the spread, it's the shape of the curve. The challenge is that the lithium curve, unlike lead, is NOT linear. The middle portion between say 10% and 90% has a very flat voltage curve.  See link above. I can attest that with my Mastervolt 110A charger set to 27.5v absorption, the alternator has stopped charging anywhere between 75-100%, averaging 85%.

If you set your charger to stop charging at 26.8v, you may be charging your batteries to only 40-50%.

And as you can see with lead, the voltage vs. SOC curve is quite linear, making it far more useful to determine SOC from voltage.
https://www.researchgate.net/profile/Jeff_Cherry2/publication/315847359/figure/fig2/AS:633864561106948@1528136496496/Typical-Open-Circuit-Voltage-OCV-of-12V-Lead-Acid-Battery.png

2) He stops charging at 3.45v per cell and has a high voltage alarm at 3.5v. His high voltage ALARM is below Victron's absorption voltage of 3.55v and below Mastervolt's absorption voltage of 3.6v. Keeping voltage low like that will help avoid high SOC induced imbalances but there are other sources of imbalance too, namely high load or imperfectly assembled batteries. And most cell balancing systems require you to charge to near full before they start balancing, as indicated by low charge current acceptance rate - namely because of the non-linear charge curve. I have had batteries that show perfect balance until you approach 3.5v+ and then one cell goes wildly out of balance.

By not charging your batteries fully every once in a while, you may end up prolonging a cell imbalance issue that, in the event that you need to draw the batteries quite deeply (big storm, can't run the genset because of sea state), could result in a cell LOW voltage problem that will destroy the battery or cause your BMS to disconnect your loads at the worst time. Further, most battery monitors determine state of charge based on calibrating to 100%. And 100% on my battery monitor is determined when charge current is below X for time period Y. That means the battery is full and no longer accepting charge. As such, Victron explicitly recommends charging fully once a month to recalibrate the SOC monitor.



--
Scott 
2007 A54 #69
SV Tengah
http://www.svtengah.com

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