This cell and three others were over charged by one of the brightest guys in DIY LFP banks. He is also an EE. $hit happens and I use the $ for an S for a reason.



Quote:

"Charging 4 x 90Ah cells in parallel with a 40 amp 12v charger, thought I'd turned the charger off, didn't discover it till a few hrs later. The cell was at 4.55v from memory and so hot the terminal bolts burn into the finger tips. The strange smell of the electrolyte vapor, but no sign of any white cloud. The heat was similar to standing in front of an oil heater on full and was still quite noticeable the following morning. Only the 2 cells in the center bulged and they are the only two that failed. The cells at either end had better cooling, they bulged a bit, but they are still part of my battery bank 12 moths later."


Let's break this down to see how easy these mistakes can happen..


*These cells were in parallel which means a 3.2V nominal pack


*A 12V charger was used instead of a power supply or charger capable of LIMITING the voltage to 3.XX


*He thought he turned the charger off. This is a prime example of HEF (human error factor). No matter how smart we are, we are still capable of making human errors or being forgetful. This is just normal human nature.


*Cells hit 4.55VPC!!!!!!!!!!!!!!!!!!!!!!!!


*Cells DID NOT EXPLODE, Catch fire or do anything other than get very hot!


*The cells did not even smoke!


*Two of the four cells actually survived this abuse!


*Simply AMAZING!!!


*Imagine how long your cells will live if you don't allow HEF into the equation and you charge them PROPERLY for a fractional "C" system?


If an EE and guy who knows more than just about anyone I know on LFP, & the subject of fractional "C" use can do this, you could too. I will mention it again, use a BMS on your bank for HVC / LVC and a bench top power supply for top balancing...


So what actually happens when you do over charge? Over charge forces lithium oxide to form on the cathode. The LFP cathode is, well, lithium iron phosphate. By causing an over charge you have now converted some of the lithium iron phosphate to Lithium oxide, not good! There is no reconversion or fixing this situation and the cell is now irreversibly damaged. Even slight over charging episodes can cause increases in internal resistance and cause a loss of capacity.


With the CC/CV charging we use in the marine market, and multiple sources of it, your best and safest bet is to limit the constant voltage stage of charging to less than 14.2V or 3.55VPC (14.0V or 3.5VPC is even safer). Remember these cells are technically "full" at 3.4 VPC or 13.6V...


If your cells rest at room temperature for 3+ hours with no load and wind up at 3.4VPC they are full. Getting them full does require a voltage higher than 3.4VPC in order to get them to 3.4VPC rested, but not much higher. Charge to 13.8V or 14.0V and allow the current to tail off to about .025C (2.5A on a 100Ah cell) and your bank is now about as full as you'll need it to be.


I charge this 400Ah Winston pack to 13.8V and 0.025C current and it still delivers 425Ah's at a .25C load! This has now been tested and repeated eleven times in complete 100% discharge capacity test to confirm the Ah capacity...



Image Courtesy: Terry