Sunday, November 9, 2014

TOP BALANCING THE BOTTOM

When I started bottom balancing the second battery module, readings were taken on each 1/2 cell. There are two individual groups of pouch cells within each cell that are hooked in series, with each side reading around 3.85 volts to the center terminal or 7.7 volts across the entire cell.


POUCH CELL


ASSEMBLED CELL CUT AWAY


CELL SHOWING THE TERMINALS


SIXTEEN CELL MODULES

When initial readings were take across the entire 16 cell module, it measured 124.5 volts and each of the half cells were within a couple hundredths of a volt of each other, so I assume the Better Place Battery Management System (BMS) was top balancing the individual cells. After draining the entire module down until the lowest cell was 2.6 volts, I let the pack sit for a couple days and took readings on each half cell again, this time trying to identify the highest half cell. The lowest had come up to 2.85 volts and the highest was up to 2.95. By the end of the day the highest cell had come up to 3.04 volts, so I hooked the two 150 watt quartz lights up and drained the pack down until the highest cell was at 2.6 volts. The lowest cell had gone down to 2.45 volts. Then I hooked up the 40 watt bulb and let the voltage start climbing back up at a lower amp draw. Once the voltage climb stopped, I turned off the 40 watt bulb and turned on a 15 watt night light. I always put a note on my computer that the batteries are discharging so I will not forget to stop the drain before going to bed. 

I may need to repeat this process several times before the half cells finally stabilize below 3 volts. Once they have set for several days and are all under 3 volts, I will use the Powerlab6 to bring them all up to 3 volts. Kind of like top balancing the bottom, as the Powerlab6 will not automatically shut off at a voltage under 3 volts when charging and if you try and discharge them below 3 volts, they just keep drifting back up. It seems that charging to bring them all to the same level at the bottom works better and gets them all to a more uniform voltage.

The LiMn2O4 cells operate at a higher voltage range that the LiFePo4 cells at at around 3.9 resting voltage (give or take) as opposed the the 3.36 volts of the Lithium Iron Phosphate cells and there isn't a lot of information on bottom balancing LiMn204 cells yet. Jack Rickard of EVTV reported that they just did not want to stabilize below 3 volts and said that is now the voltage he is using, so that is where I came up with using 3 volts.

After an hour or so with the 15 watt night light hooked up the highest cell stabilized at 3.809 volts, so the light was shut off and I will wait until Wednesday night to check all of the individual cells again. By next weekend, if they are all below 3 volts, I will start bringing them all up to 3 volts with the Powerlab6, if not, I will drain the module down until the highest cell is 2.6 volts and wait another week to see if it stabilizes below 3 volts.

Until next time,

Randy

PS. Still searching for the lost connectors!
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